Journal of Soils and Sediments

Published by Springer Nature
Online ISSN: 1614-7480
Print ISSN: 1439-0108
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  • Xiujuan ZhangXiujuan Zhang
  • Dale LiDale Li
  • Yong LiuYong Liu
  • [...]
  • Hangwei HuHangwei Hu
Purpose Bacterial diversity drives multifunctionality in ecosystems, including carbon cycling, while a comprehensive understanding of how carbon contents modulate microbial function through bacterial diversity is unknown. The exact extent of the contribution of organic matters contents to carbon cyling process will be investigated using a dilution-to-extinction methods in soils. Materials and methods We conducted a dilution-to-extinction experiment to manipulate the bacterial diversity in soils amended with five different organic matter contents. Meanwhile, the abundance and community structure of bacteria were detected by quantitative PCR and high-throughput sequencing, respectively. Furthermore, the dehydrogenase activity, cellulase enzyme activity and β-glucosidase enzyme activity were detected. Results and discussion The co-occurrence patterns of sensitive bacterial communities in the soil with 5 organic matter contents showed that organic matter content had a strong effect on sensitive bacterial network complexity. The degree of functional redundancy varied with soil organic matter contents, which was attributed to the interactions among organisms. Conclusions Soil carbon contents exerted a strong influence on ecosystem function, and bacterial diversity plays a predominant role in soil with low organic content. Our findings showed the contribution of bacterial diversity in modulating soil function and shed new light on the role of microbial groups in land restoration.
 
  • Chenguang WangChenguang Wang
  • Bo MaBo Ma
  • Yangxiu WangYangxiu Wang
  • [...]
  • Da HuoDa Huo
Purpose To explore the impacts of wheat straw length and coverage on soil infiltration, runoff, and sediment production in sloping farmland under different mulching methods. Materials and methods In this study, an indoor simulated rainfall experiment (90 mm·h⁻¹) was used to study the characteristics of runoff and sediment production in sloping farmland under different straw lengths (3 ~ 5, 8 ~ 10, and 13 ~ 15 cm), different mulching amounts (1.5, 2.5, 3.5, and 4.5 t·hm⁻²), and different mulching methods (surface mulching and mixed mulch). Results and discussion In the surface mulching, the average infiltration increased with increasing coverage and decreased with increasing length. In the mixed mulching, with the increase in coverage, the infiltration first increased and then decreased. When the coverage was less than or equal to 3.5 t⋅hm⁻², with increasing length, the infiltration first increased and then decreased. However, when the coverage was 4.5 t⋅hm⁻², with increasing length, the infiltration showed an increasing trend. The change trend of the average infiltration was opposite to the change trend of the cumulative runoff and sediment yield. When coverage and length were the same, the runoff cost of sediment control of the surface mulch was generally higher than that of the mixed mulch. Conclusions In this experiment, when the length was 3–5 cm and the amount was 4.5 t⋅hm⁻², the runoff was the lowest, and the infiltration was the highest under the surface mulch. The sediment production was the lowest when the length was 8–10 cm and the coverage was 3.5 t⋅hm⁻² under the mixed mulch. Straw mulching can reduce soil loess, which has certain significance for soil erosion control and the healthy and sustainable development of agriculture.
 
Purpose Biochar and silicate-enriched steel-slag, as agricultural and industrial waste materials, are used to improve soil physicochemical properties and soil fertility; however, there are few studies on the effects of their combined application to paddy fields on the formation of root Fe plaque. Materials and methods We tested the effects of four application rates (0, 300, 600 and 900 kg ha⁻¹) of biochar-based silicate fertilizer (BSF) to early and late rice on root Fe plaque formation. Results and discussion Application of BSF increased soil total carbon and total nitrogen concentrations in late rice, and total phosphorus concentrations were increased by 47.03% at the jointing stage and 27.73% at the mature growth stage of early rice following application of BSF at 600 kg ha⁻¹. The three application treatments all significantly increased the abundance of soil iron-reducing bacteria (IRB) in late rice, and application of 600 kg of BSF ha⁻¹ increased IRB abundance by 52.16% and 66.59% at the jointing and mature growth stage, respectively. Both IRB abundance and IP concentration were positively correlated with Fe(II) and negatively correlated with Fe(III) in late rice. Soil Fe(II) concentration was positively correlated with water content, whereas Fe(III) concentration was negatively correlated. Conclusions Our study demonstrated that moderate inputs of BSF (600 kg ha⁻¹) increased soil Fe reduction and subsequently promoted the formation of more soluble Fe(II) and iron plaque formation through soil fluidity and Fe(II) movement to roots. We suggest that the application of BSF to dry–wet rice paddies may contribute to reduction of agricultural pollution, improvement in soil conditions, and production of sustainable healthy food crops.
 
Congener profiles of PCDD/Fs in different layers
PCDD/F concentrations and toxicity equivalent concentration in a e-waste dismantling site and b stream sediment
Vertical distributions of TOC in soil/sediment core from e-waste dismantling site and stream
a Loading plot of PCA for PCDD/Fs. b The score plot for samples collected in e-waste dismantling site, adjacent stream, and paddy field. c Hierarchical cluster analysis (HCA) dendrogram for samples collected in e-waste dismantling site, adjacent stream, and paddy field
Informal electronic waste (e-waste) recycling has been identified as a significant source of polychlorinated dibenzo-p-dioxins/dibenzofurans (PCDD/Fs), which pose a health and environmental danger. The impact of PCDD/Fs in the e-waste dismantling sites depends on their concentration in environmental matrices. This study aimed to investigate the vertical distribution characteristic of PCDD/F content collected from an abandoned e-waste dismantling site in Guiyu, China, as well as the nearby paddy field and stream. One year after the e-waste dismantle site was abandoned, soil, sediment, and mud cores were taken from the site, the neighboring stream, and the paddy field. The source and distribution characteristics of PCDD/Fs in samples were revealed using a variety of statistical analytic methods. The concentrations of PCDD/Fs steadily decreased with increasing soil depth at all sampling sites. The highest concentration of PCDD/Fs was discovered in the sub-layer (10–20 cm) of the e-waste disposal site, at 263,332.72 ng/kg. PCDD/Fs concentrations in soil, sediment, and mud samples were 314.21–16,037.39, 0.01–0.11, and 6.93–97.44 ng I-TEQ/kg, respectively, according to the International Toxicity Equivalency Quantity (I-TEQ). The health risk assessment revealed the harsh working environment for the workers. The association between the three sampling sites was determined using principal component analysis (PCA) and hierarchical cluster analysis (HCA). The main source of PCDD/Fs was e-waste recycling, and one of the explanations for the similarity of PCDD/Fs distribution between the non-plough layer (0–20 cm) of paddy field and sediments was thought to be water management. Decades of informal e-waste recycling have led to PCDD/Fs reaching deep into the soil. The order of pollution levels was as follows: e-waste dismantling site > paddy field > stream. The utilization of the e-waste dismantling site and its neighboring lands necessitates meticulous planning and oversight.
 
Purpose Fe-modified biochar could promote soil organic phosphorus (Po) mineralization and phosphorus (P) bioavailability; however, the role of biochar and Fe-modified biochar in driving the mineralization of Po by phoD-harboring bacteria in soil aggregates was less known, especially in saline-alkali paddy soil. Methods A 5-year paddy field experiment was conducted in Yellow River Delta, in which four fertilization treatments were designed: no fertilization (Control), inorganic fertilizer (NPK), NPK + biochar (BC), and NPK + Fe-modified biochar (FeBC). Results Compared with NPK treatment, the proportion of macroaggregates in BC and FeBC treatments was significantly increased by 10.42% and 24.67%, respectively. And the mean weight diameter was also increased under biochar treatments. Redundancy analysis (RDA) indicated that oxalate-extractable Fe was a main factor in the stability of soil aggregates, which was significantly increased in FeBC treatment. Meanwhile, the relative abundance of phoD-harboring bacteria was increased in both BC and FeBC treatments, which was driven by the content of soil organic carbon. And the diversity of phoD-harboring bacteria was positively correlated with Po mineralization. Meanwhile, compared with NPK treatment, the activity of alkaline phosphatase in macroaggregate was increased by 39.1% in FeBC treatments, which also promoted the mineralization of soil Po. Conclusions Therefore, biochar addition, especially Fe-modified biochar combined with mineral fertilization, was a better strategy in improving soil aggregate structure and promoting Po mineralization in saline-alkaline paddy soil.
 
Soybeans in growth chambers at the time of harvest grown on potted control (right side) and sludge-amended (left side) soils. Whole plants A and the youngest fully developed tri-foliate leaves B are shown
PurposeAccumulation of toxic trace metals in agricultural soils resulting from sewage sludges represents a threat to food safety and the environment. The aim of this study was to assess the importance of field aging over several decades in attenuating trace metal bioavailability and phytotoxicity in a soil amended with sewage sludge.Methods The availability of nutrients and trace metals in a soil polluted more than 40 years earlier by heavy application of contaminated sewage sludge was estimated by measuring concentrations of soil elements leachable by water and extractable by the Modified Morgan test. A greenhouse bioassay using soybeans was conducted to measure phytoavailability of trace metals and nutrients.ResultsDespite more than 4 decades of aging in the field, Cd, Cu, P, Pb and Zn were much more available in the sludged soil when compared to a nearby uncontaminated control soil as indicated by Modified Morgan extractable levels in the sludged (8.0,11.8, 76.3, 3.0, 191 mg kg−1) compared to the control (0.0,0.15, 3.8, 1.4, 0.40 mg kg−1) soils, respectively. Uptake of Cd, P, and Zn into soybean shoots was increased in the sludged soils compared to the control soils by 8.6, 5.2 and 5.3 times, respectively. Soybeans grown in the sludged soil accumulated much greater shoot concentrations of Mo (12.5 mg kg−1) than the control soybeans (< 0.1 mg kg−1). Conversely, Pb and Cu had less plant uptake from the sludge-amended soil despite greater soil Pb and Cu concentrations. Lower Pb solubility in the sludge-amended soil compared to the control soil is attributable to Pb binding by residual soil organic matter. Phytotoxic symptoms in soybeans included stunted growth and chlorotic leaves, attributable to elevated Zn and P in the shoots and potentially reduced phytoavailability of Fe and Mn in the sludge-amended soil.Conclusions Field aging over 40 years failed to reduce availability of trace metals of concern in soils (particularly Zn, Cd, Mo) to levels that would not harm soil productivity or compromise crop safety. The results demonstrate that agricultural soils severely contaminated by trace metals are unlikely to be restored naturally to a productive condition within a time frame of several decades because long-term processes of aging and loss involved in reducing heavy metal bioavailability are very slow.
 
PurposeGrazing is the primary means of steppe management, and grazing potential represents the potential carrying capacity of steppe livestock production. In steppe ecosystem, changes in soil physicochemical properties affect soil texture, nutrient cycling, etc., which further influence vegetation productivity and change the grazing potential of steppes. Although there are many assessment methods and indicators for studies such as steppe grazing value, there is a lack of studies exploring the relationship between soil physicochemical properties and grazing potential. The main aim of our study was to test how soil physicochemical properties affect steppe grazing potential and how they interact with each other.Methods In this study, we investigated 220 typical steppe sample plots in the eastern Eurasian steppe and proposed a new method for evaluating steppe grazing potential based on plant palatability, the steppe grazing potential index (SGPI), to examine the interactions between five soil physicochemical properties and steppe grazing potential at three grazing potential levels.ResultsWe found that soil physical and chemical properties differed significantly at each grazing potential level. At both high and low grazing potential levels, the physical properties of soils played a major role in influencing SGPI. While the relationship between soil function and grazing potential was the strongest at the moderate grazing potential level, mainly, the chemical property SOC had the most significant role. Overall, SGPI is not only directly influenced by SBD and SOC, but also indirectly driven by TN.Conclusions Our study demonstrates that steppe grazing potential is very sensitive to changes in soil physicochemical properties, and its response to soil nutrients and soil texture varies at different levels. SBD and SOC are sensitive indicators that affect grazing potential. Meanwhile, the soil function and potential carrying capacity of grazing production at moderate grazing potential level are also the priority directions for future research. Exploring the relationship between soil function and steppe grazing potential can help restore livestock productivity and ecological balance in steppes with low grazing potential and ensure the sustainability of steppe ecosystem development.
 
