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Erosion, deposition and soil carbon: A review of process-level controls, experimental tools and models to address C cycling in dynamic landscapes
... Water erosion comprises three processes: (i) initiation or detachment of soil particles, (ii) transport and redistribution of sediments over the landscape, and (iii) deposition on depression or concavity sites and in aquatic ecosystems (Doetterl et al., 2016). Derived from these processes, water erosion has significant impacts on the distribution and transformation of SOC at the microwatershed scale ( discusión. ...
... La erosión hídrica comprende tres procesos: (i) inicio o desprendimiento de partículas de suelo, (ii) transporte y redistribución de sedimentos sobre el paisaje y (iii) deposición en los sitios en depresión o concávos y en ecosistemas acuáticos (Doetterl et al., 2016). Derivado de estos procesos, la erosión hídrica tiene impactos significativos sobre la distribución y transformación del COS a escala de microcuencas Wang et al., 2014b). ...
... Soil water erosion, when analyzed using the micro-watershed approach, can be considered as a CO 2 emitting source, in terms of particle initiation and transport due to the effect these processes have on soil organic matter (SOM) mineralization, but also as a source of storage at deposition sites (Doetterl et al., 2016). ...
Introduction: Soil loss caused by water erosion impacts both positive and negative fluxes of carbon to the atmosphere. Objective: To identify the main research trends related to the influence of water erosion on soil organic carbon (SOC) redistribution and its relationship with soil and water conservation practices. Materials and methods. Literature published in national and international journals was consulted in Web of Science, Scopus, SciELO, Redalyc, ResearchGate and Google Scholar. Research trends were analyzed using predefined keywords and grouped according to their affinity. Results: In the period 2012-2022, 80 % of global research focused on SOC redistribution caused by the effect of water erosion and the effect of soil management and conservation practices; however, no studies were found in this regard in Mexico. Due to water erosion, programs for the construction of soil and water conservation works have been implemented in Mexico with significant success, such as sediment control dams, but the impacts in terms of C storage have not been evaluated. Conclusions: In Mexico there are areas of opportunity to focus research at different scales: (I) analyze the redistribution of SOC caused by water erosion, (II) estimate the storage of SOC in sediments, (III) analyze the potential of mechanical soil and water conservation practices as carbon sinks, and (IV) propose a risk index of SOC loss using remote sensing.
... Soil organic carbon (SOC) represents the largest part of the terrestrial carbon reservoir (Doetterl et al., 2016). SOC determines the quality and functions of soils and is partly released into the atmosphere possibly contributing to climate change (Batjes, 1996;Lal, 2004;Meersmans et al., 2016;Wiesmeier et al., 2019). ...
... However, so far SOC storage in depositional landforms at regional scale is poorly documented. A few studies highlight the importance of type of landforms for SOC storage and it spatial distribution (Berhe et al., 2007;Doetterl et al., 2016;Rosenbloom et al., 2006). T. Li et al., 2019 state that depositional landforms store almost six times more SOC than eroded areas. ...
... above 50%) for 21% of the Especially in eroded landscapes SOC buried at depositional sites may be underestimated when one assesses SOC stocks in topsoils only. Therefore, SOC content of the entire soil profile should be assessed (Doetterl et al., 2016). Wiesmeier et al. (2013) state that in some soil types from southeast Germany, SOC is stored below the topsoil. ...
Soil erosion in loess landscapes results in soil organic carbon (SOC) redistribution and storage in SOC pools. Understanding the SOC dynamics is important because changes in the SOC stocks may have impacts on global climate change. However, the topographic‐related patterns controlling SOC storage are not well understood. Closed depressions are natural landforms in loess landscapes and preserve buried Holocene soils and SOC‐rich colluvia resulting from soil erosion and are SOC pools. The aim of this study was to quantify the impact of natural closed depressions on SOC storage in the loess landscape of the Nałęczów Plateau. Buried Holocene soils and colluvial sediments infilling five representative closed depressions were documented and the SOC stocks were calculated. Using GIS analysis SOC from all closed depressions was calculated and mapped. Between 13.5 and 229.78 Mg of SOC are in the entire soil profiles of the studied closed depressions, 10%–21% of the SOC stock is in the topsoils. The SOC stock in all closed depressions of the study area reaches 172.09–265.19 Mg ha ⁻¹ and in the soil cover outside the closed depressions is 51.20–105.40 Mg ha ⁻¹ . SOC from closed depressions varies spatially and increases the SOC stock at regional scale by 0.1–90 Mg ha ⁻¹ . For about 21% of the study area, the SOC from closed depressions increases the SOC stock in the agricultural landscape by more than 10% (up to 60%). This study highlights the importance of closed depressions for SOC storage and provides a better understanding of its spatial distribution at the regional scale.
... More generally, erosive processes also redistribute terrestrial C within the landscape, thereby affecting C fluxes as well as the spatial pattern and size of the terrestrial C reservoir itself (Berhe et al., 2007(Berhe et al., , 2008Billings et al., 2019;Doetterl et al., 2016;Yoo et al., 2005Yoo et al., , 2006. In this study, we define terrestrial C as that which is contained in soil and biomass (living and dead). ...
... Although the theoretical basis for erosion's influence on the C cycle is clear, determining how the magnitude and direction (sink or source) of that effect explicitly depend on the frequency of erosive events, their spatial distribution across a landscape, and their cumulative effects over different timescales has rarely been addressed quantitatively (Doetterl et al., 2016). Only a few studies have coupled geomorphic process-based models of erosion (sensu Dietrich et al., 2003) with carbon flux models (Billings et al., 2019;Dialynas et al., 2016;Rosenbloom et al., 2001Rosenbloom et al., , 2006Yoo et al., 2005Yoo et al., , 2006, and none of those have addressed landslides in carbon-dense temperate rainforests. ...
... For the terrestrial C cycle, global scale models typically do not include physical transport of C across a landscape and group C into just a few different reservoirs, necessarily averaging over fine-scale details (Luo et al., 2015(Luo et al., , 2017Manzoni & Porporato, 2009). More detailed models that include effects of soil aggregates, moisture, and depth on C fluxes tend to be site specific, require large amounts of fieldwork to calibrate, and have often focused on agricultural lands (Doetterl et al., 2016). Erosion models are focused on the movement of sediment, rather than C, which further complicates their coupling with C cycle models. ...
Landslides influence the global carbon (C) cycle by facilitating transfer of terrestrial C in biomass and soils to offshore depocenters and redistributing C within the landscape, affecting the terrestrial C reservoir itself. How landslides affect terrestrial C stocks is rarely quantified, so we derive a model that couples stochastic landslides with terrestrial C dynamics, calibrated to temperate rainforests in southeast Alaska, United States. Modeled landslides episodically transfer C from scars to deposits and destroy living biomass. After a landslide, total C stocks on the scar recover, while those on the deposit either increase (in the case of living biomass) or decrease while remaining higher than if no landslide had occurred (in the case of dead biomass and soil C). Specifically, modeling landslides in a 29.9 km² watershed at the observed rate of 0.004 landslides km⁻² yr⁻¹ decreases average living biomass C density by 0.9 tC ha⁻¹ (a relative amount of 0.4%), increases dead biomass C by 0.3 tC ha⁻¹ (0.6%), and increases soil C by 3.4 tC ha⁻¹ (0.8%) relative to a base case with no landslides. The net effect is a small increase in total terrestrial C stocks of 2.8 tC ha⁻¹ (0.4%). The size of this boost increases with landslide frequency, reaching 6.5% at a frequency of 0.1 landslides km⁻² yr⁻¹. If similar dynamics occur in other landslide‐prone regions of the globe, landslides should be a net C sink and a natural buffer against increasing atmospheric CO2 levels, which are forecast to increase landslide‐triggering precipitation events.
... We model the redistribution of SOC via diffusive soil transport without also simulating water erosion, because this approach well-explains spatial patterns of soil and SOC loss in agricultural fields in the Midwestern U.S. . Conceptually, our model predicts soil redistribution caused by tillage, which is the primary driver of diffusive soil transport in agricultural settings (Doetterl et al., 2012(Doetterl et al., , 2016Kwang, Thaler, Quirk, et al., 2022;Van Oost et al., 2005). The main inputs of the model are digital elevation models (DEMs) and a 3-dimensional SOC pedostratigraphy. ...
... Quantifying the impact of soil erosion on the global carbon cycle typically involves an assumption that the rate of soil and SOC redistribution is steady with time (Berhe et al., 2007;Doetterl et al., 2016;Kuhn et al., 2009;Lal, 2003;Van Oost et al., 2007). A decline in rates in SOC redistribution implies that the highest rates occurred in the past, shortly after the inception of agriculture (West et al., 2004). ...
... In other regions, where plow-based agriculture has been implemented at much longer timescales, such as Eurasia and Africa (Lal et al., 2007), rates of SOC redistribution have had more time to decline. In contrast, SOC redistribution in North America has a greater potential to affect the global carbon cycle because relatively carbon-rich soils are being actively transported (Doetterl et al., 2016). ...