Effect of agroforestry on heavy metal contents of soil and plant in bamboo plantation. A Cu in plant; B Zn in plant; C Cd in plant; D Cu in soil; E Zn in soil; F Cd in soil. Different letters (a, b, c, d) mean significantly different from each other (P ≤ 0.05). * indicates significant results (P ≤ 0.05), whereas ** indicates highly significant results (P ≤ 0.01). PNR, pure forest non-rhizosphere soil; ANR, agroforestry system non-rhizosphere soil; PR, pure forest rhizosphere soil; AR, agroforestry system rhizosphere soil
Boxplot of alpha diversity in bamboo-based agroforestry system and pure bamboo forest. The boxes indicate 25th to 75th percentiles, with mean values (blank line). Whiskers extend to the minimum and maximum values within 1.5-fold interquartile ranges (IQRs). Different letters mean significantly different from each other (P ≤ 0.05). PNR, pure forest non-rhizosphere soil; ANR, agroforestry system non-rhizosphere soil; PR, pure forest rhizosphere soil; AR, agroforestry system rhizosphere soil
Composition of microorganisms in bamboo-based agroforestry system and pure bamboo forest. A Soil microbial composition at the phylum level; B relative abundance distribution of major phyla (top 10); C relative abundance distribution of major genera (top 10); D correlations between soil properties and genera. Different letters (a, b, c, d) mean significantly different from each other (P ≤ 0.05). * indicates significant results (P ≤ 0.05), whereas ** indicates highly significant results (P ≤ 0.01). PNR, pure forest non-rhizosphere soil; ANR, agroforestry system non-rhizosphere soil; PR, pure forest rhizosphere soil; AR, agroforestry system rhizosphere soil
Fourier transform infrared (FTIR) spectra of non-rhizosphere and rhizosphere soil in a bamboo-based agroforestry system and pure bamboo forest. PNR, pure forest non-rhizosphere soil; ANR, agroforestry system non-rhizosphere soil; PR, pure forest rhizosphere soil; AR, agroforestry system rhizosphere soil
Main compounds in root exudates (top 10) in bamboo-based agroforestry system and pure bamboo forest
Purpose Bamboo forests are usually managed to create one-story canopies suitable for agroforestry practices. The aim of this study was to investigate the remediation effects of Lei bamboo (Phyllostachys praecox) in different planting systems, pure and agroforestry, on heavy metal–contaminated (Cd/Zn/Cu) soil. Methods Two different cultivation systems, i.e., a pure Lei bamboo stand and bamboo-based agroforestry system, were established in the field. Soil and plant samples were collected 5 years after planting bamboo in the experimental area. The soil nutrients, root exudates, heavy metals, microbial communities in the rhizosphere and non-rhizosphere soil, and metal uptake in the harvestable parts were determined. Results Amounts of both Cu and Cd removed from the soil per plant in agroforestry were 1.4 times greater than those in pure forest, but there was no difference in Zn in the harvestable parts of plants from the two systems. Compared with non-rhizosphere soil, the rhizosphere soil showed lower relative polysaccharide content and hydrophobicity. The most dominant phyla of bacteria were Proteobacteria, Acidobacteria, and Actinobacteria, with average relative abundances of 30.8%, 22.3%, and 11.8%, respectively. Agroforestry significantly reduced the microbial diversity in bamboo rhizosphere but significantly increased the diversity in the non-rhizosphere, compared with pure forest. There was a strong correlation between soil organic matter (SOM) and genera. Fourier transform infrared spectroscopy and root exudate analysis showed that bamboo planting systems changed the soil functional groups and chemical composition. Conclusion Bamboo roots remove heavy metals from rhizosphere soil and transfer them in large quantities to the stems. Gemmata, Kaistobacter, Arthrobacter, Anaeromyxobacter, Thiobacillus, and Candidatus Solibacter were found to be related to heavy metal soil remediation by bamboo. Rhizosphere processes are important drivers of SOM decomposition and nutrient recycling, contributing to increased heavy metal removal and microbial community diversity. These findings provide insights into the soil characterization of phytoremediation using a bamboo-based agroforestry system.
 
Purpose The increased human activities have significantly promoted the acidification of cultivated soils decreasing the soil water retention properties. This study investigated the improvement effect and mechanism of biochar on water retention properties of acidic soils. Materials and methods Biochar was mixed with acidified and molded soils in different amounts (0, 2, 5, 8, and 10%), and then soil hydrological and water infiltration characteristics were analyzed. Results and discussion The soil application of biochar, improved acidified soil capacity and porosity, and the effects were directly proportional to the applied amount of biochar. Compared to the control group, the application of 10%, biochar improved the average soil water content by 2.1–2.2 times and reduced the soil vertical infiltration rate by 41–43%. The soil vertical infiltration was 23–25% of that in the control group. Moreover, biochar-mediated improvement of soil aggregate agglomerates was found to be associated with the adsorption of soil microaggregates and the formation of water-stable macroaggregates. Conclusion This study found that applying biochar to acidified soils can relieve the problems of poor water retention with a clear improvement mechanism. Large-scale soil application of biochar preventing migration of soil ions can contribute to environmental protection and natural resources recycling.
 
Location of sampling sites (S1 is located in freshwater marsh and S2 in saline marsh)
The change of C, N, DOC content, and C:N ratios in soils during the experiment (SM means the saline marsh; FM means the freshwater marsh. The error bar represents the standard deviation of the samples (n = 4). From S1 to S5, the soil salinities decreased, while from F1 to F5, the soil salinities increased)
The change of the production rates of GHGs in the soils during the experiment (SM means the saline marsh; FM means the freshwater marsh. The error bar represents the standard deviation of the samples (n = 4). From S1 to S5, the soil salinities decreased, while from F1 to F5, the soil salinities increased)
The cumulative emissions of the GHGs in the soils during the experiment (SM means the saline marsh; FM means the freshwater marsh. The error bar represents the standard deviation of the samples (n = 4). From S1 to S5, the soil salinities decreased, while from F1 to F5, the soil salinities increased)
Spearman correlation coefficients between the GHG production rates and the soil C, N, DOC content, and C:N (n = 28)
Purpose Salinity and vegetation can significantly affect the productions and emissions of greenhouse gases (GHGs) in wetland soils. We aim to investigate the effect of salinity on the productions and emissions of GHGs in marsh soils during the decomposition of wetland plants under different scenarios. Methods Soils and wetland plants collected from a freshwater marsh (FM) and a saline marsh (SM) in the Louisiana Barataria Basin estuary were incubated under different levels of salinity treatment for 180 days. Gas and soil samples were periodically collected and analyzed for CO2, CH4, and N2O and for TN, TC, and DOC contents, respectively. Results CH4 and CO2 production rates reached maximums on day 30 as the decomposition processed. The increment of CH4 and CO2 production rates in SM soils was greater than in FM soils. The CH4 and CO2 production rates were generally more significantly correlated with DOC than with TC. Reducing salinity inhibited the productions and emissions of CO2 and N2O but significantly promoted those of CH4 in SM soils, while in FM soils, increasing salinity significantly decreased the CO2 production and emission but had no significant effect on CH4 and N2O. The CO2-equivalent cumulative emissions increased by 41% as the salinity decreased by 17.7% in saline marsh, and the percent contribution of CH4 emissions to CO2-equivalent cumulative emissions increased from 15 to 47%. Conclusion Our results suggested the potential that the diversion of Mississippi River water to the wetlands might promote the production and emission of CH4 in saline marsh.
 
Purpose Environmentally friendly mulching material and appropriate tillage practice are needed to solve plastic film residues in agricultural production in ridge-furrow rainwater harvesting technology (RFRHT) in the Loess Plateau in China. Materials and methods A field experiment in randomized block design was conducted to (1) investigate the runoff coefficient for three ridge widths (30, 45, and 60 cm) using three ridge mulching materials (ridges compacted with soil (RCS), maize straw biochar-soil mixture (SBM), and cow dung biochar-soil mixture (DBM)); (2) the effects of three ridge widths using three ridge mulching materials on soil moisture, temperature, nutrients, fodder yield, quality, and water use efficiency (WUE) of sainfoin and conventional flat planting (FP) as a control, during two consecutive sainfoin-growing years: 2017 and 2018. Results The predicted runoff coefficient for RCS30, RCS45, RCS60, SBM30, SBM45, SBM60, DBM30, DBM45, and DBM60 (subscripts 30, 45, and 60 referred to ridge widths) was 0.31, 0.33, 0.34, 0.26, 0.30, 0.31, 0.22, 0.24, and 0.25, respectively, over 2 years. DBM had a higher concentration of total nitrogen and organic matter compared to SBM, while SBM had a higher concentration of Olsen phosphorus and available potassium compared to DBM. The higher runoff coefficient and soil moisture in SBM led to higher fodder yield, WUE, and condensed tannin content of sainfoin, compared to DBM. Compared to FP, in RCS, fodder yield and WUE of sainfoin decreased by 8.8–17.8% and 0.6–2.6 kg ha⁻¹ mm⁻¹, respectively. Condensed tannins concentration of sainfoin for RCS, SBM, and DBM increased by 4.1 −9.0%, 11.4 −21.8%, and 9.4 −15.2%, respectively. Fodder yield in SBM and DBM increased by 14.3 −19.5% and 7.1 −10.0%, respectively, while WUE in SBM and DBM increased by 6.7 −8.5 and 4.7 −5.5 kg ha⁻¹ mm⁻¹. Conclusion Ridges compacted with biochar-soil mixture, especially with maize straw biochar-soil mixture, increased fodder yield, WUE, and condensed tannin content of sainfoin. The optimum ridge width in SBM and DBM for sainfoin production was 46–49 and 41 cm, respectively.
 
Form transformation of soil Cd during the main growth stages of rice. FI flooding irrigation, II intermittent irrigation, WI wet irrigation, F1 acid extractable, F2 reducible, F3 oxidizable, F4 residual. a, b Tillering stage. c, d Jointing stage. e, f Maturity stage. CK, BC20, BC40, and BC60 refer to the biochar dosage of 0 g kg⁻¹, 20 g kg⁻¹, 40 g kg⁻¹, and 60 g kg⁻¹, respectively. Values (means ± SD, n = 3) with different lowercase letters indicate a significant difference (p < 0.05) between treatments in the same fraction
Form transformation of soil Cu during the main growth stages of rice. FI flooding irrigation, II intermittent irrigation, WI wet irrigation, F1 acid extractable, F2 reducible, F3 oxidizable, F4 residual. a, b Tillering stage. c, d Jointing stage. e, f Maturity stage. CK, BC20, BC40, and BC60 refer to the biochar dosage of 0 g kg⁻¹, 20 g kg⁻¹, 40 g kg⁻¹, and 60 g kg⁻¹, respectively. Values (means ± SD, n = 3) with different lowercase letters indicate a significant difference (p < 0.05) between treatments in the same fraction
Cd and Cu transfer coefficient in different parts of rice (2019). FI flooding irrigation, II intermittent irrigation, WI wet irrigation. CK, BC20, BC40, and BC60 refer to the biochar dosage of 0 g kg⁻¹, 20 g kg⁻¹, 40 g kg⁻¹, and 60 g kg⁻¹, respectively. Values (means ± SD, n = 3) with different lowercase letters indicate a significant difference (p < 0.05) between treatments in the same rice organs
Purpose Biochar has become a research hotspot in soil heavy metal pollution remediation. However, there are few studies on the effect of biochar on the heavy metal accumulation in rice under different irrigation regimes, and there is very limited information on the soil heavy metal speciation changes during whole rice growth period. This study aims to clarify the effect and mechanism of biochar on Cd and Cu accumulation in rice grains under different irrigation regimes. Materials and methods The rice straw biochar (0 g kg⁻¹ (CK), 20 g kg⁻¹ (BC20), 40 g kg⁻¹ (BC40), and 60 g kg⁻¹ (BC60)) was applied to the paddy soil (contaminated with Cd and Cu) under three irrigation regimes (flooding irrigation (FI), intermittent irrigation (II), and wet irrigation (WI)), and a rice pot experiment was carried out for two consecutive years. The speciation changes of Cd and Cu in the soil during the key stage of rice growth and the metal content of each part of the rice were analyzed. Results and discussion Biochar promoted the formation of reducible and oxidizable Cd throughout the rice growth period, while the acid-extractable and residual forms were dynamic and affected by irrigation regimes. A significant reduction in acid-extractable Cu content was observed, and biochar mainly converted the acid-extractable Cu to oxidizable Cu, while reducible Cu was sensitive to soil water conditions. As biochar reduced soil available Cd content, BC60 decreased Cd content in brown rice by 50.41%, 70.32%, and 81.52% under FI, II, and WI, respectively, but the Cd content in brown rice was the lowest in FI. Biochar decreased the Cu content in brown rice under FI (maximum 31.2%), while significantly increased Cu content under II (maximum 74.33%) and WI (maximum 49.3%) because biochar increased soil available Cu content after rice heading stage and encouraged its transport from root to shoot. Conclusions Rice straw biochar can be used to control Cd pollution in rice under various irrigation regimes, and the effect is better when combined with flooding irrigation. The application of rice straw biochar to control crop Cu pollution is not suitable for water-saving irrigation.
 
Purpose Substituting chemical fertilizers with organic manure may alleviate soil acidification in vegetable fields, but the optimum organic manure substitution ratio (SR, the ratio of organic nitrogen (N) rate to total N application rate) may vary due to different climatic conditions (mean annual precipitation (MAP), mean air temperature, and climate zone), initial soil properties (soil texture, initial soil pH, and soil organic carbon content), and management practices (vegetable type, cultivation type, planting season, growth cycle, planting years, and N application rate). Methods Based on a meta-analysis of 348 pairwise observations collected from 74 studies in Chinese vegetable fields, we quantified the responses of topsoil (0–20 cm) pH and vegetable yields to various organic manure (commercial and farmyard manure) substitution ratios (low, SR ≤ 35%; medium, 35% < SR ≤ 70%; and high, SR > 70%) with chemical fertilizer alone as the control. Results The soil pH increased overall with the increase of SRs. Compared with the control, low, medium, and high SRs increased the soil pH by 2.6%, 5.6%, and 9.0% and the vegetable yield by 11.0%, 12.6%, and 3.2%, respectively. The soil pH in open (33.7%) and greenhouse (66.3%) vegetable fields under medium SR increased by 6.3% and 5.0%, respectively. The initial soil pH, MAP, and climate zone are the key factors affecting soil pH in the vegetable field, and their total contribution ratio on soil pH change accounted for 32.1% of all variables. When the initial soil pH ≤ 6, the soil pH increased with the increase of SRs. Conversely, when the initial soil pH > 8, all SRs decreased the soil pH. The soil pH also tended to increase with increasing MAP and subtropical climate zone. The contribution rate of the N application rate on soil pH change was 7.5%. The growth cycle, planting season, and vegetable type had the least impact on soil pH change. Conclusion In general, manure substitution can significantly improve the soil pH and vegetable yield. The optimal SR ranges from 35 to 70%. However, the interactions (among factors) and trade-offs during the process remain complicated, which requires further validation based on local conditions.
 
Overview of the study area in Mu Us sandy land, Inner Mongolia, China. Panel a shows the map of China. Panel b shows the map of the Inner Mongolia Autonomous Region. Panel c shows the 29 study sites in the Mu Us sandy land. The number beside each site represents the year of aerial seeding over the period 1983–2017
Landscape photos showing ecological restoration from aerial seeding at various stages in the Mu Us sandy land, Inner Mongolia, China. Panel a shows the initial stage of aerial seeding at the beginning of restoration (2014), panel b shows restoration from aerial seeding after nearly 10 years (2006), panel c shows restoration from aerial seeding after nearly 20 years (1998), and panel d shows restoration from aerial seeding after nearly 35 years (1984)
Changes in plant biomass (a), relative abundance of soil bacterial (b) and fungal (c) communities at the phylum level along restoration chronosequences following aerial seeding in the Mu Us sandy land, Inner Mongolia, China
β-diversity of the plant (a), soil bacterial (b), and fungal (c) communities following aerial seeding along restoration chronosequences in the Mu Us sandy land, Inner Mongolia, China. Points in the figure represent Jaccard distance between each two sites. Blue in the figure represents plant; orange represents soil bacteria; green represents soil fungi. Significant regression lines are shown in solid at p < 0.05 and p < 0.01 level
Changes of normalized stochasticity ratio in the plant (a), soil bacterial (b), and fungal (c) communities following aerial seeding along restoration chronosequences in the Mu Us sandy land, Inner Mongolia, China. Box plots show the quartile, and midlines show the median levels. Blue in the figure represents plant; orange represents soil bacteria; green represents soil fungi. Dotted lines show the boundary between stochastic and deterministic processes (value = 0.5)
PurposeThe restoration of degraded ecosystems is an important issue in applied ecology. Understanding community assembly during restoration can help facilitate ecological restoration. Aerial seeding is a widely used method to restore degraded ecosystems threatened by desertification. However, there is limited understanding of how aerial seeding affects community assembly processes and the driving factors of plants, soil bacteria, and fungi during ecosystem restoration.Methods This study analyzed the community composition, β-diversity (community difference degree), community assembly processes, and influencing factors of plants, soil bacteria, and fungi using aerial seeding restoration chronosequences from 1983 to 2017 in Mu Us sandy land, China.ResultsOur results showed that plant biomass and relative abundances of beneficial flora in soil bacterial and fungal communities all increased since restoration commenced. β-Diversity of plant communities first decreased and then increased with increasing restoration time. Community assembly was dominated by stochastic processes in early stages of restoration, deterministic processes in middle stages, and stochastic processes in later stages. Dispersal limitation and environmental filtering (soil total nitrogen, soil total organic carbon, and mean annual precipitation) influenced stochastic and deterministic processes in plant communities, respectively. The β-diversity of soil bacteria continuously decreased, and community assembly was almost entirely driven by stochastic processes in the entire chronosequences. Homogenizing dispersal was the key factor driving community assembly. We found no significant changes in soil fungal β-diversity. Deterministic and stochastic processes simultaneously drove soil fungal community assembly. Both environmental filtering (soil total organic carbon and mean annual precipitation) and dispersal limitation affected the community assembly.Conclusion We confirmed the effectiveness of aerial seeding restoration by studying the community composition of plants, soil bacteria, and fungi. Our study highlights that the community assembly processes and driving factors varied for plants, soil bacteria, and fungi during restoration chronosequences. It is necessary to carry out research simultaneously on the community assembly of aboveground plants and underground soil microorganisms.
 
Purpose Volcanic, humid tropical landscapes are characterized by short-term geomorphic transformations due to volcanism and seismic activity, landslides, and other frequent mass movements. These landscape-forming processes are amplified by high temperatures, high annual precipitation rates, and intense rainstorms. The latter can result in significant surface runoff and sediment mobilization, even under pristine rainforest cover. However, knowledge about sediment sources and the magnitude of the associated erosion and accumulation rates remains limited in these systems. Methods This study explores the use of radioisotopes (U-235, Bi-214, Pb-214, total Pb-210, and K-40) and of the fallout radionuclide (FRN) Cs-137 to address that knowledge gap in a pristine, tropical rainforest catchment in northern Costa Rica. We analyzed FRN and radioisotope activities from two reference soil profiles and compared them with those of 17 superficial soil samples collected on two hillslopes and of three streambed sediment samples. Results Modeled hillslope erosion and accumulation rates ranged from 6 t ha⁻¹ year⁻¹ erosion to 6.7 t ha⁻¹ year⁻¹ deposition with up to ± 60% uncertainty reflecting spatially variable interception of rainfall inputs. Preliminary sediment fingerprinting results suggested that deeper soil material, likely originating from in-stream bank erosion and channel incision, was the dominant source of stream sediment (79 ± 19%), whereas superficial soil present on the hillslopes only contributed 22 ± 18% to the stream mixture. Conclusion Our exploratory work highlights the potential importance of channel erosion processes in the sediment yield of steep rainforest catchments, even when hillslopes and streams have a strong hydrological connection.
 
Purpose The main aim of this study was to investigate factors influencing the long-term distribution of ¹³⁷ Cs activity concentrations in the bottom sediments of the dam lake, Koronowo Reservoir, 32 years after the Chernobyl nuclear power plant accident. For this purpose, selected properties of the collected sediment samples, such as grain size, mineralogical composition, and organic matter (OM) content, were investigated. Materials and methods The samples of lake sediments were collected with a Kayak-type gravity corer. The spatial and vertical distributions of ¹³⁷ Cs and ⁴⁰ K activity concentrations in the bottom sediments were investigated based on gamma spectrometry measurements. The particle size distribution of surface lake sediments was determined using a laser particle size analyzer. SEM and XRD were used for the mineralogical analysis of the collected sediment samples. Additionally, the content of organic matter was examined in all samples using an elemental analyzer. Results The ¹³⁷ Cs content was significantly elevated in the case of fine-grained (< 63 µm) surface lake sediments (classified as silts, which are deposited in the profundal zone of Koronowo Lake) and ranged from 12.5 ± 4.1 to 29.2 ± 4.0 Bq kg ⁻¹ . It was found that the increased concentration of ¹³⁷ Cs activity is more closely related to the content of the silt fraction (2–63 µm) than to the clay fraction (< 2 µm) in the collected surface lake sediments. The content of clay minerals also showed a significant positive correlation with ¹³⁷ Cs activity concentration in the surface lake sediments of Koronowo Lake. A similar relationship was noticed for the OM content, but it may be suspected that it is the result of radiocesium-bearing particle accumulation in OM-rich sediments. Conclusion The most important factor influencing the spatial distribution of ¹³⁷ Cs activity concentrations in the surface lake sediments of Koronowo Lake, apart from the bottom morphology and grain size of sediments, is the content of clay minerals. Moreover, the increased detrital inflow to the lake after the construction of the dam could have probably affected the vertical distribution of ¹³⁷ Cs activity concentrations in the bottom sediments, as evidenced by, e.g., the measurements of ⁴⁰ K activity concentration.
 
Purpose Tillage practices affect greenhouse gases’ exchange from soil to atmosphere and thereby change global soil carbon (C) cycle. However, there is limited understanding on how tillage practices affect CO2 production by altering C-cycling functional gene composition of the soil microbes. Methods A long-term field tillage experiment was initiated in 2012 and consisted of no-tillage (NT) and moldboard plowing (MP) both with residue returned, and conventional tillage (CT) with residue removed. In 2020, we sampled the soil and quantified soil physicochemical properties, CO2 emissions, and the microbial C-cycling functional genes which were determined by the quantitative microbial element cycling (QMEC) method. Results The results showed that NT and MP significantly increased CO2 accumulative emissions compared with CT, but CO2 accumulative emissions were not markedly different between NT and MP. Both NT and MP greatly increased soil C-cycling functional genes at the depth of 0–20 cm, while MP decreased C-cycling functional genes, dissolved organic carbon (DOC), and microbial biomass carbon (MBC) concentrations, resulting in a reduction of C allocation at the soil 20–30 cm depth. Furthermore, DOC and NH4⁺-N concentrations were the most important environmental factors for changing microbial function potential. And the ratio of absolute abundance C degradation genes and C fixation genes were both significantly related to CO2 accumulative emissions (R² = 0.37, P < 0.05), suggesting that NT may have great soil potential to accumulate C. Conclusion These findings suggest that no-tillage will be an appropriate strategy to promote the potential of soil C cycle by altering composition and functionality of soil microbial communities.
 
Purpose Microbially mediated nitrate (NO3⁻) reduction coupled to ferrous Fe (Fe[II]) oxidation (termed NRFO) and anaerobic ammonium (NH4⁺) oxidation coupled to ferric Fe (Fe[III]) reduction (termed Feammox) represent two known processes driving Fe redox cycle, and the associated interactions between nutrients and contaminants in paddy soils. Thus, we hypothesized that NRFO and Feammox may play important roles in Cd availability, especially in paddy soils characterized by dynamic redox processes. Materials and methods An acidic paddy soil from a red soil region of southern China was amended with NO3⁻ or NH4⁺ in strictly anoxic incubation experiments. The microcosm technique was used with some modification to achieve the conditions that would facilitate the gradual change in the magnitude of redox. The pH and redox potential (Eh) of soil suspension were measured in situ. Gas samples were withdrawn to analyze nitrous oxide (N2O) concentration. Soil slurries were sampled to determine NO3⁻, nitrite (NO2⁻), NH4⁺, and Fe(II) concentrations and to study Fe and Cd distributions among soil liquid and solid phases. Stoichiometric calculations of redox reactions were carried out at the time point of maximum levels of NO2⁻ production and N2O emission in all soil slurries during the anaerobic incubation. Results and discussion Our study demonstrated the occurrence of NRFO in soil slurries. Of the Fe(II) produced, 30% was contributed by Fe(II) oxidation via NRFO. The occurrence of Feammox in soil slurries was also observed, in which 17% of Fe(II) produced was attributed to Fe(III) reduction via Feammox. Both aqueous and solid-phase Fe(II) oxidation by NO3⁻ and NO2⁻ during NRFO lowered Fe concentrations in soluble, MgCl2-extractable, and NaOAc-extractable fractions, and promoted the formation of amorphous Fe oxides, which provided reactive surfaces for Cd adsorption, thus decreasing the soluble Cd fraction. In contrast, enhanced Fe(III) reduction due to Feammox and subsequent Fe(II) adsorption and precipitation accelerated Fe transformation from MgCl2-extractable to NaOAc-extractable fractions, which triggered Cd transformation from soluble to NH2OH·HCl-extractable fractions. Conclusions All NRFO and Feammox affected the Cd distribution by controlling the Fe redistribution among the soil liquid and solid phases. In the context of high N inputs and high Fe content in paddy soils of southern China, we suggest that NRFO and Feammox could be two potentially important mediating pathways for manipulating the bioavailability of Cd-contaminated soils.
 
Purpose In this study, the effectiveness of in situ sediment capping was investigated as a remediation option for total mercury (THg) and methylmercury (MeHg) in the Hyeongsan River estuary, South Korea. Materials and methods A small plot study was conducted by capping four different sites with different capping materials (HS1 with zeolite, HS2 with activated carbon (AC)/zeolite, HS3 with AC/sand, and HS4 with zeolite/sand), and one uncapped site (HS5) was used as a control. The capping effectiveness was evaluated ex situ by collecting sediment cores after 2 months, 1 year, and 2 years of cap residence. Results and discussions Capping in HS1, HS2, and HS3 reduced > 90% of the THg sediment after 2 months and > 83% of the MeHg after 1 year. HS4 reported a relatively low reduction (56% for THg and 83% for MeHg). In pore water, after 2 months, HS1, HS3, and HS4 led to higher reductions (93–95% for THg and 34–41% for MeHg) than HS2 (77% for THg and 20% for MeHg). The capping effectiveness declined after 1 year and 2 years, presumably because of the redistribution of contaminated sediments onto the small capped demonstration area. Despite reduced effectiveness over time, the capped sites reported lower concentrations of THg and MeHg compared to the uncapped site, which may be attributed to the combined effects of the treatment and natural attenuation. Conclusions Overall, the results imply that AC and zeolite as active sorbents and sand as passive caps can be potential mercury remediation options for contaminated sediments.
 
Purpose Returning carbon (C) and nitrogen (N) into soils through harvest residue retention has great potential to mitigate land degradation in production forestry. However, the long-term effect of harvest residue retention on soil microbial communities with consequences for C and N sequestration needs further clarifications. Materials and methods We therefore revisited a 17-year field trial implementing three experimental contrasts, i.e., residue retention (forming into windrows), tree gaps, and reforested sites, in sloping hoop pine (Araucaria cunninghamii) plantations, subtropical Australia. We determined total and labile C and N pools, diversity and composition of bacterial community, the abundance of N-cycling functional genes, and basic soil properties in the topsoil (0–20 cm). Results Windrowed residue retention significantly increased soil total C content by ~ 141% compared to those of the gap and/or reforested sites. Significant increases in microbial biomass N (MBN, ~ 242%) and its proportion to total N (~ 105%) coupling with lower microbial biomass C/N ratios (MBC/MBN) suggest enhanced microbial N immobilization under windrowed residue retention. Windrowed residue retention significantly decreased bacterial alpha diversity and altered bacterial community composition by favoring copiotrophic dominance, as determined primarily by MBC/MBN, water-extractable C, and MBN. Shifts in their putative function and the abundance of N-cycling functional genes further indicate that a copiotroph-dominated bacterial community under windrowed residue retention may confer a higher potential of soil C and N sequestration. Conclusions These results highlight the importance of managing microbial-mediated N cycling and bacterial life strategy in various post-harvest management paradigms.
 