Soil is the source of the vast majority of food consumed on Earth, and soils constitute the largest terrestrial carbon pool. Soil erosion associated with agriculture reduces crop productivity, and the redistribution of soil organic carbon (SOC) by erosion has potential to influence the global carbon cycle. Tillage strongly influences the erosion and redistribution of soil and SOC. However, tillage is rarely considered in predictions of soil erosion in the U.S.; hence regionwide estimates of both the current magnitude and future trends of soil redistribution by tillage are unknown. Here we use a landscape evolution model to forecast soil and SOC redistribution in the Midwestern United States over centennial timescales. We predict that present‐day rates of soil and SOC erosion are 1.1 ± 0.4 kg ⋅ m−‐2 ⋅ yr−‐1 and 12 ± 4 g ⋅ m⁻² ⋅ yr⁻¹, respectively, but these rates will rapidly decelerate due to diffusive evolution of topography and the progressive depletion of SOC in eroding soil profiles. After 100 years, we forecast that 8.8 (+1.9/−2.1) Pg of soil and 0.17 (+0.03/−0.04) Pg of SOC will have eroded, causing the surface concentration of SOC to decrease by 4.4% (+0.9/−1.1%). Model simulations that include more widespread adoption of low‐intensity tillage (i.e., no‐till farming) determine that soil redistribution, SOC redistribution, and surficial SOC loss after 100 years would decrease by ∼95% if low‐intensity tillage is fully adopted. Our findings indicate that low‐intensity tillage could greatly decrease soil degradation and that the potential for agricultural soil erosion to influence the global carbon cycle will diminish with time due to a reduction in SOC burial.
... Subsequently, after passing through the ground litter, NP infiltrates into the soil (In) and is stored in the topsoil layer, and/or gives rise to surface runoff (R). Nutrients distribution through infiltration and runoff is important in connecting plant and soil systems (Doetterl et al., 2016;Vania et al., 2016). Thus, rainfall redistribution across the plant canopy and topsoil layer alters the spatial and temporal dynamics of water and nutrients, leading to significant differences in DC and DN between TF, SF, In and R (Alves et al., 2022;Li et al., 2019;Song et al., 2022). ...
... F P 62.9 ± 9.2 45.1 ± 7.5 17.8 ± 3.3 37.7 ± 8.2 F TF 93.9 ± 11.9 73.2 ± 9.9 20.7 ± 3.0 37.9 ± 7.0 F SF 21.3 ± 3.3 18.6 ± 2.9 2.7 ± 0.5 5. concentrations of nutrient materials contributed less to the dynamics of the corresponding fluxes than the water fluxes themselves (Doetterl et al., 2016;Li et al., 2019). ...
... DC fluxes have usually been seen as a lesser component than carbon fluxes associated with primary productivity or heterotrophic respiration in terrestrial systems (Kalbitz and Kaiser, 2008). However, the important role played by DC fluxes in ecosystems cannot be ignored, as DC fluxes are carriers of carbon loss in topsoil (Kindler et al., 2011) and are an important source of organic biomass in deep soils (Doetterl et al., 2016;Rumpel and Kögel-Knabner, 2010). Due to difficulties involved in direct measurements, DC fluxes in soil layers, particularly DC in In from topsoil to deeper soils, are not considered when calculating carbon accumulation (Zhang et al., 2013;Doetterl et al., 2016;Li et al., 2019). ...
Dissolved carbon (DC) and dissolved nitrogen (DN) driven by rainfall partitioning are critical components of carbon and nitrogen cycles in terrestrial ecosystems, especially in vegetation restoration areas. However, the variations of DC and DN fluxes in the input (throughfall and stemflow) and output (infiltration and surface runoff) water pathways of shrub ecosystem in drylands were poorly understood. In this study, the rainfall partitioning, dissolved organic and inorganic carbon (DOC and DIC) and DN fluxes in 52 rainfall events affecting a xerophytic shrub ecosystem on the heavily eroded Chinese Loess Plateau were measured over the 2019–2021 rainy seasons. The contributions of plants and topsoil (0–5 cm) to the variations in DC and DN fluxes along water pathways were quantified. We found that the DC and DN fluxes in throughfall accounted for more than 80% of net input flux, and the topsoil infiltration of DC and DN fluxes into soil layer below 5 cm depth were the most important output flux (contribution more than 98%). The mean event plant-derived DC and DN fluxes were 52.2 and 5.3 mg m−2, respectively, accounting for 45.4% and 12.2% of net DC and DN input fluxes (115.2 and 43 mg m−2). The soil-buffered DC and DN fluxes (455.6 and 29.5 mg m−2) were 3.95 and 0.69 times greater than the net input fluxes, respectively. The DC and DN fluxes in infiltration had the best correlation and increased most quickly with rainfall depth in the four different water pathways. Furthermore, the soil carbon content in 0–50 cm depth and soil nitrogen content in 0–10 cm depth in shrub-covered land was significantly higher than those in bare land (p < 0.05). This study highlights the important roles of rainfall partitioning in plant-sourced and soil-buffered DC and DN fluxes, which should be considered in soil carbon and nitrogen accumulation in drylands.
... Current models increasingly place microbially mediated processes as key drivers of soil carbon turnover and stabilization (e.g. [10,11]), but we currently lack diagnostic markers that reflect microbial processes and that serve as tracers of soil inputs to downstream carbon reservoirs [12]. Radiocarbon signatures of organic molecules that reflect the functioning and export of the soil organic matter pool would thus provide crucial constraints on carbon cycling in the environment. ...
... All basin properties influence the stability, erosion and mobilization of OC (e.g. [12,[40][41][42][43][44]). Relations between catchment properties and compound or fraction dated are explored using a spearman rank-order correlation [45]. ...
Compound- and compound class-specific radiocarbon analysis of source-diagnostic ‘biomarker’ molecules has emerged as a powerful tool to gain insights into terrestrial carbon cycling. While most studies thus far have focused on higher plant biomarkers (i.e. plant leaf-wax n-alkanoic acids and n-alkanes, lignin-derived phenols), tracing paedogenic carbon is crucial given the pivotal role of soils in modulating ecosystem carbon turnover and organic carbon (OC) export. Here, we determine the radiocarbon (¹⁴C) ages of glycerol dialkyl glycerol tetraethers (GDGTs) in riverine sediments and compare them to those of higher plant biomarkers as well as markers of pyrogenic (fire-derived) carbon (benzene polycarboxylic acids, BPCAs) to assess their potential as tracers of soil turnover and export. GDGT Δ¹⁴C follows similar relationships with basin properties as vegetation-derived lignin phenols and leaf-wax n-alkanoic acids, suggesting that the radiocarbon ages of these compounds are significantly impacted by intermittent soil storage. Systematic radiocarbon age offsets are observable between the studied biomarkers, which are likely caused by different mobilization pathways and/or stabilization by mineral association.
This article is part of the Theo Murphy meeting issue 'Radiocarbon in the Anthropocene'.
... Soils with a sand content like our site or higher comprise 26.5% o German agricultural soils, and around one th o European agricultural land, but store relatively little carbon, although having high variation (European Commission, Joint Research Centre, 2013;Huang & Hartemink, 2020;Poeplau et al., 2020). In order to better understand the mechanisms behind C sequestration in sandy soils and the infuence o dierent C management practises (Huang & Hartemink, 2020), uture works should now comprise long term eld studies on C cycling o eroded soils in dierent landscapes and dierent climates (Doetterl et al., 2016). Not only or modelling the interplay o erosion and C cycling across larger scales (Doetterl et al., 2016), but particularly or restoring degraded land by common agricultural practice. ...
... In order to better understand the mechanisms behind C sequestration in sandy soils and the infuence o dierent C management practises (Huang & Hartemink, 2020), uture works should now comprise long term eld studies on C cycling o eroded soils in dierent landscapes and dierent climates (Doetterl et al., 2016). Not only or modelling the interplay o erosion and C cycling across larger scales (Doetterl et al., 2016), but particularly or restoring degraded land by common agricultural practice. Supporting economic incentives will re-increase yields and C returns into these sites. ...
... SOC represents the largest OC stock found in the terrestrial ecosystem (Stocker et al., 2013;Luo et al., 2019), and it is sensitive to environmental disturbances, such as agricultural activities and soil erosion (Schmidt et al., 2011;Luo et al., 2017). Erosion of SOCenriched topsoil from the upslope to the downslope or downstream considerably affects the total carbon stocks and persistence of different landform positions in soil (Doetterl et al., 2016). Enhanced vulnerability of OC to microbial decomposition and/or dissolution in water owing to soil aggregate breakdown and the subsequent exposure of previously physically protected organic matter can increase the decomposition rate of OC during its mobilization and transportation (Rainer et al., 2018;Wang et al., 2014). ...