Purpose Soil erosion by water yields sediment to surface reservoirs, reducing their storage capacities, changing their geometry, and degrading water quality. Sediment reuse, i.e., fertilization of agricultural soils with the nutrient-enriched sediment from reservoirs, has been proposed as a recovery strategy. However, the sediment needs to meet certain criteria. In this study, we characterize sediments from the densely dammed semiarid Northeast Brazil by VNIR-SWIR spectroscopy and assess the effect of spectral resolution and spatial scale on the accuracy of N, P, K, C, electrical conductivity, and clay prediction models. Methods Sediment was collected in 10 empty reservoirs, and physical and chemical laboratory analyses as well as spectral measurements were performed. The spectra, initially measured at 1 nm spectral resolution, were resampled to 5 and 10 nm, and samples were analysed for both high and low spectral resolution at three spatial scales, namely (1) reservoir, (2) catchment, and (3) regional scale. Results Partial least square regressions performed from good to very good in the prediction of clay and electrical conductivity from reservoir (< 40 km ² ) to regional (82,500 km ² ) scales. Models for C and N performed satisfactorily at the reservoir scale, but degraded to unsatisfactory at the other scales. Models for P and K were more unstable and performed from unsatisfactorily to satisfactorily at all scales. Coarsening spectral resolution by up to 10 nm only slightly degrades the models’ performance, indicating the potential of characterizing sediment from spectral data captured at lower resolutions, such as by hyperspectral satellite sensors. Conclusion By reducing the costly and time-consuming laboratory analyses, the method helps to promote the sediment reuse as a practice of soil and water conservation.
 
Schematic model of N loss and availability affected by climate extremes and land degradation as fingerprinted by tree ring N isotopic composition (δ.¹⁵ N)
Mean annual growing season rainfall (GroRain, mm) during 1958–2016 (n = 57) for the study site in Northern China
Impacts of climate extremes and bushfires during lifespan of Pinus sylvestris on mean annual ¹⁵ N natural abundance (ẟ.¹⁵ N, ‰) in tree ring samples (averaged at 3-year interval) for sample tree 1 (81 years old in 2018) (a), sample tree 2 (57 years old in 2018) (b), sample tree 3 (57 years old in 2018) (c) and sample tree 4 (57 years old in 2018) (d) in Northern China
Relationship between total nitrogen (N) concentration (%) and ¹⁵ N natural abundance (δ.¹⁵ N, ‰) in tree rings of P. sylvestris in Northern China
Purpose Climate extremes, such as droughts and floods, have become intensified and more frequent due to intensifying climate change. Increased atmospheric carbon dioxide (CO2) and warming-induced water limitation, as well as climate extremes, may alter carbon (C) and nitrogen (N) cycling in forest ecosystems. This provides a brief review of stable nitrogen isotopic composition (δ¹⁵N) in tree ring in relation to climate extremes and bushfires in context of N availability and losses in forest ecosystems. Material and methods Tree rings were extracted from four Pinus sylvestris and four Larix gmelinii sample trees, located in a boreal plantation forest of Mohe City, Heilongjiang Province, China. Tree rings were measured to obtain mean annual basal area increment (BAI), while tree ring δ¹⁵N and total N concentrations were measured on mass spectrometer at 3-year intervals. The tree ring δ¹⁵N data were related to possible climate extremes and bushfires. A brief review of the relevant literature was also undertaken to support our preliminary research findings. Results and discussion Globally, increasing atmospheric CO2 concentration and water limitations have led to a warmer-drier climate. This has also been associated with increases of climate extremes such as drought and floods as well as bushfires. These extremes have been recorded with detrimental effects on plant and soil structures within forest ecosystems and play an important role in regulating N availability and losses in forest ecosystems. Studies of N deposition within forest ecosystems using soil and plant δ¹⁵N also showed that N losses under various climate extremes can occur through direct changes in N cycling, such as increasing soil nitrification and denitrification or leaching. It is highlighted that tree rings δ¹⁵N has the potential to fingerprint the intensity and frequency of climate extremes and bushfires in the forest ecosystems, but more such tree ring δ¹⁵N research needs to be done in diversified forest ecosystems to confirm the potential of using tree ring δ¹⁵N for quantifying the frequency and intensity of climate extremes and bushfires at both regional and global scale. Conclusion The variation and trend of δ¹⁵N in the soil–plant-climate systems are closely linked to the N cycling in forest ecosystems, and tree ring δ¹⁵N has the great potential to fingerprint both intensity and frequency of climate extremes such as drought and floods as well as bushfires.
 
Map showing the location of sampling sites along the lower Yellow River
Spatial–temporal variations in suspended particulate matter (SPM) (a), the median grain size (MGS) (b), the particulate organic carbon (POC) (c), the mass percentage of POC (POC%) (d), the ratio of C and N (C/N) ratios (e), and the organic carbon isotopes (δ¹³Corg) (f) in surface water from the lower Yellow River (red circle represents samples collected before WSR, blue square represents samples collected after WSR)
Correlation analysis of POC with SPM, MGS, and POC%, respectively, in surface water from the lower Yellow River (red circle represents samples collected before WSR, green square represents samples collected after WSR)
Analysis of the source of POC in the surface water from the lower Yellow River. A Samples collected before WSR, B samples collected after WSR
Purpose Large rivers play an important role in the global carbon cycle through the transportation of particulate organic carbon (POC) from the continent to the ocean. Human disturbance such as water–sediment regulation (WSR) significantly changes the downstream fluxes of sediment and POC. However, the sources and variations of POC affected by WSR in the lower Yellow River remain poorly understood. Methods The surface water samples were collected in the lower Yellow River, and the concentration and the median grain size (MGS) of suspended particulate matter (SPM) and the particulate organic carbon (POC) were measured. In addition, the C/N ratios and stable carbon isotopic composition (δ¹³Corg) of the POC in the surface waters of the lower Yellow River before and after the WSR of the Xiaolangdi reservoir were also analyzed to explore the spatio-temporal changes of POC transport. Results Both SPM and POC concentrations were significantly higher before WSR than after WSR and showed different deposit and scour features along the lower Yellow River, mainly due to the different water discharge and particulate contents. Correlation analysis shows that POC% and MGS were the main factors that influence the variation of POC before WSR, while SPM was the main factor after WSR. The C/N ratio in the surface water of the lower Yellow River before WSR was significantly higher than that after WSR, while δ¹³Corg showed the opposite trend. Application of a two-end-member mixing model of δ¹³Corg suggests that, on average, about 56.4% and 82.0% of the POC is derived from terrestrial soil and that approximately 43.6% and 18% of the POC is derived from C3 plant detritus before and after WSR, respectively. Conclusion Our research indicates that SPM and POC transport in the lower Yellow River was significantly affected by the WSR of the Xiaolangdi Reservoir. Therefore, the effect of WSR on the transport and environmental implication of POC should be considered in the management of the Xiaolangdi Reservoir in the future.
 
The sampling sites
Percentage of each phosphorus fractions in sediments of the Muyang River (A1–A6) and Lengshui River (B1–B5) using SEDEX method
Sediment phosphorus adsorption kinetics of the Muyang River (A1–A6) and the Lengshui River (B1–B5)
Sediment adsorption isotherm of the Muyang River (A1–A6) and the Lengshui River (B1–B5)
Purpose Sediment phosphorus fractionation, adsorption kinetics, and adsorption isotherm characteristics were studied in two rivers, Muyang River and Lengshui River in China, emptying into a drinking water source reservoir, Songhua Reservoir. The relationship between phosphorus fractions and adsorption characteristics was further investigated to explore possible management options for maintaining river water quality. Methods Sediment samples were collected from six and five sampling sites located along the Muyang and Lengshui Rivers, respectively. Sediment physical–chemical properties and phosphorus fractions were quantified. Adsorption characteristics were modeled with adsorption kinetics and adsorption isotherms. Results The results from sedimentary sequential chemical extraction (SEDEX) method showed that organic phosphorus (Org-P) had the highest portion of total phosphorus in sediments, followed by detrital phosphorus and authigenic phosphorus (Auth-P), residual phosphorus, iron-bound phosphorus (Fe–P), aluminum-bound phosphorus (Al-P), and exchangeable phosphorus. Notably, the Lengshui River had a higher portion of Fe–P. The kinetics of phosphorus adsorption in sediments was simulated using a pseudo-second-order kinetic model. The results showed that sediment phosphorus adsorption in the upper reaches of the rivers was primarily dominated by single-layer adsorption, whereas that in downstream reaches gradually became irregular and complex deep adsorption. Adsorption isotherms could be well fitted using the modified Langmuir model and the Freundlich model, which indicated that the Lengshui River sediment had higher maximum phosphorus adsorption capacity and strength than Muyang River sediments. The EPC0 values of sediments in both rivers indicated that sediments acted as the source of phosphorus in surface water. Principal component analysis showed that (i) the portion of Fe/Al-P in sediments positively correlated with equilibrium adsorption capacity, rate, and strength; (ii) Auth-P in sediments could increase the speed and amount of physically adsorbed phosphorus in sediments; and (iii) Org-P positively impacted the maximum adsorption capacity of phosphorus in sediments. Conclusion Sediment phosphorus fractions and sorption could be affected by human disturbance. At the same time, sediment phosphorus fraction features correlated with several P adsorption characteristics. The findings from this study provide useful guidance for water quality protection of the drinking water source reservoir.
 
Purpose Traditional measurement for soil properties is time-consuming and costly, while visible–near-infrared spectroscopy enables the rapid prediction of soil properties. In this study, visible–near-infrared spectroscopy was used to predict these four soil properties including OC (organic carbon) content, TN (total nitrogen) content, pH value, and clay content in rare earth mining areas based on different spectral transformation and calibration methods. Materials and methods A total of 232 soil samples were collected from unexploited, in situ leaching, and heap leaching mining areas in southern Jiangxi Province, China. The chemical properties and reflectance spectra of air-dried samples were measured. Spectral transformations including first-order derivative (FOD), continuum removal (CR), and continuous wavelet transform (CWT) were selected to improve the prediction accuracy of the model. Partial least-squares regression (PLSR), the support vector machine (SVM), and extreme gradient boosting (XGBoost) were used to construct prediction models. Results and discussion The highest prediction accuracies in terms of the coefficient of determination (R²), root mean square error (RMSE), and relative prediction deviation (RPD) were obtained using CWT spectra with XGBoost for organic carbon content (R² = 0.89, RMSE = 0.24, RPIQ = 4.67), total nitrogen content (R² = 0.86, RMSE = 0.01, RPIQ = 4.14), and pH value (R² = 0.73, RMSE = 0.19, RPIQ = 1.66). The best prediction result for clay content was obtained using CWT spectra with the SVM (R² = 0.67, RMSE = 6.45, RPIQ = 2.75). Conclusions The CWT coupled with a non-linear model, such as XGBoost, is an effective method for the accurate prediction of soil properties in rare earth mining areas.
 
Evolution of total As and AsIII concentration in soil leaching experiments with P soil. T1day 29: just before the first amendment; T2day 62: just before the second amendment; T3day 96: end of experiment. Error bars represent the standard error (n = 3). Significance of differences between As concentration was using a non-parametric Kruskal and Wallis test
Evolution of the MPN of AsIII-oxidizing microorganisms in the NP and P soils. T1day 29: just before the first amendment; T2day 62: just before the second amendment; T3day 96: end of experiment. Error bars represent the standard error (n = 3). Significance of differences between amendment conditions was evaluated separately for each incubation time using the non-parametric Kruskal and Wallis test
Quantification of arsB, aioA and 16S rRNA genes in the NP soil (in blue) and P soil (in orange). T1day 29: just before the first amendment; T2day 62: just before the second amendment; T3day 96: end of experiment. Error bars represent the standard error (n = 3)
Bar plot: total final dry plant biomass for each pot: stems, leaves and grains of barley grown at the end of experiment (T3day 96) for NP and P soils. Significance of differences between dry biomass mean of NP and P was using a non-parametric Kruskal and Wallis test (letters A and B; n = 3). Red dots: Omega-3 Index measured in barley grown (on day 42) on NP and P soils supplied with different concentrations of ammonium sulphate. Error bars represent the standard error (n = 8). Significance of differences between Omega-3 Index values was analyzed using a non-parametric Kruskal and Wallis test (letters a and b)
Concentrations of the main extracted species in the grains of barley grown on soils during the pot experiment. NP: non polluted soil; P: polluted soil. Error bars represent the deviation of the analysis. In the condition NP × 10, As concentration was lower than the quantification limit, i.e. 15 µg kg⁻¹
Purpose Arsenic (As) is one of the most widespread toxic elements, affecting human health through consumption of contaminated water or food. This work studied the effects of ammonium sulphate (NH4)2SO4 on mobility, speciation, and toxicity of As in a soil polluted by the destruction of chemical ammunition. Methods Effects of fertilization on mobility and speciation of As, abundance of active AsIII-oxidizing and AsV-reducing microorganisms, abundance of bacterial 16S rRNA genes, and aioA and arsB genes involved in As biotransformation were monitored during barley growth in polluted (P) and non-polluted (NP) soils in pots maintained in outdoor conditions, with either the usual dose of (NH4)2SO4, 10 × or 100 × this dose. As transfer to plants and phytotoxicity were evaluated through quantification and speciation of As in grains and lipid peroxidation in plant leaves. Results As mobility was significantly reduced by the highest dose of (NH4)2SO4. Fertilization tended to increase the Omega-3 Index in barley with the P soil. Abundance of AsIII-oxidizing microorganisms was higher and plant biomass was lower in P than in NP soil. Neither pollution level nor fertilization dose affected As speciation in soils or gene abundance. The sum of the concentrations of extracted AsIII and AsV species in grains was in the range 0–150 µg kg⁻¹ with NP soil and 2000–3500 µg kg⁻¹ with P soil. Conclusions Results underline the importance of developing surveys of As dynamics in agricultural contexts with diverse types of crops, with a focus on As speciation mechanisms, linked to soil microbial activities and plant physiology.
 