... However, most studies on OC dynamics in eroding landscapes have focused on the quantitative analysis of OC transported in the context of erosion and physical protection mechanisms of soil carbon. Further, several studies have investigated the stability and stabilization mechanisms of OC in eroding and depositional landscapes (Doetterl et al., 2016;Wang et al., 2014). ...
The physicochemical interactions between soil organic carbon (SOC) and minerals are important for the long-term stabilization of OC. However, in the dynamic landscape, the mechanisms of OC stabilized by different types of minerals remains unclear. Herein, we investigated the effects of erosion and deposition on the distribution of SOC pools and the interaction of OC with different minerals in an agricultural Mollisol landscape. We quantified the SOC fractions using a physically separated technique and measured the polyvalent cation bound-OC using a mineral reduction release method. Sorption experiments and specific mineral surface area (SSA) were determined. Results show that 77.6%−96.9% of SOC was mineral-associated OC (MAOC). MAOC contents in the topsoil (0–20 cm) and subsoil (60–80 cm) of the depositional sites were 1.17 and 1.64 times higher than those in the topsoil and subsoil of the eroding sites, respectively. A higher MAOC content in the depositional sites suggests that MAOC in the middle slope migrates directly to the downslope through erosion, and subsequent depositional and burial processes promote OC interactions with minerals. Furthermore, Fe and Al oxides were the main minerals that stabilized OC compounds at each slope, while Ca played a vital role in OC interactions in the depositional sites. The Fe- and Al-bound OC (Fe/Al-OC) showed higher aromaticity, humification, and molecular weight than Ca-OC and bulk soil dissolved OC, which remained unaffected by erosion and deposition. These OC interactions with Fe/Al increased the chemical structural stability of SOC. Furthermore, the depositional sites showed the highest MAOC contents, MAOC loading, BET-C constant, adsorption capacity, and the lowest SSA covered by SOC (%SSASOC-covered). These results indicate that depositional sites have the highest capacities for OC sequestration. Based on these results, the deposition or burying of eroding OC plays a key role in stabilizing soil OC.
... Lateral loss of SOC is a result of changes in several environmental factors, including climate, geomorphology, soil, and land use, which result in the lateral movement of carbon [14,15]. For example, it was reported that lateral movement of organic carbon from land to water in northern Europe increased annually by 1.4% 1990 to 2013 [16]. ...
... Remote Sens. 2023,15, 1775 ...
Erosion-induced soil organic carbon (SOC) loss substantially affects the redistribution of global organic carbon. The Chinese Loess Plateau, the most severely eroded region on Earth, has experienced notable soil erosion mitigation over the last few decades, making it a hotspot for soil erosion studies. However, the overall rate of SOC loss and spatiotemporal evolution under changing environments remain unclear. In this study, we investigated SOC loss from 1982 to 2015 in the severely eroded Hetong region of the Chinese Loess Plateau by combining the Revised Universal Soil Loss Equation (RUSLE) model and the localized enrichment ratio function derived from field observations and attributed the changes in SOC loss to climate- and human-induced vegetation changes. The results showed that SOC loss in the Hetong region was 64.73 t·km−2·yr−1, 16.79 times higher than the global average. Over the past 34 years, SOC loss decreased by 23.84%, with a total reduction of more than 105.64 Tg C since the change-point year. Moreover, our study found that vegetation changes dominated the changes in SOC loss in the Hetong region, contributing 89.67% of the total reduction in SOC loss in the Hetong region. This study can inform carbon accounting and sustainable catchment management in regions that have experienced large-scale ecological restoration.
... However, this view has shifted over the past decades to one where context dependent physical or chemical (in)accessibility of OM for decomposers is seen as key regulating mechanism of decomposition (Schmidt et al. 2011;Lehmann and Kleber 2015). Specifically, the susceptibility of OM to degradation is thought to be regulated by a combination of the following prerequisites: the accessibility of the OM, the presence of decomposers and suitable abiotic conditions such as pH, temperature and moisture (Doetterl et al. 2016). The same conditions apply for an efficient composting process. ...
Composting is recognized as a sustainable waste management strategy. However, little is known about green waste, and specifically rose waste, degradation patterns during composting. This study aimed (1) to gain insight in the underlying decomposition patterns during rose waste composting and (2) to identify co-metabolisms of ligneous material. Five different compost mixtures were tested ranging from pure rose waste to mixtures with tomato waste, kalanchoe waste or mature compost added. Samples were taken during a six-month experiment and analyzed by pyrolysis-GC/MS. The temporal trends in the relative abundance of 10 different compound groups were measured. Lignin and aliphatic compounds together accounted for ≥ 50% of the quantified pyrolysis products, but with changing contributions during composting. The relative abundance of polysaccharides and terpenes strongly decreased with more than 60% in the first 2 months. The simultaneous decrease in relative abundance of lignin and polysaccharides during initial composting phase indicated co-metabolism of lignin. The results from this study showed that while the presence of lignin is commonly regarded as a challenge in composting, it actually undergoes degradation through distinct mechanisms at the various composting stages.
... The manipulative field experiment was originally established to study the feedbacks of a dynamic disequilibrium in the carbon cycle of arable lands. Deep tillage or soil erosion lead to an admixture of subsoil material into the plow layer (Doetterl et al., 2016), which alters topsoil properties (such as SOC or clay content). The resulting changes in the main rooting zone might reduce crop growth (Öttl et al., 2021). ...
In the light of the ongoing global climate crisis and the related increases in extreme hydrological events, it is crucial to assess ecosystem resilience and – in agricultural systems – to ensure sustainable management and food security. For this purpose, a comprehensive understanding of ecosystem water cycle budgets and spatiotemporal dynamics is indispensable. Evapotranspiration (ET) plays a pivotal role in returning up to 90 % of incoming precipitation back to the atmosphere. Here, we studied the impacts of soil types and management on an agroecosystem's seasonal cumulative ET (ETsum) and agronomic water use efficiency (WUEagro, the dry matter per unit of water used by the crop). To do so, a plot experiment with winter rye (17 September 2020 to 30 June 2021) was conducted in an eroded cropland which is located in the hilly and dry ground moraine landscape of the Uckermark region in northeastern Germany. Along the experimental plot (110 m × 16 m), two closed chambers were mounted on a robotic gantry crane system (FluxCrane as part of the AgroFlux platform) and used to determine ET. Three soil types representing the full soil erosion gradient related to the hummocky ground moraine landscape (extremely eroded: Calcaric Regosol; strongly eroded: Nudiargic Luvisol; non-eroded: Calcic Luvisol) and additional topsoil dilution (topsoil removal and subsoil admixture) were investigated (randomized block design, three replicates per treatment). Five different modeling approaches were used and compared in the light of their potential for reliable ETsum over the entire crop cultivation period and to reproduce short-term (e.g., diurnal) water flux dynamics. While machine-learning approaches such as support vector machines (SVMs) and artificial neural networks (with Bayesian regularization; ANN_BR) generally performed well during calibration, SVMs also provided a satisfactory prediction of measured ET during validation (k-fold cross-validation, k=5). We found significant differences in dry biomass (DM) and small trends in ETsum between soil types, resulting in different WUEagro. The extremely eroded Calcaric Regosol showed an up to 46 % lower ETsum and up to 54 % lower WUEagro compared to the non-eroded Calcic Luvisol. The key period contributing to 70 % of ETsum spanned the beginning of stem elongation in April to the harvest in June. However, differences in the ETsum between soil types and topsoil dilution resulted predominantly from small differences between the treatments throughout the cultivation rather than only during this short period of time.
... They include regular tillage that adversely affects soil microorganisms by destroying microhabitats and fungal hyphal networks 45 . Tillage also leads to soil oxygenation and reduces soil organic carbon by removing root organic releases and by increasing susceptibility to erosion 46,47 . In addition, considerably more fertilizers are used on arable land with annual crops compared to grasslands 48 . ...
Soils provide essential ecosystem functions that are threatened by climate change and intensified land use. We explore how climate and land use impact multiple soil function simultaneously, employing two datasets: (1) observational – 456 samples from the European Land Use/Land Cover Area Frame Survey; and (2) experimental – 80 samples from Germany’s Global Change Experimental Facility. We aim to investigate whether manipulative field experiment results align with observable climate, land use, and soil multifunctionality trends across Europe, measuring seven ecosystem functions to calculate soil multifunctionality. The observational data showed Europe-wide declines in soil multifunctionality under rising temperatures and dry conditions, worsened by cropland management. Our experimental data confirmed these relationships, suggesting that changes in climate will reduce soil multifunctionality across croplands and grasslands. Land use changes from grasslands to croplands threaten the integrity of soil systems, and enhancing soil multifunctionality in arable systems is key to maintain multifunctionality in a changing climate.
... Mostly dissolved OM and finer-textured (<63 µm) particles were transferred with the runoff that was caused by smaller rainfall intensities of about < 7.5 mm h − 1 (e.g., Hjulström, 1935;Doetterl et al., 2016). The particle size-dependent and density-dependent segregation during deposition of solid and liquid runoff fractions was found to be reflected in the soil landscape by the spatially-distributed patterns of sedimentation (e.g., Deumlich et al., 2010;Sommer et al., 2008). ...
Heavy rain events can cause soil material to be widely transported by surface runoff in arable soil landscapes. While soils of the eroded hilltop and backslope areas are affected by a loss of material, soil properties in deposition areas are affected by sedimentation of particulate and dissolved mineral and organic substances. During the event, the composition of mineral and organic matter (OM) fractions changes dynamically with runoff velocity. However, changes in OM composition during runoff have mostly been neglected. The objective was to identify changes of OM composition of runoff fractions with time and flow velocity during a single rainfall-runoff event. By the comparison of runoff from a conventionally-tilled (CT) plot and a no-till (NT) plot, we aimed at discussing process-based explanations for dynamic OM composition in runoff. Surface runoff was collected from CT and NT plots at the toe-slope of an experimental hillslope. Time-dependent runoff samples were compared with cumulative bulk samples obtained by Coshocton wheels and separated in a coarser sediment (sed>63 µm) and a finer fraction of < 63 µm-sized particles plus dissolved OM (<63 µm&diss). The OM composition was determined with Fourier transform infrared (FTIR) spectroscopy. It was found to change with runoff volume due to contact with litter (NT) or soil minerals (CT) mainly for finer fractions from both CT and NT plots. Even during a single runoff event, the OM mass and composition in runoff changes characteristically. The OM composition dynamics was dominated by OM cat /C-O-C ratios in runoff from CT and by C-H/C-O ratios in runoff from NT due to contact with soil minerals (CT) and litter (NT). The time-dependent collection schemes suggested that OM composition in runoff reflected the surface properties and could help to further improve explanations for spatial distribution of OM-related soil properties such as the potential wettability in arable soil landscapes.
... Several hillslope and catchment-scale studies have suggested a link between SOC and sediment transport (Gregorich et al. 1998;Starr et al. 2000;VandenBygaart 2001;Ritchie and McCarty 2003;Page et al. 2004;Van Oost et al. 2005;Li et al. 2006Li et al. , 2007Yoo et al. 2006;Huang et al. 2007;Mabit et al. 2008;Kuhn et al. 2009;Quinton et al. 2010;Doetterl et al. 2012Doetterl et al. , 2015Doetterl et al. , 2016Fissore et al. 2017). The above studies suggest that (1) 137 Cs and SOC are moving along the same physical pathways, (2) 137 Cs and SOC are moving by the same physical mechanisms and (3) 137 Cs can be used to predict the spatial distribution of SOC. ...
Context Quantifying soil organic carbon (SOC) depth distribution and its vertical transport is needed for both improved understanding of soil properties and behaviour as well as enhanced organic carbon sequestration. This is a global issue, that if better understood, could result in both more agriculturally productive soils as well as enhanced environmental outcomes. Aims Quantify whole soil-profile SOC and down-profile movement at a series of sites in south-east Australia. Methods Soil is sampled at regular intervals using cores and assessed for SOC and environmental tracer (137Cs) concentration. Key results Soils that have a high clay content (Vertosols) and crack (i.e. self mulching) have the highest SOC content. In high clay content soils, 137Cs is present at depths well below that at which it would be present by diffusive processes. Conclusions Surface soil, labelled with 137Cs is moving down the soil profile by advective processes to depths well below that possible by diffusive processes alone. Using local erosion rates and carbon input, it is estimated that less than 1% of SOC is delivered to the cracking soils by erosional processes and that the majority of SOC must be produced in situ. Implications Given that 137Cs is a relatively new environmental tracer (1945 onwards), this suggests that surface labelled soil is reaching depths of up to 80 cm at decadal time scales. The methods and findings here have global applicability and provide insights into potential enhancement of carbon sequestration in both cropping and grazing landscapes.
... The timescale of evaluation matters, as some authors have argued that after erosion of topsoil, new C stocks may form that, in combination with the persistence of SOC from eroded soil trapped in riparian sediments, can lead to a net increase over time (118)(119)(120)(121). Yet, for the policy-relevant timeframe of current land use, erosion is likely to be a net loss factor for SOC and its control is relevant, even though there may be "positive leakage" and net effects at the landscape scale may be smaller than what plot-level measurements suggest (122). ...
The few percent of soil organic carbon (SOC) among mineral components form the interface of climate, plant growth, soil biological processes, physical transport infrastructure, and chemical transformations. We explore maps, models, myths, motivation, means of implementation, and modalities for transformation. Theories of place relate geographic variation in SOC to climate, soil types, land cover, and profile depth. Process-level theories of biophysical change and socioeconomic theories of induced change explain SOC transitions that follow from land use change when a declining curve is bent and recovery toward SOC saturation starts. While the desirability of recovering from SOC deficits has been mainstreamed into climate policy, the effectiveness of proposed measures taken remains contested. Process-level requirements for transitions at plot and landscape scales remain uncertain. Expectations of policy-induced SOC transitions have to align with national cross-sectoral C accounting and be managed realistically with land users (farmers) and commodity supply chains (private sector, consumers).
Expected final online publication date for the Annual Review of Environment and Resources, Volume 48 is October 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
... Even if we can try to go back as far as Paleolithic times to evaluate the amount of ground moved intentionally and unintentionally by humans (e.g., Hooke, 2000), it is considered that the development of agriculture, at the onset of the Bronze Age, as well as growing industrialization and urbanization since the 19th century are responsible for global environmental changes inducing a dramatic increase of continental surface erosion (Ruddiman, 2003(Ruddiman, , 2007. Based on global RUSLE models, Borrelli et al. (2017) estimated that the worldwide potential soil erosion reached 35-40 Gt in 2012, in agreement with other estimates (Doetterl et al., 2016;Pimentel & Burgess, 2013;Van Oost et al., 2007). They also observed an overall increase of 2.5% during the first decade Reuter et al., 2007), showing the regional distribution of the sampled watersheds (open circles and black polygons). ...
Land degradation resulting from increased continental surface erosion is a worldwide and systemic phenomenon due to unsustainable human activities. Already fragile, the intertropical zone is likely to be further affected by climate change like increased aridity, an aggravating factor of soil erosion and land degradation. A major challenge is thus to provide the necessary knowledge to not only deepen our understanding of the Earth's system and its critical thresholds but also to help achieving sustainability. Understanding the factors that control the properties and processes of the critical zone, and especially what will be its responses to ongoing climate and land use changes, requires multidisciplinary efforts to tackle time scales that are compatible with morphogenesis and soil development as well as environmental disturbances of anthropogenic origin. Due to its prominent ecological importance, the Brazilian Cerrado biome is an ideal natural laboratory were to gauge the consequences of recent and intense agricultural activities on continental surface erosion. We focused on the region of Brasília where our approach allows confronting the temporal scales of long‐lived and stable cosmogenic nuclides with that of short‐lived radioactive isotopes, through a comparison of natural and anthropogenically disturbed land surfaces. Our results indicate that long‐term, background denudation rates are lower than 10 mm Kyr⁻¹ whereas recent erosion rates due to human activities may reach rates at least 160 times higher, exceeding by far the sustainability of the soil resource.
... Wind action first removes the near-surface soil fine particles, leaving gravel and other larger size materials. In severely disturbed grazing areas, the surface is coarse-grained, and wind-eroded soil particles transported from these locations have significantly less fine sand content, whereas soil TC and TN content is significantly positively correlated with soil fine particle content (Doetterl et al., 2016). Hence, wind erosion increased along the stocking rate gradient, but its C and N content decreased with a significant increase in C and N fluxes carried by wind-eroded soil particles. ...
A high stocking rate can intensify wind erosion in grasslands, and strong wind can carry away soil surface particles, soil carbon (C), and nitrogen (N), which leads to soil barrenness. In this research, the dust flux, total carbon (TC) and total nitrogen (TN) contents, and fluxes of aeolian sediment were determined in the nongrowing season (mid‐October to mid‐April of the following year) and growing season (mid‐April to mid‐October) from 2017 to 2020 at a continuous grazing gradient experiment platform established in 2004 in a desert steppe in Inner Mongolia, China. The results were as follows: (1) As the stocking rate increased, fluxes of aeolian sediment at 10 cm (H10), 30 cm (H30), and 100 cm (H100) were greatly increased ( p < 0.05). Aeolian sediment fluxes followed the order control (CK) < light stocking rate (LG) < moderate stocking rate (MG) < heavy stocking rate (HG). (2) The TC and TN contents of aeolian sediment decreased with an increase in stocking rate, and higher grazing intensity resulted in a difference in TC content, which decreased between the two seasons. The TC flux/aeolian sediment flux increased with an increase in stocking rate, and the ratio of TC flux increased between two seasons due to the influence of heavy grazing. Grazing disturbance had little effect on TN flux. As grazing intensity increased, wind erosion resulted in an increase in soil available carbon loss, and this loss was more severe in the nongrowing season. (3) The C/N ratio increased with increase in aeolian sediment flux. The aeolian sediment flux and C/N showed an inverse relationship with the increase in grazing intensity. Regression lines for the two seasons showed an intersecting trend, and the intersection points moved backward as height increased, indicating that higher stocking rates could reduce the influence of grassland vegetation on C/N. As height increased, the regression intercept increased, indicating that the loss of wind‐eroded soil changed from a low C/N loss to a high C/N loss. In general, high stocking rates aggravated soil wind erosion, resulting in more C pool loss than N loss in terms of soil elements and more available C loss in the nongrowing season. In the nongrowing season, vegetation characteristics are poor, coupled with grazing interference, and wind erosion in the nongrowing season is serious in general; however, the change law of C and N in wind‐eroded soil is directly related to its particle size. C and N content in fine particles is higher, and C content in soil is much higher than N, so the TC loss is relatively large; therefore, reducing the intensity of grazing can reduce the occurrence of soil wind erosion, and implementing measures to protect grassland in spring and other periods vulnerable to wind erosion would be conducive to the sustainable usage of grassland resources.