Purpose The enclosure and reclamation of wetland sediments altered soil properties and heavy metal levels in coastal areas. This study focuses on the effects of anthropogenic enclosure and reclamation activities on the physicochemical properties and the biotoxicity of heavy metals in sediments/soils in the Yangtze Estuary. And, the purpose was to explore the factors affecting heavy metals in different reclamation stages and to provide reasonable suggestions for stabilizing heavy metals in farmland transformed from enclosed land. Materials and methods According to the reclamation situation, sampling areas were divided into natural wetland (NW), the enclosure area (EA) separated from NW by a 2-m-high embankment, the reclamation area with short-term (3 years) rice cultivation history (RAS), and the reclamation area with long-term (10 years) rice cultivation history (RAL). A total of 360 heavy metal concentration data were used to assess heavy metal levels. The acid volatile sulfide (AVS) and the simultaneously extracted metals (SEM) were analyzed to compare the toxicity of heavy metals in sediments/soils at different reclamation stages. The ratios of SEM-Hg/THg (total mercury) and SEM-As/TAs (total arsenic) were used to assess the effect of Fe/S on the stability of Hg and As. Correlation matrix analysis and multiple linear regression models were performed to investigate the correlations between heavy metals and soil physicochemical properties. Results and discussion The cycle of leaching and drying in EA could effectively remove heavy metals for further use. And, there were no significant variations in sediment/soil properties and heavy metals between EA and RAS, suggesting a weak influence of short-term farming activities. However, due to long-term human cultivation activities and the influence of soil total organic carbon (TOC), electrical conductivity (EC), clay size fraction content, and redox conditions, the concentrations of Zn (95.27 mg·kg⁻¹), Cu (26.42 mg·kg⁻¹), Ni (39.93 mg·kg⁻¹), Fe (6.34%), and As (9.80 mg·kg⁻¹) significantly increased in RAL compared with NW, EA, and RAS. In NW and RAL, insoluble sulfide had a strong immobilization effect on As, while Fe(III) (oxyhydr)oxides were responsible for the stabilization of Hg in EA and RAS. Fe/S speciation, soil moisture content (MC), and TOC were key factors controlling the mobility and biotoxicity of Hg and As. And, these effects mainly occurred in 0–15 cm of surface soils. Conclusion Paddy or upland farming was suggested to control As or Hg risks in reclaimed soils/sediments.
 
Map of research localities in the investigated area (Slovakia)
Results of granulometric analysis of samples AS1-AS3, PGVS1-PGVS3
TG/DTG and DTA curves of samples AS1-AS3, PGVS1-PGVS measured in air. Heating rate 10 °C/min
XRD patterns of samples AS1-AS3, PGVS1-PGVS3
Purpose To evaluate the ecological risk, physico-chemical properties, and the impact of anthropogenic sources on permanent eighteen samples of grass vegetation soils (PGVS) and agricultural soils (AS) near the U.S. Steel company in Košice city, Eastern Slovakia. Methods In this study, permanent grass vegetation soils (PGVS) and agricultural soils (AS) samples were analyzed. Samples were taken from the following localities (L): PGVS1a-1c; AS1a-1c from U.S. Steel-slag heap (L1), PGVS2a-2c; AS2a-2c from Gomboš (L2), PGVS3a-3c; AS3a-3c from U.S. Steel plant (main gate of U.S. Steel) (L3). The physico-chemical properties were determined using methods, such as thermal analysis (TG, DTG/DTA), X-ray fluorescence spectrometry (XRF), CHNS spectrometry, granulometry, and X-ray powder diffractometry (XRPD). Results The environmental quality index of Cd and Co in all samples was found to be ˃3 and indicated high pollution of the studied area by these elements. This assumption was confirmed by XRF spectrometry. For all the analyzed samples, the permitted limits for the elements Cd, Cr, and Co were largely exceeded, that also corresponded to the area from which they were taken. In all the studied samples, the C, H, N, and S concentrations were higher in the permanent grass soil samples than the concentrations in agricultural soil samples. The quartz was found as the major mineral; the others such as chlorite, albite, muscovite, or microcline were detected in smaller amounts. All samples were thermaly stable up to 40 °C. Conclusions In the surveyed area, the major source of heavy metal pollution was from industrial discharge. This demonstrates that effective management strategies could be developed to safeguard the environmental quality of agricultural soils and permanent grass vegetation in the studied area.
 
Purpose Understanding the mechanisms of interaction between humic acids (HAs) and metal ions in soil media is essential for integrated environmental and agricultural management. This study aims to select the best isotherm model and explore the correlation between physicochemical properties of humic acids isolated from forest soils and metal ion sorption parameters. Materials and methods The HA fractions of ten forest soils were initially isolated and well characterized. Afterward, the sorption of four metal ions (Cu, Fe, Mn, and Zn) on the isolated HAs was investigated. The experimental data was initially fit to the three isotherm models to find the optimal model. Subsequently, the correlation between physicochemical HA properties and metal ion sorption parameters was explored using correlation and clustering analysis. Results and discussion Results showed that despite employing identical extraction procedures, isolated HAs from soils differed statistically in their descriptive characteristics. The adsorption data generally fit the Freundlich isotherm model best. However, isotherm model performance was affected by both metal ion and HA types. Six descriptive HA attributes including H/C and O/C molar ratio, the internal oxidation degree (ω), carboxylic acidity, E4/E6 ratio, and the Δlog-K were closely associated with sorption parameters. Conclusions Since the performance of isotherm models was affected by both metal ion and HA type, the isotherm model should be selected with regard to target metal ions and HA type. Highly correlated variables with sorption parameters of HA-metal ions screened in this study could be reliably used to develop sorption predictive models for HA-metal ions in soil and aquifers. Graphical abstract
 
Measured (point) and modeled (line) concentrations of NH4⁺-N (a) and NO3⁻-N (b), ¹⁵ N enrichment of NH4⁺-N (c), and NO3.⁻-N (d) during the whole incubation experiment. Control, no fertilisation; NPK, chemical fertilisation; 70F + 30 M, 70% of chemical N plus 30% of organic N; 50F + 50 M, 50% of chemical N plus 50% of organic N; 30F + 70 M, 30% of chemical N plus 70% of organic N. Error bars represent the standard deviations of the mean (n = 3)
The relation between gross MNrec rates and soil organic matter (a), AHN (b) and C/N (c), between gross MNlab rates and the ratio of AAN to AHN (d) and the ratio of AHUN to AHN (e). Error bars refer to standard deviations. MNrec, mineralisation of recalcitrant organic-N to NH4⁺; MNlab, mineralisation of labile organic-N to NH4.⁺; AHN, acid-hydrolysable N; C/N, the ratio of soil organic C and total N; AAN, amino acid N; AHUN, acid-hydrolysable unknown N
The relation between gross autotrophic nitrification rates and the concentration of NO3⁻-N (a) and pH (b), between gross NH4⁺ immobilisation rates and gross mineralisation rates (c), the ratio of AAN to AHN (d) and the ratio of AHUN to AHN (e), between consumption rate of NO3.⁻ and gross autotrophic nitrification rates (f), the content of AHN (g) and pH (h). Error bars refer to standard deviations. AAN, amino acid N; AHN, acid-hydrolysable N; AHUN, acid-hydrolysable unknown N
Dynamic of double-cropping yield in different fertilisation treatments in Nanchang, Jiangxi province of China from 1984 to 2019 (a) and the relationship between rice yield and gross MNrec rates (b). MNrec, mineralisation of recalcitrant organic-N to NH4.⁺; control, no fertilisation; NPK, chemical fertilisation; 70F + 30 M, 70% of chemical N plus 30% of organic N; 50F + 50 M, 50% of chemical N plus 50% of organic N; 30F + 70 M, 30% of chemical N plus 70% of organic N
Purpose Knowledge of nitrogen (N) dynamics of organic fertilisers as partial substitutes for chemical fertilisers could improve the retention of mineral N and optimise fertilisation practices in paddy soils. Materials and methods ¹⁵ N tracing was performed to quantify the effect of 36 years of partial substitution of chemical N by organic N on gross N transformation rates in soils, and its relationship to organic N fractions. Treatments were chemical fertilisation (NPK), substitution of chemical N with 30%, 50% and 70% organic N (70F + 30 M, 50F + 50 M and 30F + 70 M), and no fertilisation (control). Results and discussion Partial substitution elevated N mineralisation, ammonium immobilisation, autotrophic nitrification and nitrate consumption rates by 58.4–609.3%, 36.0–304.2%, 0.5–320.0% and 51.4–112.6%, respectively, compared to NPK treatment. However, gross N transformation rates differed with varying organic N substitution proportions. Fifty percent substitution increased mineralisation and immobilisation of the labile organic N pool by increasing the level and distribution of amino acid N fractions in acid-hydrolysable N, resulting in the highest N supply and retention capacity and the lowest N loss potential. Thirty percent substitution also increased the supply and retention capacity of nitrogen, but its integrated effect was inferior to the 50% substitution treatment. However, 70% substitution increased the potential loss of nitrate by increasing the ratio of gross nitrification rate to gross ammonia immobilisation rate (N/I). Partial substitution also increased the yield by 4.26–6.91% (p < 0.05), but there were no significant differences between organic N substitution proportions. Conclusion Thus, accumulating active fractions of the soil organic N pool enhanced soil N supply and mineral N retention in acidic rice fields. Given optimal sustainable agricultural practices, and considering yield, N retention and potential N losses, a 50% substitution of chemical N with organic N can be recommended for the study region.
 
Purpose Rare-earth elements (REEs) have been listed as emerging pollutants, and REEs often occur together with heavy metals (HMs) in the environment. Large amounts of REEs and their coexisting HMs enter into the surrounding soils through dust, surface runoff, and leachate, causing serious REE and HM co-contamination and resulting in ecological crisis. The ecological effects caused by REEs have been gradually concerned, but ignore the synergistic effect of coexistence with HMs. Soil microorganisms are closely related to the soil ecosystem stability. At present, under long-term REE–HM disturbance, the response of bacterial and fungal communities and the effects of community functions remain unclear. In this study, the response of bacterial and fungal communities to different REE and HM co-contamination levels and the community predicted function were analyzed. Methods 16S rRNA and ITS1 high-throughput sequencing were performed for bacteria and fungi, respectively. The bacteria community functions were predicted using the PICRUSt2 method. Results The co-contamination caused decreases in bacterial and fungal community richness and diversity, with significant changes in community structure and composition, especially in the most serious co-contaminated soils. With the increase in the pollution levels, the bacterial communities became reorganize, whereas fungal communities had a certain buffer capacity. The microbial symbiotic pattern changed under severe co-contamination conditions, and microorganisms enhanced interactions to advance the dominant taxa adaptability or resistance. Bacterial communities developed more competitive relationships, whereas fungal communities developed more symbiotic relationships. Thus, bacteria are more sensitive than fungi. PICRUSt2 prediction results showed that bacterial community could resist cell damage caused by exotic REEs and HMs by strengthening the efflux transport system, DNA repair function, cell defense mechanism, and detoxification mechanism. The co-contamination might enhance bacterial antibiotic resistance; dissimilatory and assimilatory nitrate reduction and denitrification might be the nitrogen metabolic predominant processes. Conclusion For the first time, we systematically confirmed that bacteria and fungi respond differently under long-term REE and HM exposure: bacterial are more sensitive than fungi. The ecological functions of bacterial community were changed, and the ecological risk of desert steppe soil environment might be increased. Our results have important significance for ecological risk assessments in co-contamination environment.
 
Purpose Runoff caused by rainfall events contributes to the watercourses pollution through the export of OM, nutrients, and sediments from soils. This study aimed to assess the loss of OM, nitrogen, phosphorus, and sediments by the runoff of soils under agriculture and cattle farming. Materials and methods Three pedons located on the top of the slope, in the backslope, and one close to the alluvial plain (CoAoMo) were selected as study sites. Experiments using a drop-forming rainfall simulator on runoff microplots were carried out under laboratory conditions. The physical and chemical characteristics of soils, sediments, and runoff waters were analyzed. Results and discussion The rainfall took part in the soil acidification process through hydric erosion and lixiviation. Hydric erosion produced the loss of divalent cations associated with sediments transported by runoff waters, whereas the loss of monovalent ions (sodium) was linked with a leaching process. The losses of sediment, divalent cations, OM, NK, and P Bray were more marked in soils with a higher position in the landscape and higher slope. On the other side, CoAoMo showed the lowest sediments loss, OM, and NKj due to its high sodium concentration that decreases sediment detachment during the runoff processes. However, CoAoMo, which presented the highest TRP into runoff waters, would be the product of exogenous contamination. Sediments were enriched in fine materials, OM (EF: 1.16–1.32) and NK (EF: 1.31–1.69). Conclusion According to these results, the particulate fraction represents the major proportion of nutrients in the runoff waters. Non-conservative management of these soils could conduct to a loss of plant cover that became them susceptible to intense erosion.
 