... The flow of soil soluble carbon through rivers into aquatic ecosystems is thought to be a crucial link in the global carbon cycle (Harrison et al., 2005;KINDLER et al., 2011). Surface flow, interflow, and soil erosion, which commonly occur during natural rainfall events, are three major routes for the lateral transport of soil organic carbon (SOC) from soils to aquatic ecosystems (Doetterl et al., 2016;Hua et al., 2016). The annual cumulative carbon loss caused by interflow and surface runoff is up to 909.20 mg C⋅m − 2 , accounting for 29% of the total annual organic carbon loss (Keke and Bo, 2018). ...
Soil carbon in karst critical zones is an essential part of the global carbon pool, and the migration of dissolved carbon is a vital component of the carbon cycle in such regions. The unique “dual hydrological structure” makes the path and process of migration of soil dissolved carbon driven by rainfall unclear in karst slopes, which ultimately hinders the accurate evaluation of the carbon sink effect in such regions. Therefore, based on field monitoring of natural rainfall, this study evaluated the process and characteristics of the spatial migration of soil dissolved carbon in bare karst slopes and its response to natural rainfall (rainfall duration and amount). The results showed that the concentration of dissolved carbon transported in surface runoff was significantly lower than that transported within the soil layer; however, carbon loss displayed the opposite trend. Rainfall factors (rainfall amount and duration) and runoff volume significantly affected soil dissolved carbon, contributing 46.91% to soil dissolved carbon. Rainfall factors significantly affect soil dissolved carbon migration by influencing runoff. In addition, the hydrological path is an important factor affecting soil dissolved carbon transport. Our study shows that the soil and bedrock interface (SBI) and shallow karst fissures (SKF) are the main routes of dissolved carbon migration in soils in bare karst slopes. Dissolved carbon migrating along SKF is a vital contributor to the “missing carbon sink” in karst regions, and dissolved carbon migrating along the SBI is potentially an important contributor to the “missing carbon sink.” This study provides primary data and is expected to serve as a reference for revealing the mechanism of karst carbon migration and transformation and accurately evaluating the carbon sink effect in karst areas.
... Briefly, 20 g of dried soil were treated by 12 M hydrochloric acid to remove inorganic C from the soils (Harris et al., 2001), mixed with 200 ml of distilled water for 15 h to break up nonwater stable aggregates and then wet-sieved through a 250 µm sieveshaker using a microaggregate isolator. Thereafter, the residual part of the soil (<250 μm) was placed on a 53 µm sieve and sieved to isolate the 250 -53 μm and < 53 μm fractions (Doetterl et al., 2016). Finally, the residue part of the soils was analysed for OC (Vario EL III, CHNS Elementar, Germany). ...
... It is well documented that agricultural practices have led to significant losses of C (Guo and Gifford, 2002;Deng et al., 2016;Tang et al., 2019), an observation that has instigated many other studies looking into the effects of different land managements on the addition, distribution, and decomposition of C (Six et al., 1999;Post and Kwon, 2000;Luo et al., 2010;Deng et al., 2016). Conventional agriculture is responsible for larger rates of erosion than those of soil production (Montgomery, 2007), and the disruption of surficial soils not only redistributes C across the landscape but also leads to soil structural changes that can promote SOC mineralization to CO 2 (Lal, 2003;Quinton et al., 2010;Doetterl et al., 2016). Changes in SOC at surface will thus affect the availability of C to be transported to deep soils, with consequences for C stabilization and its overall depth distribution. ...
The depth distribution of soil organic carbon (SOC) is governed by the interaction of many ecosystem features, including differential C inputs in shallow and deep soils and the redistribution of C via water flow through the profile. In C-rich Mollisols in particular, we need to better understand the degree to which the conversion of native prairie to cultivated lands is changing C loss and retention. We probed multiple mechanisms driving these processes using two approaches: one leverages a regional-scale dataset derived from the Natural Resources Conservation Service (USDA-NRCS) National Cooperative Soil Survey (NCSS) Characterization Database; and a second focusses on a local-scale, more detailed dataset representative of the climatic and land-use gradients invoked in the larger database. The first approach focused on parameterizing SOC depth distributions of Mollisols across a climatic gradient in the US Midwest to investigate how land use and effective precipitation affects vertical gradients of SOC. The second approach furthered the investigation of SOC depth distribution drivers by quantifying biological, physical, and chemical properties of multiple soil profiles across Kansas, US. SOC declined more gradually with depth as water availability increased in native prairie soils, prompting the hypothesis that increased water flow through the profile carries C to deep layers, particularly where high root abundances promote soil porosity. Analyses of multiple soil profiles indicate that surficial changes driven by land conversion propagate their influence to deep soil horizons in ways significant for the coupling of C cycling across depths. Our findings support the hypothesis, and specifically suggest linkages between decreased root abundances and increased flows of soluble C downward under agriculture, and associated changes in soil structure that affect the propensity of SOC to form aggregates. The interplay between rooting depth abundances and water availability in different land uses thus appears to influence the arrangement of soils particles and voids in ways important for vertical water flow and C transport. Our work illuminates the convergence of multiple important mechanisms driving changes in the shape of SOC depth distributions across timescales shorter than typically assumed, with consequences for projecting soil C cycling and storage in the Anthropocene.
... In addition to the chemical and molecular composition of soil organic matter (SOM) influencing SOC longevity itself (Kögel-Knabner et al., 2008;Lavallee et al., 2020;Six et al., 2000), soil texture, soil chemistry, thickness, aggregate abundance, and mineralogy set the accommodation space and protection mechanisms necessary for SOC storage (Kramer and Chadwick, 2016;Lehmann et al., 2020;Masiello et al., 2004;Slessarev et al., 2022). In general, these soil properties depend on the rates of fresh mineral supply (soil production), weathering, and erosion (Doetterl et al., 2016;Mudd and Yoo, 2010). Over millennia, these processes control soil properties required for significant SOC storage suggesting that analysis and quantification of soil development can advance our understanding of the persistence and preservation of SOC (Lehmann et al., 2020). ...
Characterizing the distribution and dynamics of organic carbon in soil is critical for quantifying changes in the global carbon cycle. In particular, weathering controls on near-surface and deep (>1 m) soil organic carbon (SOC) dynamics have been proposed but limited data prevents us from predicting SOC over topographically complex landscapes and quantifying how changes in climate and perturbations, such as wildfire or land management, influence SOC stocks. To advance our understanding of how weathering alters soil geochemistry and influences SOC storage, we synthesize previous data with a new analysis of the Siuslaw River soil chronosequence from terraces in the Oregon Coast Range, a region that harbors the richest SOC inventories in the continental US. We analyze how the relationships between soil geochemistry, physical properties, and SOC storage vary with weathering status and pathways across soils that span 0.041 to 990 kyr and vary in depth from 1 m to >10 m. To distinguish the key properties and processes influencing SOC storage at different depths, we break our analysis into three depth intervals: 0–30, 30–100, and >100 cm. Our results suggest that the processes that control SOC stocks vary systematically with time and depth owing to weathering impacts on soil properties and pedogenic development. At 30 kyr we observe a peak in SOC stock in the top 100 cm coincident with a peak in oxalate extractable Al and Fe concentrations, representing secondary poorly crystalline minerals, which is consistent with previous studies. We also observe a decline in shallow SOC stock for >30 kyr soils as poorly crystalline minerals are replaced by more stable crystalline forms and soils become clay dominated. At 120 kyr, SOC below 100 cm starts to contribute significantly to the total SOC profile inventory and by 990 kyr, this fraction composes >40% of the total SOC stock. Taken together, our results indicate that total SOC stock increases with soil age as the increased intensity of bedrock weathering deepens the critical zone, creating accommodation space for deep SOC storage. These findings reveal the intimate link between poorly crystalline minerals and SOC and suggest that systematic analysis of soil development in the critical zone provides a first-order constraint on SOC stocks.
... Sediment deposition in reservoirs, lakes, and rives well documented historical hydrological events (Mills et al., 2017;Radbourne et al., 2017), and hydrological components (e.g., rainfall, runoff, and sediment) can be interpreted based on the sediment depositions at annual or event scales, providing a powerful tool for inferring historical hydrological processes in ungagged headwater regions (Nafchi et al., 2021;Wang et al., 2019b;Wei et al., 2017b;Xiao et al., 2023). Meanwhile, physically-based hydrological models have been widely used for simulating hydrological processes for a continuous time period, but data shortage limited parameter estimation for models, especially for headwater regions with few observations (Doetterl et al., 2016). Therefore, the integration of sediment deposition records and hydrological models can be a promising method for investigating lateral carbon movement from land to inland waters, because hydrological models are good at continuously simulating spatiotemporal carbon loss, and sediment deposition can provide necessary support for model calibration and validation. ...
... The location and movement of SOC has been shown to be related to soil erosion and deposition. Therefore, quantifying SOC soil redistribution requires an understanding of erosion and deposition processes at multiple spatial and temporal scales (Moore et al. 1993;Knighton 1998;Lal 2001Lal , 2003Lal , 2004Berhe et al. 2007;Hancock et al. 2010;Quinton et al. 2010;Ruiz Sinoga et al. 2012;Gaspar and Navas 2013;Berhe et al. 2014;Kirkels et al. 2014;Murphy 2015;Doetterl et al. 2016;Hoyle et al. 2016). ...
Context It is well recognised that soil organic carbon (SOC) can be transported and deposited along the same pathways as those of soil erosion and deposition. Aims To examine the viability of environmental tracers 137Cs and unsupported 210Pb (210Pbex) as tools to inform soil erosion and deposition patterns as well as that of the distribution of SOC. Methods Multiple soil cores were collected along two transects of similar length and aspect in a steep-slope soil mantled environment in south-east Australia. Key results Average SOC concentration was high for both transects (~6% and 4%). SOC decreased moving downslope suggesting loss of SOC by erosion. There were strong and significant positive relationships of SOC with 137Cs and 210Pbex (both r > 0.77, P < 0.0001). At this site, SOC concentration appears related to erosion and deposition patterns. Conclusion The hillslope distribution of 137Cs and 210Pbex were very similar, indicating that both tracers were viable in this environment (r = 0.9, P < 0.0001). The different origins and half-lives of 137Cs and 210Pbex also demonstrate that the patterns of erosion and deposition are consistent at decadal time scales. Implications The use of 210Pbex provides an alternative method for understanding erosion and deposition patterns as well as that of SOC, given that the viability of 137Cs (half-life of 30.1 years) is now questionable due to no new replenishment.
... With the rapid growth of population, food demand is increasing day-byday [2,3]. Conserving soil by controlling erosion is crucially important [4]. Therefore, understanding the laws and influencing factors of N loss on the soil of sloping farmland is highly significant for controlling soil and water loss as well preventing agricultural non-point source pollution. ...
Surface flow (SF) and subsurface flow (SSF) are important hydrological processes occurring on slopes, and are driven by two main factors: rainfall intensity and slope gradient. To explore nitrogen (N) migration and loss from sloping farmland in the Miyun Reservoir, the characteristics of total nitrogen (TN) migration and loss via SF and SSF under different rainfall intensities (30, 40, 50, 60, 70, and 80 mm/h) and slope gradients (5°, 10°, and 15°) were studied using indoor stimulated rainfall tests and mathematical models. Nitrogen loss via SF and SSF was found to increase exponentially and linearly with time, respectively, with SSF showing 14–78 times higher loss than SF. Under different rainfall intensities, SSF generally had larger TN loss loading than SF, thereby indicating that SSF was the main route for TN loss. However, the TN loss loading proportion via SF increasing from 14.03% to 35.82% with increasing rainfall intensity is noteworthy. Furthermore, compared with the measurement data, the precision evaluation index Nash-Suttcliffe efficient (NSE) and the determination coefficient (R2) of the effective mixing depth model in the numerical simulation of TN loss through SF in the sloping farmland in the Miyun Reservoir were 0.74 and 0.831, respectively, whereas those of the convection-dispersion equation for SSF were 0.81 and 0.811, respectively, thus indicating good simulation results. Therefore, this paper provides a reference for studying the mechanism of N migration and loss in sloping farmland in the Miyun Reservoir.
... Furthermore, SOC in the smaller fractions tend to be more stable and presumably harder for microbes to decompose (Table 2, Fig. 1). Because smaller soil fractions are likely to experience longer transport distances during erosion processes (Figs. 1 and 2; Starr et al. 2000), the more stable SOC in the EQS < 63 µm fractions tends to be more persistent during transport processes and may also contribute to stabilizing SOC in depositional sites or water body enriched with finesized fractions (Berhe and Kleber 2013;Doetterl et al. 2016;Polyakov and Lal 2008). Hence, our findings partly support the results observed from field plot experiments in Luvisols and Alfisols, where stable SOC is more preferentially mobilized by interrill erosion than labile SOC (Nyawade et al. 2018;Wang et al. 2013). ...
Purpose
Rivers transfer eroded sediment with soil organic carbon (SOC) from terrestrial land to ocean basins, playing a key role in the global carbon cycle. During sediment delivery, the coarse particles can be preferentially settled and the fine particles may be transported to downstream depositional sinks. However, the size-specific quantification of soil microbial communities and SOC composition and their roles in SOC stability have not been fully elucidated.
Materials and methods
In this study, two soils with similar textures but different aggregate structures were fractionated using a settling tube apparatus into three size classes according to their settling velocities. The individual soil particle-size aggregates and unfractionated soil were analyzed to determine (1) the diversity and composition of the soil microbial communities via 16S rRNA and ITS1, and (2) the SOC composition using mid-infrared diffuse reflectance spectroscopy.
Results
The results showed that as the soil aggregate size decreased, the ratio of alkyl groups to carboxyl groups (C−H/COO, C−H/C=O) and soil CO2 emission per gram of SOC significantly decreased; however, the richness and diversity of the soil bacterial and fungal communities increased. Meanwhile, the aggregates of smaller size had lower relative abundances of Proteobacteria and Basidiomycota but higher relative abundances of Actinobacteria and Ascomycota. Rhizobiales was an indicator taxon for bigger soil aggregate communities, while Deinococcales, Capnodiales, and Venturiales were indicator taxa for communities with smaller soil aggregates. SOC composition showed a stronger direct effect on SOC stability in bigger soil aggregates (path coefficient, 0.68). However, SOC composition exerted a greater indirect impact on SOC stability by regulating soil bacterial and fungal community composition in smaller size aggregates (path coefficient, 0.65).
Conclusions
Overall, our study provides a fundamental understanding of the chemical and microbial control of SOC stability at the aggregate level and highlights the potential impacts of aggregate-scale SOC decomposition on the fate of eroded SOC.
... The soil DOC leaching further influences riverine DOC and thus greenhouse gas outgassing Tang et al., 2023;Zhang et al., 2023). Although DOC represents only a very small part of the global carbon budget (Doetterl et al., 2016;Mchunu et al., 2011), its dynamic change is a key process that intensively impacts the global carbon cycle (Lal, 2003;Li et al., 2020). Generally, the quantity and quality of DOC jointly determine the abovementioned DOC biogeochemical processes (Hosen et al., 2014;Mao et al., 2017;Williams et al., 2016). ...
Underscoring how crop mulching changes the dissolved organic carbon (DOC) in runoff is essential to understanding the sustainable application of cornstalk mulching. This study examined the runoff DOC quantity and quality in plots with and without cornstalk mulching under various rainfall intensities in a representative steep fallow land of southwestern China. Field rainfall simulations within a 60 min duration were conducted on the triplet plots (1.5 m  1 m for a single plot) with bare soils under 30-and 90-mm h À1 intensities and with cornstalk mulching (air-dried whole plants with 60% coverage and weight 1.65 kg per plot) under 90 mm h À1 intensity, respectively. The concentration and loss of runoff DOC and its quality variables, including SUVA 254 (aromaticity of DOC), C:C (proportion of colored humic substances in DOC), and E4:E6 (fulvic acid to humic acid in DOC), are determined. Results showed that runoff DOC concentration and loss were both significantly higher at 90 mm h À1 intensity than at 30 mm h À1 intensity (p < 0.05), simultaneously, they were 52.6% and 53.2% higher in mulched plots than in bare plots (p < 0.05), respectively. Runoff SUVA 254 and C:C in bare plots were 90% and 88.9% higher in 30 mm h À1 intensity than in 90 mm h À1 intensity (p < 0.05), respectively. Whereas, E4:E6 of runoff was 12.3% higher in mulched plots than that in bare plots (p < 0.05). Our findings imply that cornstalk mulching enhances the export of runoff DOC with a decline in aromaticity and the molecular weight of humic substances.