Bulk density at different soil depths under Red Pine, Oak, and degraded forest ecosystems in the Göksu catchment, Turkey
Relationship of equivalent depth soil organic, passive, active C, and total nitrogen stocks with equivalent mass stocks of soil organic (SOCeq), passive (PCeq), active C (ACeq), and total nitrogen (TNeq) under Red Pine, Oak, and degraded forest ecosystems in the Göksu catchment, Turkey
Relationship of carbon pool index (CPI) with nitrogen management index (NPI) in soils under Red Pine, Oak, and degraded forest ecosystems in the Göksu catchment, Turkey
Relationship of carbon pool index (CPI) with carbon lability (CLI) in soils under Red Pine, Oak, and degraded forest ecosystems in the Göksu catchment, Turkey
Purpose Deforestation is one of the ecosystem disservices associated with accelerated loss of soil organic carbon (SOC) and nitrogen (TN). The objective of our study was to evaluate the impact of deforestation on the partition of SOC pools, TN content, and the SOC lability when compared with the well-stocked Red Pine (Pinus brutia Ten.) and Oak (Quercus coccifera) forests. Methods Geo-referenced replicated soils under Red Pine and Oak and their adjacent deforested shrubby sites (control) were sampled at 0 to15 am and 15 to 30 cm depths from the Göksu catchment in Mediterranean region of Turkey. Soils were analyzed for SOC, active C (AC), passive C (PC), and TN contents. Results SOC content under both forests was higher compared to deforested shrubby sites; however, SOC under Red Pine was 22% higher than under Oak. A similar pattern in AC, PC, and TN pools was observed with a higher partition of PC:AC under both forests than under shrubby sites. The equivalent mass SOC, AC, PC, and TN stocks linearly and positively accounted for > 95% of the variability (R²) in their stocks based on equivalent depth. However, the equivalent depth overestimated 5.6 ± 0.71 Mg/ha to predict SOC storage based on equivalent mass. While the C and N pool indices (CPI and NPI) were higher under both forests compared to shrubby sites, SOC lability did not vary consistently among themselves. The CPI non-linearly and inversely accounted for 57% of the variability (R²) in SOC lability, suggesting increasing SOC accumulation significantly decreased its lability. In contrast, the CPI accounted for 42% of the variability (R²) in the NPI with a slope of only 0.57; suggesting both SOC and TN are disproportionately coupled in soil organic matter (SOM) under existing forest ecosystems. Conclusion Deforestation affected both SOC and TN stocks. However, increasing SOC sequestration under well-stocked forests is responsible for decreasing SOC lability (higher PC:AC) and partially decoupled C:N stoichiometry in SOM. Future research is needed to evaluate the impact of forest management practices on SOC and TN stocks and their lability across geographic landscape-scale under Mediterranean climates.
 
Changes of soil enzymatic activities. (a) Catalase, (b) acid phosphatase, (c) sucrase, and (d) urease
The principal coordinate analysis (PCoA) (a bacterial community; b fungal community) and hierarchical clustering analysis (c bacterial community; d fungal community) of soil microbial communities. The principal coordinate analysis and hierarchical clustering analysis based on Bray–Curtis distances matrices in OTUs levels
(a) Soil bacterial and fungal community composition; those phyla whose relative abundance were lower than 1% were merged into “Other”. (b) Relative abundance differences among different treatments at phylum levels; only those phyla which had significant differences among different treatments were showed, and different letters indicate significant differences of relative abundance in same phylum among different treatments at p < 0.05 following LSD test
Cladogram showed the biomarker microbes of the microbial lineages from phylum to genus among the three different treatments (a bacterial community; b fungal community); the strip painted with different colors represented different treatments; yellow dot represented the microbes with no statistical differences among the three treatments; the dot with other colors represented the indicator microbes with LDA scores greater than 4.0 with in the corresponding treatment
Redundancy analysis (RDA) for the relationship between environmental factors and microbial community, and the relationship between activities of the soil enzyme and microbial community (a environmental factors and bacteria, b environmental factors and fungi, c enzyme activity and bacteria, and d enzyme activity and fungi)
Purpose Phosphorus (P)-containing passivators have a stabilizing effect on cadmium (Cd)-contaminated agricultural soils to be safely used, offering good potential for risk control of Cd-contaminated agricultural soils to be strictly controlled. In this study, an incubation experiment was conducted to evaluate the risk control effects of using hydroxyapatite (HAP) and monocalcium phosphate (MCP) on Cd-contaminated agricultural soils to be strictly controlled. Materials and methods Samples of topsoil were collected (0–20 cm) from agricultural land near a lead–zinc mine in Southwestern China containing 32.07 mg kg⁻¹ Cd with a pH of 7.28. The amounts of passivators added were equal to approximately 3% of the soil by weight. The soil Cd content, physicochemical properties, enzyme activity, and microbial community were analyzed. Results The results showed that the application of HAP and MCP decreased the activity and mobility of Cd in soils to be strictly controlled. HAP was more effective in decreasing the exchangeable Cd (CdEx) than MCP (rate of decrease was 48.1% for HAP and 24.4% for MCP). According to the results of the geometric mean (GMean) and the integrated total enzyme activity (TEI) index, the total soil enzyme activity of the HAP treatment was higher than that of CK and MCP treatment. HAP and MCP significantly decreased the Chao and Shannon bacterial community indices and the Shannon index of the soil fungal community. HAP increased Actinobacteria abundance, which is beneficial to soil fertility enhancement and plant growth, and MCP increased Rhizobiales abundance, which promotes soil P cycling and plant growth. Primary driving factors for the changes in bacterial and fungal community composition in the stabilized soils were CEC and CdEx for bacteria and Cd bound to carbonates (CdCar) and residual Cd (CdRes) for fungi. Conclusions HAP is more suitable for risk control of Cd-contaminated agricultural soils to be strictly controlled than MCP from the perspective of soil Cd activity and mobility, soil enzyme activity, and diversity and composition of the soil microbial community.
 
Purpose This study was conducted to investigate the effect of biosurfactant and nutrient amendment on the bioremediation of heavy oily sludge. The biosurfactants exhibit characteristics of amphiphilicity, emulsifying activity, and biodegradability. The application of biosurfactant, appropriate water and nutrition, combined with mechanical stirring effectively enhanced bioremediation, providing a promising scheme for sustainable development of the ecosystem. Methods Here, biosurfactant was produced by strain Pseudomonas sp. CH1. Biosurfactant (80 mg L⁻¹) was periodically added to heavy oily sludge for 10 weeks. The effects of biosurfactants on petroleum hydrocarbon content, catalase activity, and microbial community composition were characterized. Results Compared with the control (water addition only), treatment supplemented with biosurfactants exhibited better performance in degrading total petroleum hydrocarbons and n-alkanes, while regular cumulative addition of biosurfactants inhibited biodegradation. Moreover, the analysis of microbial community structure showed that the biosurfactant selectively enriched microorganisms that are capable of degrading petroleum hydrocarbons. Conclusion The results demonstrate the potential of biosurfactants for heavy oily sludge bioremediation. The application of biosurfactants can improve catalase activity and enrich the degrading populations, thereby promoting the biodegradation of total petroleum hydrocarbons, especially the aliphatic fractions, whereas high biosurfactant content inhibited the biodegradation. Graphical abstract
 
Purpose Runoff and soil erosion reflect the interactions of soil properties and rainfall. However, few researchers have investigated the forms of nitrogen lost, the first flush and the relationships between nitrogen losses and rainfall, runoff duration, different slope gradients and fertilizer rates on red soils and paddy soils. Materials and methods This study examined the nitrogen losses (NH4+-N, NO3−-N and TN) in runoff under simulated rainfall conditions with an intensity of 80 mm/h. The slope angles were set at 0°, 5°, and 15° for the red soil, and 0° and 5° for the paddy soil. The fertilizer was applied to the soils at 3 rates, i.e., 60, 180, and 300 kg N/hm2. Results and discussion The results showed that the cumulative rainfall required to generate runoff differed significantly for different slope gradients, and the value for the red soil was 46.46 mm at 0°, which was 3.46 and 4.62 times of the rainfalls at 5° and 15°. The value for the paddy soil was 20.09 mm (average of 0° and 5°), which was half of red soil’s value. Of the TN lost in a 90-minute event, 57.25±12.62% was lost in the first 20 min of runoff generation, with the losses decreasing as the runoff generation time increased. The NH4+-N/TN and NO3−-N/TN differed significantly at different fertilizer N levels (P < 0.01). As the fertilizer application rate increased, the NH4+-N/TN tended to increase while the NO3−-N/TN tended to decrease. The NH4+-N/TN decreased exponentially, while NO3−-N/TN increased logarithmically, as the runoff duration time increased. A model, which included parameters to predict the initial losses of precipitation, was established to simulate the processes driving TN losses under different conditions. It is useful to analyze the TN losses from different soil types, slope gradients, and fertilizer rates. Conclusion The threshold slope gradient, first flush effect and the TN loss process of the red and paddy soil were determined in this study. This information derived can potentially contribute to developing measures for reducing agricultural diffuse pollution and improving resource allocation.
 
Purpose Salinization is one of the main challenges for sustainable development of irrigated agriculture in arid areas, and its formation process is closely related to the movement of soil water and salts. Therefore, understanding soil water-salt dynamics is an important prerequisite for prediction and control of salinization. The purpose of this study is to investigate water-salt dynamics in oasis farmland soil during maize growth period. Materials and methods A soil column experiment was conducted in the study, and the soil used in the experiment came from irrigated farmland in an oasis area in Northwest China. To evaluate the effect of cultivation years on soil water-salt dynamics, soils from farmlands with cultivation years of 20 years (Till20) and 50 years (Till50) were selected. Soil water content (SWC), electrical conductivity (EC), and soil temperature were continuously monitored by Decagon 5TE sensor during maize growth period. Results and discussion The results showed that water-salt dynamics were different between the two farmlands. In Till20, SWC and EC increased with soil depth overall, while they increased first and then decreased with soil depth in Till50. Except for deep soil layers, SWC, EC, and their coefficient of variation (CV) were higher in Till50. The water storage of most soil layers in both croplands decreased after the growth period. In Till20, soil salts showed an increasing trend after the growth period, with an average increase of 29%. However, in Till50, salts only increased (by 34.7%) in the topsoil, while the other soil layers showed a desalting trend. There was a significant positive correlation between salinity and water content in both farmlands, while the correlation between salinity and soil temperature was quite different between Till20 and Till50. Conclusions Soil salinity dynamics in oasis farmland was closely related to water content during crop growth period, and it was quite variant between croplands with different cultivation years. The soil salinity in farmland with shorter cultivation years increased more obviously in most soil layers after crops growth period, while the farmland with longer cultivation years tended to accumulate salts in topsoil. Therefore, the development of soil water and salt control measures in irrigated agriculture should take into account the influence of cultivation years.
 
3D pore space features that have been measured, and the method by which the data was collected, along with a brief description of how the data was processed into the pore space parameters required
Porosity (%) distribution measured by indirect 2D (top) and direct 3D (bottom) methods. A normal distribution line has been included in both plots. Inset top panel: representation of conventional 2D method applied to a floc. Inset bottom panel: rendered image of 3D-measurement method applied to the same natural floc
Floc size and porosity relationship measured by 2D methods (top panel) and 3D methods (bottom panel). Floc Feret diameter and effective density-inferred porosity are plotted in the top panel, and 3D directly quantified floc volume and porosity are plotted in the bottom panel. The grey (micro-flocs) and gold (macro-flocs) boxes indicate the divide in the sampling strategy
Effective (green) and isolated (pink) pore volume % proportion distribution in natural sediment flocs. Inset: 3D renderings of effective and isolated pore volume from the same natural sediment floc (grey) volume. The “145” annotation on the isolated porosity plot indicates the number of data points that fall within the lowest bin category of 0–2% porosity
Pore space shape distribution in natural flocs, divided into hydraulically effective and isolated pore spaces. Circle in effective pore shape plot indicates a zone where a large portion of the n is situated
Purpose Flocculated cohesive suspended sediments (flocs) play an important role in all aquatic environments, facilitating the transport and deposition of sediment and associated contaminants with consequences for aquatic health, material fluxes, and morphological evolution. Accurate modelling of the transport and behaviour of these sediments is critical for a variety of activities including fisheries, aquaculture, shipping, and waste and pollution management and this requires accurate measurement of the physical properties of flocs including porosity. Methods Despite the importance of understanding floc porosity, measurement approaches are indirect or inferential. Here, using μCT, a novel processing and analysis protocol, we directly quantify porosity in natural sediment flocs. For the first time, the complexity of floc pore spaces is observed in 3-dimensions, enabling the identification and quantification of important pore space and pore network characteristics, namely 3D pore diameter, volume, shape, tortuosity, and connectivity. Results We report on the complexity of floc pore space and differentiate effective and isolated pore space enabling new understanding of the hydraulic functioning of floc porosity. We demonstrate that current methodological approaches are overestimating floc porosity by c. 30%. Conclusion These new data have implications for our understanding of the controls on floc dynamics and the function of floc porosity and can improve the parameterisation of current cohesive sediment transport models.
 