... SOC stability, which is defined as SOC resistance to loss (Doetterl et al. 2016), is controlled by a combination of soil and environmental conditions (Bai et al. 2020), including biochemical stability (Baldock et al. 1992;Plaza et al. 2013), physical protection (Lavallee et al. 2020), chemisorption (Wang et al. 2021) and microorganisms (Kravchenko et al. 2019). SOC stability is the consequence of multiple mechanisms, among which physical protection from decomposition is considered of primary importance; the study of SOC pools yielded by physical fractionation has therefore surged as popular indicator for overall SOC stability (Sokol et al. 2022;Xue et al. 2018). ...
Aims
As the largest terrestrial carbon (C) pool, soil organic carbon (SOC) plays a critical role in the global C cycle. Particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) are currently popular methods for the fractionation and analysis of SOC. Nitrogen (N) addition affects SOC dynamics. However, the characteristics of POC and MAOC and their potential drivers under N (inorganic and organic N) addition remain unclear.
Methods
We conducted a meta-analysis based on data from 50 studies on terrestrial ecosystems in China to investigate the responses of SOC, POC, and MAOC to N addition, and to reveal the factors influencing POC and MAOC.
Results
The results showed that organic N addition significantly increased the SOC (effect size: 0.35), POC (effect size: 0.68), and MAOC (effect size: 0.20), whereas inorganic N addition significantly increased SOC (effect size: 0.10) and POC (effect size: 0.21). POC and MAOC concentrations increased with fertilization duration (years); however, the physical stability of SOC remained unchanged. No correlation was observed between the SOC sequestration rate and duration of fertilization under inorganic N addition, whereas the SOC sequestration rate showed a trend of first increasing and then decreasing with organic N addition.
Conclusions
The main factors affecting POC and MAOC were microbial biomass carbon (MBC) and soil pH, and the driving factors of POC and MAOC were different under inorganic and organic N additions. Additionally, N addition may decrease the stability of the SOC pool.
... The latest compilation indicated that sediment and OC burial in terrestrial depositional environments are predicted to increase due to accelerated erosion and reduced sediment export fluxes caused by climate change and human activities 6 . Understanding the carbon fate in depositional areas is an important prerequisite for shedding light on the debate on whether the erosion-transport-deposition process is a net atmospheric C source or sink [7][8][9] , and is critical to refining our knowledge of the carbon cycle 10 . ...
Understanding the processes governing lateral terrestrial organic carbon transfer is confounded by the fact that organic carbon deposits on land have not yet been fully explored. Despite recent advances in understanding organic carbon deposition in aquatic ecosystems, the burial of organic carbon in dry depositional environments remains unclear. Here, combining large-scale field surveys and remote sensing techniques, we provide a robust estimate for sediment retention and organic carbon burial of check dams on the Chinese Loess Plateau. We find that the 50,226 active check dams have intercepted 10.2 ± 0.6 Pg eroded sediment during 1970-2020, which equals to 46% of the sediment load of Yellow River. Based on 86 deep sediment cores, we estimate that 21.6 ± 9.9 Tg of organic carbon was buried over the past 50 years by check dams with a burial rate of 468 ± 204 g C m−2 yr−1, approximately one order of magnitude higher than that of global lakes/reservoirs. We also find that the organic carbon burial efficiency of check dams (~80%) is significantly higher than in other depositional environments. We argue that organic carbon burial by check dams represents a significant terrestrial carbon sink and must be accounted for in global carbon budget. Check dams on the Chinese Loess Plateau are estimated to trap and bury approximately 10.2 ± 0.6 Pg of eroded sediment and 21.6 ± 9.9 Tg of organic carbon over the past 50 years, according to large-scale field surveys and remote sensing.
Rationale and scope: Although soil erosion was recognised as a serious problem in antiquity and research into erosion started in the early 20th century, it remains a substantial problem for agriculture and the environment across the globe. It disrupts agricultural production, threatening food production, increases the severity of floods and droughts and impacts on soil biology and biogeochemical cycling. This review describes the different processes and manifestations of erosion on arable land and the availability of global data. It points out that while there is a good understanding of the processes of erosion, the causes are complex and even if agronomic and landscape solutions are available, their implementation is challenging and needs tailored approaches to account for the specific local socio-economic, political, and institutional contexts.
Depositional profiles often feature sorted layers with stratified porosity and water retention, but the vertical partitioning of CO2 production and transfer remain unclear. In this study, fine soil and coarse sand were refilled to form three layering patterns: Layer-Mix (soil and sand well-mixed), Layer-Thin (8 thin layers interlaid), and Layer-Thick (4 thick layers interlaid). Three doses of ¹³C-labelled glucose were respectively added to the top, middle, and bottom. The results show that the Layer-Thin and Layer-Thick cumulatively released 62% and 67% less CO2 than the Layer-Mix. The ¹³C-CO2 contributed 14.1~60.3% to the total CO2 released from the Layer-Mix, but was only responsible for 7.3~48.8% of that from the Layer-Thin and 7.0~37.0% of that from the Layer-Thick. The peaks of δ¹³C-CO2 of the two interlaid columns were lowered and lagged by 1~2 days, but the δ¹³C residue remaining in the soil were on average 3~6‰ more negative than that of the Layer-Mix. The ¹³C-CO2 contributed more to the total CO2 when the glucose was added at the top, but the δ¹³C-soil was 3‰ more negative when added at the bottom. Overall, the lagged outgassing and lower share of ¹³C-CO2 from the two interlaid columns did not match with the more negative ¹³C residue remaining in the soil. Such inconsistency collectively highlights that the interlaid layers did not inhibit the decomposition of ¹³C-labelled glucose (i.e., potentially abundant CO2 produced), but the low diffusivity of the fine layers significantly impeded CO2 transfer through the heterogeneously structured soil profile.
Purpose
Mangrove ecosystems which play a critical role in global C sequestration have recently become threatened by global warming and sea level rise. This study aimed to explore the responses of SOC mineralization and C pool stability in mangrove ecosystems to warming and flooding conditions.
Methods
Laboratory incubation and solid-state ¹³C nuclear magnetic resonance (¹³C-NMR) analysis were used to evaluated soil organic carbon (SOC) mineralization and stability under warming (ST), flooding (SW), warming in coincidence with flooding (SWT), and soil under normal temperature taken from a subtropical mangrove ecosystem in southeastern China.
Results
The cumulative SOC mineralization of ST increased by 24%, while SWT and SW decreased by 59% and 68% relative to CK, respectively. Furthermore, the ¹³C-NMR analysis showed that O-alkyl C was the dominant SOC chemical composition in all treatments (37.1–43.1%), followed by the aromatic C and alkyl C. The aromaticity index (AI) decreased, and the ratio of alkyl and O-alkyl to aromatic (Alip/Arom) increased in all treatments compared with CK, while the alkyl C/O-alkyl C ratio (A/O-A) and hydrophobicity index (HI) were higher for ST and SW but lower for SWT. Warming significantly facilitated SOC mineralization, and lowered SOC stability due to the high cumulative CO2-C production, while flooding had the opposite effects. However, warming in coincidence with flooding led to low SOC decomposition and a more recalcitrant substance.
Conclusion
The theoretical elevated CO2 release from mangrove soils under warming in coastal zones might be partially offset by prolonged flooding. The different effects on the chemical fractions, chemical compositions, and stability of the SOC pool in mangrove soil under warming and flooding might further complicate the research on coastal carbon sinks.
Over 70% of soil organic carbon (SOC) is stored at a depth greater than 20 cm belowground. A portion of this deep SOC actively cycles on annual to decadal timescales and is sensitive to global change. However, deep SOC responses to global change likely differ from surface SOC responses because biotic controls on SOC cycling become weaker as mineral controls predominate with depth. Here, we synthesize the current information on deep SOC responses to the global change drivers of warming, shifting precipitation, elevated CO 2 , and land use and land cover change. Most deep SOC responses can only be hypothesized because few global change studies measure deep soils, and even fewer global change experiments manipulate deep soils. We call on scientists to incorporate deep soils into their manipulations, measurements, and models so that the response of deep SOC can be accounted for in projections of nature-based climate solutions and terrestrial feedbacks to climate change.