PurposeWater scarcity and soil salinity are among the major factors responsible for lower plant growth and production. Many studies have reported beneficial effects of biochar application in mitigating salt and drought stress in soil. This study was conducted to investigate the efficiency of poultry manure-derived biochar (BC) and silica modified BC (SBC) to mitigate the waning effects of soil salinity and water scarcity on safflower (Carthamus tinctorius L.) plant growth.Materials and methodsA pot experiment was conducted in greenhouse with soil collected from local agricultural farm. Amendments were added to soil at three rates: 0%, 1%, and 3% (w/w), and three levels of salinity (0, 50, and 100 mM) were applied, while in a separate pot experiment, plants were irrigated at 2-level water contents, i.e., 50% and 100% of field capacity (FC). Results and discussionPlants grown in soil with BC and SBC showed comparatively higher growth against control treatment with no added amendments. Soil receiving SBC displayed an increase in pH value by 0.07–0.47, lower electrical conductivity value (0.86–1.17 dS m−1), and higher uptake of P (878.57 to 1753.58 µg plant−1). Furthermore, higher K+ and lower Na+ availability and plant uptake were found in SBC-amended soil compared with control. Overall, 3% SBC application performed outclass in improving plant growth under 50% FC.Conclusion In nutshell, BC and SBC displayed substantial potential in reducing soil salinity and drought stress and improved nutrient availability and plant uptake.
 
Concentration of soil NH4⁺-N (a) and NO3⁻-N (b)
The average of soil NH4⁺-N and NO3⁻-N concentration for 6 sampling days during incubation period
Soil N2O uptake (a), gross N2O emission (b), and net N2O flux (c) rate during incubation time
Soil cumulative gross N2O emission, N2O uptake, net N2O flux and uptake/gross emission ratio
Relationship between soil cumulative gross N2O emission and net N2O flux (n = 24)
PurposeNitrous oxide (N2O) is an important greenhouse gas. Soils are the main source and an important sink of N2O. Gross rates of soil N2O emission and uptake determine the net flux and concentration of N2O in the atmosphere. Denitrification is generally considered a key pathway of N2O consumption, during which N2O is reduced to N2 by nitrous oxide reductase (N2OR) encoded by nosZ genes. Although the soil pH is an important factor in denitrification, its effect on N2O uptake across the soil‒atmosphere interface is still unclear.Materials and methodsThis study subjected acid red soil to eight different pH treatments (including 3.80, 4.47, 4.98, 6.08, 6.69, 6.99, 7.15, and 7.32) via lime application. An incubation experiment was conducted using a 15N2O dilution technique to quantify soil gross N2O emission, N2O uptake, and net N2O flux.Results and discussionThere are two contrasting gross N2O emission patterns in the tested pH range. When pH ≤ 4.98, the cumulative gross N2O emission decreased as the soil pH increased. In contrast, when pH > 4.98, the increasing pH promoted gross N2O emission. The significant positive correlation between gross N2O emission and net N2O flux indicated that the net flux of N2O is driven by gross emission rather than uptake. There was no observable effect on the cumulative N2O uptake in the 3.80–6.99 pH range, but it decreased at 7.15 and 7.32 pH. However, the copies of nosZ genes (both nosZ clades I and II) were inhibited at low pH and considerably increased with increasing soil pH. In addition, the cumulative N2O uptake/gross emission ratio was negatively correlated with soil pH.Conclusions Our findings show that low pH inhibits soil nosZ without affecting N2O uptake. However, this study provides direct evidence for N2O absorption across the atmospheric soil interface.
 
Soil physicochemical properties at 0–2, 2–5, and 5–10 cm depths under different treatments based on two-way ANOVA. P < 0.05 indicates significant effect of experimental treatment on soil physicochemical properties
Treatment effects on bacterial (a, b, and c) and fungal (d, e, and f) diversity (OTU richness and phylogenetic diversity) at 0–2, 2–5, and 5–10 cm depths based on two-way ANOVA. P < 0.05 indicates significant effect of experimental treatment on bacterial OTU richness and phylogenetic diversity
Phylum or class-level bacterial and fungal community structure in relation to treatments at each soil depth (a and b). The treatment effects on the relative abundance of bacterial and fungal taxa based on two-way ANOVA (c), and P < 0.05 indicates significant effect of experimental treatment on bacterial or fungal taxa
Redundancy discriminate analysis (RDA) between bacterial (a, b, and c) or fungal (d, e, and f) community composition and soil properties and at 0–2, 2–5, and 5–10 cm depths, respectively. Red arrows indicate significant factors
Treatment effects on bacterial and fungal connectivity patterns based on two-way ANOVA (a). Paired comparison between treatments of bacterial and fungal connectivity patterns (b). PC, positive cohesion; NC, |negative cohesion|. BP/N positive cohesion: |negative cohesion| ratio of bacteria; FP/N, positive cohesion: |negative cohesion| ratio of fungi; BTotal, bacterial total cohesion; FTotal, fungal total cohesion. * (P < 0.05), ** (0.001 < P < 0.01), and *** (P < 0.001). The results of two-way ANOVA are preferred
Purpose Nitrogen (N) deposition and warming may influence microbially mediated processes and functioning of ecosystem. Our study aimed to examine how soil bacterial and fungal community composition, diversity, and interactions respond to N deposition and warming. Materials and methods High-throughput sequencing and bioinformatic analysis were performed to explore microbial community composition and diversity, and cohesion analysis was adopted to assess the microbial interactions after 11 consecutive years ammonium nitrate supplementation and warming in a desert steppe of Inner Mongolia, Northern China. Results Our results demonstrated nitrogen supplementation, warming, and N supplementation plus warming affected the bacterial and fungal community structures, and the effects were soil depth-dependent. N supplementation improved the relative abundance of copiotrophic groups (Bacteroidetes and Betaproteobacteria) and restrained oligotrophic groups (Chloroflexi, Deltaproteobacteria, and Acidobacteria) at 0–2 cm depth. N supplementation plus warming significantly increased the relative abundance of Ascomycota at 2–5 cm depth. N supplementation and N supplementation plus warming significantly reduced bacterial diversity. The redundancy analysis demonstrated that pH and N availability significantly contributed to the variation in bacterial and fungal community, respectively. N supplementation and warming simplified bacterial connectivity and improved positive cohesion: negative cohesion ratio, while warming increased fungal connectivity and reduced positive cohesion: negative cohesion ratio. Conclusion The effects of N supplementation and warming on bacterial and fungal community structure were soil depth-dependent. N supplementation and N supplementation plus warming reduced bacterial diversity. N supplementation and warming simplified bacterial interactions. The bacterial community was more sensitive to nitrogen supplementation and warming than fungal community.
 
Purpose Poor soil structure and instability are primary problems in salt-affected soil, which is formed by excessive amounts of exchangeable Na⁺ and inadequate cementing substances in the soil. Long-term cattle manure application is a management practice for salt-affected soil that can change the distribution of soil aggregates and increase the stability of soil aggregates. Methods Experiments were carried out in a randomized block design comprising 5 treatments according to the number of years of cattle manure application. Soils to which cattle manure was applied for 7, 12, 15 and 18 years were used as the experimental treatments, and soil without cattle manure application was used as the control treatment. Soil aggregate destruction, organic matter contents, and exchangeable cations were measured, and the soil percentage of aggregate destruction was calculated. Results The results indicated that the application of cattle manure to the salt-affected soil significantly increased the organic matter content and the exchangeable Ca²⁺ and Mg²⁺ in water-stable aggregates and significantly decreased the exchangeable Na⁺ and soil pH compared to those in untreated soil in all fractions (P < 0.05). The abundance of water-stable microaggregates (WSAs < 0.25 mm) was reduced, while that of mechanically stable microaggregates (MSAs < 0.25 mm) increased with increasing years of cattle manure application. Conclusion We concluded that applying cattle manure to salt-affected soil led to a reduction in exchangeable Na⁺ and increases in exchangeable Ca²⁺ and organic matter contents in soil aggregates; these changes caused the agglomeration of soil aggregates and thus a decrease in the soil percentage of aggregate destruction and an increase in soil aggregate stability, ultimately resulting in an increase in macroaggregate (WSAs > 0.25 mm) abundance.
 
Purpose The benefits associated with the conventional use of neonicotinoids on greenhouse vegetables have lasted for several decades. Extensive use of neonicotinoids could result in their accumulation in the soils, thereby potentially threatening human health through ingestion, dermal contact and inhalation. This study aimed to clarify the pollution characteristics and non-dietary human cumulative risk of neonicotinoids in vegetable greenhouse soils. Materials and methods A total of 283 soil samples were collected from celery, cucumber, pepper and tomato greenhouses across Shandong Province in China and analysed for nine widely used neonicotinoids. Furthermore, the potential health risks for both adults and children were assessed. Results Among all the soil samples, imidacloprid, clothianidin and thiamethoxam were the top three detected neonicotinoids, with detection frequencies of 96.82–99.65%. The three neonicotinoids had higher average concentrations in the soils, with average concentrations of 27.55–157.64 µg/kg. All the soil samples contained at least two neonicotinoids, but most of the detected residues were at low levels with concentrations ranging from 0.02 to 1816.67 µg/kg. The levels of total neonicotinoids (calculated based on a relative potency factor method) in tomato and pepper soils were statistically higher than those in cucumber and celery soils. Although the exposure risk to children was far higher than that to adults, the health risk assessment for each neonicotinoid or total neonicotinoids was within the established safe limits (hazard index range, 1.07 × 10⁻¹⁰ to 1.95 × 10⁻³, < 1). Despite the low health risk, potential hazards of exposure to neonicotinoid-contaminated soils should be continuously assessed due to the low-dose adverse effects and potential accumulation in human tissues. Conclusions Our findings indicate that attention should be given to the neonicotinoids in vegetable greenhouse soils due to their ubiquity and toxicokinetic characteristics.
 
Purpose Rock desertification is the most serious ecological problem in karst areas and can easily cause soil structure instability. The aim of this work was to analyze the influences of organic carbon, carbon components and pore structural differences on the stability of aggregates in rocky desertification areas. Materials and methods Soil samples were collected from the karst peak-cluster depression area in Southwest China. Soil aggregate stability was determined by the Le Bissonnais (LB) method. The relationship between variables was analyzed via the structural equation model (SEM). Results and discussion There were significant differences in the stability of aggregates among different land uses in the study area: secondary forest (SF) > coppice forest (CF) > plantation forest (PF) > citrus plantation (CP) > cultivated land (CL). The main crushing mechanisms of aggregates were slaking and mechanical failure. Aggregate pores were mainly < 30 μm storage pores, with pore throat diameters concentrated in the 0–20 μm range, which was associated with the cohesive soil texture of the study area. The pore connectivity of aggregates was better in SF and CF, followed by PF, while CP and CL had lower colloidal material contents, isolated pore structure and poor connectivity due to the heavy application of inorganic fertilizers and influences from human disturbances. Conclusions SEM analysis showed that SOC and POC determined aggregate stability, mainly by directly or indirectly influencing aggregate porosity and the number of storage pores. The POC was the main variable in organic carbon composition to determine the pore characteristics and stability of aggregates. To meet the bidirectional demand of production and improvement of soil structure, it is necessary to rationally distribute fertilizer and reduce disturbance frequency in agricultural production land.
 
The distribution of sampling sites in the dry valleys of southwestern China
Latitudinal patterns of soil organic carbon density (SOCD) and soil inorganic carbon density (SICD) at different depths
Relative contributions of climate, soil and vegetation properties to soil organic carbon density (SOCD) and soil inorganic carbon density (SICD) across different depths. Variation partitioning analysis was conducted to identify the variance in the SOCD and SICD explained by these three groups of biotic and abiotic factors. Climate parameters include MAT, mean annual temperature; MAP, mean annual precipitation; soil properties include pH, silt, clay, exCa, exchangeable calcium; exMg, exchangeable magnesium; vegetation variables include: SSWI, shrub Shannon–Wiener index; HSWI, herb Shannon–Wiener index; SFRB, shrub fine root biomass; SC, shrubs cover; HC, herb cover
Mean variable importance for the prediction of soil organic carbon density (SOCD) obtained from random forest analysis. The importance is expressed as percentage of increase in mean square error (% IncMSE). Significance denoted by *P < 0.05. MAT, mean annual temperature; MAP, mean annual precipitation; SSWI, shrub Shannon–Wiener index; HSWI, herb Shannon–Wiener index; SFRB, shrub fine root biomass; SC, shrubs cover; HC, herbs cover; exCa, exchangeable calcium; exMg, exchangeable magnesium
Mean variable importance for the prediction of soil inorganic carbon density (SICD) obtained from random forest analysis. The importance is expressed as percentage of increase in mean square error (% IncMSE). Significance denoted by *P < 0.05. MAT, mean annual temperature; MAP, mean annual precipitation; SSWI, shrub Shannon–Wiener index; HSWI, herb Shannon–Wiener index; SFRB, shrub fine root biomass; SC, shrubs cover; HC, herbs cover; exCa, exchangeable calcium; exMg, exchangeable magnesium; SOC, soil organic carbon
PurposeDrylands account for 47.2% of land area and contain 15.5% of global carbon (C). However, the variation in organic and inorganic C stocks across latitudinal gradients in arid and semiarid shrubland ecosystems remains understudied, and we lack in-depth understanding of the main drivers of C variation at this spatial scale.Methods Here, we sampled soils from 95 sites across a latitudinal gradient to explore both the latitudinal patterns and potential drivers of soil organic carbon density (SOCD) and soil inorganic carbon density (SICD). We also assessed variation in SOCD and SICD down the soil profile, by sampling soils at four depths (0 – 10 cm, 10 – 20 cm, 20 – 30 cm, and 30 – 50 cm).ResultBoth SOCD and SICD exhibited a binomial relationship with latitude (P < 0.01). Soil properties accounted for the greatest variation in SOCD, with the most important explanatory factor being exchangeable calcium, followed by mean annual temperature, pH, plant diversity, and silt content. Soil pH and plant diversity were more important in explaining variation in SOCD in the subsoil (> 20 cm depth) than the topsoil. For SICD, soil properties explained the greatest variation at all depths. Soil pH explained the most variance in SICD, followed by exchangeable calcium and mean annual temperature in the topsoil (i.e., 0 – 10 cm and 10 – 20 cm). In the subsoil (i.e., 20 – 30 cm and 30 – 50 cm), exchangeable calcium was the most important predictor, followed by soil organic carbon, mean annual temperature, and pH.Conclusion Our study shows that soil properties are a strong predictor of latitudinal patterns of soil organic and inorganic C in arid and semiarid shrubland ecosystems. We also identified differences in potential drivers of SOCD and SICD with depth, advancing our understanding of large-scale patterns of C storage in arid and semiarid soils.
 
Purpose The biogeochemical cycling of carbon (C) is essential for maintaining plant productivity and thus plays a vital role in soil carbon sequestration. However, limited information is available for addressing the effects of N input on SOC sequestration and discriminating between the differential effects on bulk soil and the rhizosphere. The aims of this study are to explore the responses of SOC fractions to different N addition levels and their potential environmental drivers. Materials and methods A 20-month pot experiment was conducted to investigate the responses of soil organic C (SOC) fractions to varying levels of N addition (0, 25, 50, 75, 100, and 150 kg N·ha⁻¹·year⁻¹) in rhizosphere and bulk soil by using a nylon mesh. The key roles of bacterial community composition and C-acquisition enzyme activities were also deciphered as the driver factors of SOC fractions. Results and discussion Carbon levels (i.e., particulate organic C (POC), light and heavy C fractions, dissolved organic C, and easily oxidizable organic C) in the rhizosphere increased with increasing N addition. Greater variation was observed in the response of POC to N addition, when compared to other SOC fractions, indicating relatively higher sensitivity of POC to elevated soil N levels. In the rhizosphere, N addition increased bacterial diversity and the activities of C-acquisition enzymes but only at lower levels (≤ 50 kg N·ha⁻¹·year⁻¹). The effect of N addition on rhizospheric SOC was strongly associated with increased root biomass and inorganic N content. Meanwhile, in bulk soil, changes in SOC fractions were largely driven by soil NO3⁻-N level. Conclusions The results of the present study indicate that N addition increases rhizospheric SOC content by regulating soil N levels and bacterial activities and highlight the differential responses of SOC fractions from rhizosphere and bulk soil to N addition, which will be helpful to understand the role of SOC sequestration in response to N deposition.
 
Geographic distribution of study sites. Dots with different colors represent different driving factors of SOC dynamics. The background map is colored according to global SOC stock (units: kg/m²) in 0–30 cm depth. SOC stock data from Zenodo (https://zenodo.org/)
Sensitivity of soil respiration to temperature (Q10) across different climates and different altitudes. (a) Q10 for different forest types (Zhang et al. 2021a), (b) Q10 across different altitudes (Kong et al. 2022)
SOC stocks and standard deviation at different plant successional stages. (a) SOC stocks of three succession stages (Lasanta et al. 2020), (b) SOC stocks of different forest types (Sokolowska et al. 2020)
Mineral SOC content and standard deviation with different burning stages and burning intensity. (a) low intensity wildfire and includes three combustion stages: BF (unburnt soil), DF (during fire) and 1AF (one year after fire) (Dymov et al. 2021), (b) different burning intensities (Li et al. 2020c)
Effects of grazing intensity and grazing strategy on SOC. (a) Effects of different grazing intensities on SOC stocks (Vaieretti et al. 2021), (b) effects of grazing strategies on SOC content; G, F15, and F30 represent free grazing, 15 years in fenced, and 30 years in fenced, respectively (Zhang et al. 2020a)
PurposeMountains have unique microclimates and rich plant diversity, resulting in different patterns and dynamics of soil organic carbon (SOC) across plant communities and elevations. Nevertheless, few studies have systematically reviewed the drivers of the dynamics of global SOC in mountainous regions.Materials and methodHere, we collected relevant published literature to analyze the main drivers of the dynamics of global SOC at different elevations and plant communities. Specifically, we analyzed the impact of natural variability and human activity on SOC.Results and discussionWe found that natural factors mainly included climate change, plant succession, and wildfires. Anthropogenic factors mainly included land use changes and grazing practices. SOC stocks at low elevations were more susceptible to grazing, precipitation, and land use changes. Conversely, higher elevations were more susceptible to warming and plant community succession. Notably, montane forests and permafrost, which are important terrestrial carbon sinks, were more easily regulated by wildfires and climate change. However, grazing had different effects on SOC in montane grasslands.Conclusions This review highlights the synergy of multiple drivers that should be fully considered when investigating mechanisms underlying montane SOC. We recommend that future work explore the impact of extreme weather events on montane SOC.
 
Purpose Vegetation restoration types influence the physicochemical properties and near-surface characteristics of soil and likely affects soil resistance. However, variations in the characteristics of soil resistance under different vegetation restoration types have not been characterized in the severely eroded subtropical regions of South China. The objective of this study was aimed to identify the mechanism of soil resistance under different vegetation restoration types and simulate the prediction equation. Materials and methods This study examined four typical vegetation restoration types that were restored by artificial planting forest (AF), orchard (AO), scrubland (AS) and grassland (AG) and compared them with unrestored eroded land (EL) as a control. The soil detachment rate was measured by scouring the soil with steel cutting rings with flowing water under five shear stress intensities (3.40–18.11 kPa). This study quantified the effects of typical types of vegetation restoration on soil resistance in severely eroded subtropical regions of South China by identifying the major factors and simulating a prediction equation. Results The results showed that the eroded land had the maximum soil detachment rate (0.474 kg m⁻² s⁻¹), and then was the most easily detached. It was followed sequentially by the artificial grassland, the artificial scrubland, the artificial orchard and the artificial forest. The soil detachment rate was negatively correlated with clay content, capillary porosity, soil organic matter content, soil cohesion (p < 0.01) and soil initial water content (p < 0.05), and it was a positively correlated with silt content (p < 0.01). Near-surface characteristics also inhibited the soil detachment process to some extent. A similar trend was observed for rill erodibility, which was dominated mainly by soil organic matter content, root surface density and sand content (NSE = 0.806). The critical shear stress was significantly positively related to soil cohesion and biological crust thickness (NSE = 0.632). Conclusions Based on the Water Erosion Prediction Project (WEPP) model, the soil detachment rate was simulated from easily measured factors, such as sand content, soil organic matter content, soil cohesion, biological crust thickness and root surface density (R² = 0.812, NSE = 0.827). This observation provides a reference regarding the soil–water erosion process in severely eroded subtropical regions in South China.
 
Map of the study area with sampling locations (The nomenclature of sampling points was ordered by the FDA hydrolytic rate)
Optimization for determination method of FDA hydrolytic rate. (a) Acetone volume for sediment, (b) Acetone volume for sterilized sediment, (c) Chloroform/methanol (2:1 V/V) volume for sediment, (d) Chloroform/methanol (2:1 V/V) volume for sterilized sediment, (e) Ultrasound time for sediment, (f) Ultrasound time for sterilized sediment. Other conditions: 1.8 g fresh sediment; 15 mL of 50 mM phosphate buffer solution (pH: 7.2); 0.2 mL of 3 g/L FDA; incubation for 1 h at 30 °C in a shaker at 50 rpm
Correlation between different environmental factors and FDA hydrolytic rates by the improved determination method of FDA. (a) OM, (b) TN, (c) AVS and (d) moisture content in the sediments
Measured and predicted values of regression models. (a) Comparison, (b) equiline scatter plot
PurposeThe fluorescein diacetate (FDA) method has been widely used to quantify microbial activity rapidly and sensitively. However, due to high silt–clay ratios and high organic matter contents in urban river sediments, the current FDA methods for soil/coastal sediments may produce errors when applied to urban river sediments. Here, the FDA method was optimized to assess the effects of pollutants on microbial activity.Materials and methodsBased on their microbial activities, five urban river sediments with different contamination levels were selected for method optimization. The effects of pollutants on microbial activity were assessed using multiple linear regression analysis based on the optimized method.ResultsReducing the method’s variation coefficient from 1.9% ~ 3.8% to 0.7% ~ 3.3% by increasing the amount of terminator was critical for improving the method’s precision and reproducibility. The fluorescein adsorbed to the sediment was released by ultrasonication in an ice-water bath for 20 min, improving measured microbial activity values. Using the optimized method, we found significant positive correlations between microbial activity and physicochemical indicators. According to multiple linear regression analysis, the influence of different variables on microbial activity diminished in the following order: organic matter > total nitrogen > acidified volatile sulfides > moisture content.Conclusions The optimized method reduced the coefficient of variation and improved the measured microbial activity values. Based on the optimized method, multiple linear regression analysis will be helpful for the prediction of pollutant concentrations in urban river sediments.
 
Dominant soil bacterial phyla under different treatments a. Principal component analysis of bacterial communities under different treatments b. Bray–Curtis dissimilarity among different treatments c. In b, the x- and y-axes in panels are indicated by the first and second coordinates, respectively, and the values in parentheses show the percentages of the community variation explained. CK, soil only; SBR, soil amended with biogas residue; SBC300, soil amended with biochar from biogas residue under 300 °C; SBC600, soil amended with biochar from biogas residue under 600 °C; SBC800, soil amended with biochar from biogas residue under 800 °C
Van Krevelen (VK) plots for CHO molecular formulas assigned to the ESI FT-ICR MS spectral peaks in a CK, b SBR, c SBC300, d SBC600, and e SBC800 treatments. CK, soil only; SBR, soil amended with biogas residue; SBC300, soil amended with biochar from biogas residue under 300 °C; SBC600, soil amended with biochar from biogas residue under 600 °C; SBC800, soil amended with biochar from biogas residue under 800 °C
The rates a and cumulative b of CO2 release from CK, SBR, BC300, BC600, and BC800 treatments. Error bars show the standard deviations (n = 3). Different lowercase letters in each line indicate significant differences (P < 0.05) among treatments using Duncan’s multiple range test. CK, soil only; SBR, soil amended with biogas residue; SBC300, soil amended with biochar from biogas residue under 300 °C; SBC600, soil amended with biochar from biogas residue under 600 °C; SBC800, soil amended with biochar from biogas residue under 800 °C
Correlation heat map between bacteria community (horizontal axis) with soil organic carbon (SOC) mineralization and dissolved organic carbon (DOC) structure (vertical axis) based on Pearson bilateral significance test. Chloroflexi, Actinobacteria, Proteobacteria, Acidobacteria, Planctomycetes, Firmicutes, Bacteroidetes, Verrucomicrobia, WPS-2, Gemmatimonadetes, and Others are the dominant microbial flora in the top ten, which are indicated in italics. * and ** indicated P < 0.05 and P < 0.01
Structural equation modeling (SEM) of the effects of submergence on soil organic carbon (SOC) mineralization as a result of submergence-induced changes in soil dissolved organic carbon (DOC) and soil microorganisms. Square boxes denote variables included in the models. Values associated with solid arrows represent standardized path coefficients (SPCs) and asterisks mark their significance: *P < 0.05; **P < 0.01; ***P < 0.001. Solid arrows denote the directions and effects that were significant (P < 0.05), and the thickness represents the magnitude of the path coefficients. Dashed arrows represent the directions and effects that were non-significant (P < 0.05). (DOC chemical structure = factor 1 of PCA, represents the overall information of lipids, proteins, carbohydrates, unsaturated hydrocarbons and lignins of soil dissolved organic carbon, because the first principal coordinate explained 76.6% of the variations. Exogenous carbon properties = submergence or no submergence, including pH, EC, TOC, TN, C/N, and SSA of different exogenous carbon. CFI = 0.937 is result from the chi-squared value is less than the degrees of freedom in this SEM
As a high-yield by-products of biogas engineering, biogas residue (BR) are limited in the application of soil fertility due to their ecological threat. Therefore, conversing BR into biochar is to be considered. However, whether BR biochar still had ecological risk to soils (especially the widely distributed soils such as Ultisol soil) remains to be probed. Considering that soil microbial communities and carbon (C) pools play crucial roles in soil ecological environment, the soil bacterial communities, dissolved organic C (DOC) molecular structure, and bulk C mineralization in soils with BR (SBR) and BR biochar (produced at 300 °C, 600 °C, and 800 °C) addition (SBC) were explored in a microcosm incubation experiment using the Ultisol soil (clay loam soil). Soil without biochar addition was set as control (CK). The key technology of Fourier-transform ion cyclotron resonance mass spectrometry (ESI-FT-ICR-MS) was adopted in DOC molecular structure determination. SBC300 and SBC600 significantly increased Shannon and Simpson indices by 4.3–7.7% and 2.3–2.6% than SBR. The abundance of Chloroflexi, Acidobacteria, Planctomycetes, and Firmicutes phyla were ordered with CK > SBC > SBR. ESI-FT-ICR-MS results showed that the DOC in SBC and SBR had more proteins, carbohydrates, and unsaturated hydrocarbons than in CK, and the highest increases emerged in SBC600. The soil C mineralization ability showed SBR > SBC > CK. The increase of SOC mineralization and the simplification of DOC molecular structure were significantly related to the increase of Proteobacteria and the decrease of Firmicutes. The structural equation modeling showed DOC concentration (SPC = − 0.300), bacterial community richness (SPC = 0.271), and diversity (SPC = − 0.939) were the important abiotic and biotic factors regulating C mineralization. In conclusion, the application of BR biochar had lower ecological risk than direct application of BR in the Ultisol soil.
 
Top-cited authors
Hailong Wang
  • Foshan University
Yong Sik Ok
  • Korea University
Zhihong Xu
  • Griffith University
Ji-Zheng He
  • Chinese Academy of Sciences
Jean-Louis Morel
  • University of Lorraine - INRA