Expected final online publication date for the Annual Review of Ecology, Evolution, and Systematics, Volume 54 is November 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
Livestock grazing is the most extensive land use in global drylands and one of the most extensive stressors of biological soil crusts (biocrusts). Despite widespread concern about the importance of biocrusts for global carbon (C) cycling, little is known about whether and how long-term grazing alters soil organic carbon (SOC) stability and stock in the biocrust layer. To assess the responses of SOC stability and stock in the biocrust layer to grazing, from June to September 2020, we carried out a large scale field survey in the restored grasslands under long-term grazing with different grazing intensities (represented by the number of goat dung per square meter) and in the grasslands strictly excluded from grazing in four regions (Dingbian County, Shenmu City, Guyuan City and Ansai District) along precipitation gradient in the hilly Loess Plateau, China. In total, 51 representative grassland sites were identified as the study sampling sites in this study, including 11 sites in Guyuan City, 16 sites in Dingbian County, 15 sites in Shenmu City and 9 sites in Ansai District. Combined with extensive laboratory analysis and statistical analysis, at each sampling site, we obtained data on biocrust attributes (cover, community structure, biomass and thickness), soil physical-chemical properties (soil porosity and soil carbon-to-nitrogen ratio (C/N ratio)), and environmental factors (mean annual precipitation, mean annual temperature, altitude, plant cover, litter cover, soil particle-size distribution (the ratio of soil clay and silt content to sand content)), SOC stability index (SI) and SOC stock (SOCS) in the biocrust layer, to conduct this study. Our results revealed that grazing did not change total biocrust cover but markedly altered biocrust community structure by reducing plant cover, with a considerable increase in the relative cover of cyanobacteria (23.1%) while a decrease in the relative cover of mosses (42.2%). Soil porosity and soil C/N ratio in the biocrust layer under grazing decreased significantly by 4.1%–7.2% and 7.2%–13.3%, respectively, compared with those under grazing exclusion. The shifted biocrust community structure ultimately resulted in an average reduction of 15.5% in SOCS in the biocrust layer under grazing. However, compared with higher grazing (intensity of more than 10.00 goat dung/m2), light grazing (intensity of 0.00–10.00 goat dung/m2 or approximately 1.20–2.60 goat/(hm2·a)) had no adverse effect on SOCS. SOC stability in the biocrust layer remained unchanged under long-term grazing due to the offset between the positive effect of the decreased soil porosity and the negative effect of the decreased soil C/N ratio on the SOC resistance to decomposition. Mean annual precipitation and soil particle-size distribution also regulated SOC stability indirectly by influencing soil porosity through plant cover and biocrust community structure. These findings suggest that proper grazing might not increase the CO2 release potential or adversely affect SOCS in the biocrust layer. This research provides some guidance for proper grazing management in the sustainable utilization of grassland resources and C sequestration in biocrusts in the hilly regions of drylands.
Chemical structure of organic matter (OM) pools in the arable layers of agrochernozems (noneroded, eroded, and depositional has been studied by solid-state 13C-NMR spectroscopy. As has been shown, two competing processes simultaneously take place in the erosional zone: decomposition of the OM of the underlying horizon exposed due to erosion, and the stabilization of fresh OM having entered with the crop residues (OM dynamic replacement). Analytical data suggest that the OM dynamic replacement in the erosional zone efficiently compensate for the OM decomposition, as is evidenced by the highest C/N ratio of all studied OM pools in the eroded agrochernozem along with the absence of statistically significant differences in the integral characteristics of their chemical structure. However, a continuous removal of the upper soil layer from the eroded agrochernozem with each erosion event does not fully compensate for the quantitative OM losses there. The most labile OM part can be mineralized during transportation of the eroded material to the depositional zone. Accordingly, the OM entering the depositional zone is to a greater degree transformed as compared with that of the eroded agrochernozem. Nevertheless, characteristic of the depositional agrochernozem is an increased accumulation of organic carbon in the bulk soil and all examined OM pools. Correspondingly, the continuous OM inputs from the slope position subject to erosion with its subsequent burial after each consecutive erosion event, as well as the repacking/aggregation of the newly deposited OM, efficiently
contribute to the deposition of organic carbon in the depositional zone.
Soil organic carbon (SOC) strongly contributes to the operation of the global carbon cycling, and topographical factors largely influence its spatial distribution. However, SOC distribution and its leading topographical impact factors in subtropical forest ecosystems (e.g., the Zhejiang Province in China) have received relatively limited attention from researchers. In this study, 255 forest soil samples were collected from the Zhejiang Province to quantify the spatial variation in SOC and impact factors in subtropical forests. The SOC contents over soil profiles were 35.95 ± 22.58 g/kg, 20.98 ± 15.26 g/kg, and 13.77 ± 11.28 g/kg at depths of 0–10 cm, 10–30 cm, and 30–60 cm, respectively. The coefficient variations at different depths were 62.81% (0–10 cm), 72.74% (10–30 cm), and 81.92% (30–60 cm), respectively. SOC content shows a moderate intensity variation in the Zhejiang Province. The nugget coefficients of the SOC content for the three depths were 0.809 (0–10 cm), 0.846 (10–30 cm), and 0.977 (30–60 cm), respectively. Structural factors mainly influence SOC content. SOC content is positively correlated with elevation and slope, and negatively correlated with slope position (p < 0.05). However, the SOC content was negatively correlated with slope in mixed coniferous and broad-leaved forest. The distribution of the SOC content was relatively balanced between different slope positions. However, the differences became obvious when forest types were distinguished. Topographical factors affected the SOC content differently: elevation > slope > slope position. Slope becomes the main influencing factor in 30–60 cm soil. Forest type significantly influenced the SOC content but with a low statistical explanation compared to topographical factors. Topography has different effects on SOC of different forest types in subtropical forests. This reminds us that in future research, we should consider the combination of topography and forest types.
Although dissolved organic matter (DOM) has a substantial influence on biogeochemical processes relevant to erosion in agricultural ecosystems, the mechanisms underlying the dynamics and biodegradation of DOM in eroding landscapes are yet to be clarified. In this study, we examined the changes in dissolved organic carbon (DOC), profile distribution, biodegradation, and chemical characteristics associated with erosion in a Mollisol agricultural landscape. The results of this showed that DOC content in the mid-slope was significantly higher than that in the down-slope, particularly at depths of 40–100 cm. The higher humification index alongside a low biological index indicated that the DOM in the depositional position was characterized by a higher humic content compared to the up-slope positions. The up-slope samples were found to have contained greater levels of protein-like substances, whereas the mid- and down-slope soils comprised a larger proportion of anthropogenic humic-like materials, thus indicating that DOM in the up-slope soil was primarily microbial-derived. For the topsoils, the activities of β-glucosidase and cellulase were highest in the down-slope both before and after incubation, thereby suggesting that the deposition was conducive to the secretion of these enzymes by soil microbes. Overall, our observations indicated that erosion and deposition had a significant effect on the chemical composition and biodegradation pathways of DOM in soil profiles. These findings provide detailed information on the heterogeneous DOM characteristics at the molecular level, as well as DOC effects on environmental behavior in agricultural eroding environments.
Forage supply and soil organic carbon storage are two important ecosystem functions of permanent grasslands, which are determined by climatic conditions, management and functional diversity. However, functional diversity is not independent of climate and management, and it is important to understand the role of functional diversity and these dependencies for ecosystem functions of permanent grasslands. Especially since functional diversity may play a key role in mediating impacts of changing conditions. Large-scale ecosystem models are used to assess ecosystem functions within a consistent framework for multiple climate and management scenarios. However, large-scale models of permanent grasslands rarely consider functional diversity. We implemented a representation of functional diversity based on the CSR theory and the global spectrum of plant form and function into the LPJmL dynamic global vegetation model forming LPJmL-CSR. Using a Bayesian calibration method, we parameterised new plant functional types and used these to assess forage supply, soil organic carbon storage and community composition of three permanent grassland sites. These are a temperate grassland, a hot and a cold steppe for which we simulated several management scenarios with different defoliation intensities and resource limitations. LPJmL-CSR captured the grassland dynamics well under observed conditions and showed improved results for forage supply and/or SOC compared to LPJmL 5.3 at three grassland sites. Furthermore, LPJmL-CSR was able to reproduce the trade-offs associated with the global spectrum of plant form and function and similar strategies emerged independent of the site specific conditions (e.g. the C- and R-PFTs were more resource exploitative than S-PFTs). Under different resource limitations, we observed a shift of the community composition. At the hot steppe for example, irrigation led to a more balanced community composition with similar C-, S- and R-PFT shares of above-ground biomass. Our results show, that LPJmL-CSR allows for explicit analysis of the adaptation of grassland vegetation to changing conditions while explicitly considering functional diversity. The implemented mechanisms and trade-offs are universally applicable paving the way for large-scale application. Applying LPJmL-CSR for different climate change and functional diversity scenarios may generate a range of future grassland productivity.
Due to frequent human disturbance and the influence of crop growth and development, the migration of soil organic carbon (SOC) in sloping farmland is considerably different to those in other ecosystems. The impacts of maize over its entire growth period on the SOC loss in sloping farmland on purple soils under different erosion stages were investigated, in 2016. This was undertaken using rainfall simulation tests on 15° slopes with a rainfall intensity of 1.5 mm·min−1, in Sichuan Province, China. In this study, erosion development, fluctuating increasing trends in the surface runoff yield, interflow runoff yield, sediment yield, and dissolved organic carbon (DOC) migration flux were observed. Opposite trends were observed in the DOC mass concentration, total soil organic carbon (TOC) content of the sediment, the SOC content of sediment particle state, the DOC content of the sediment, and the SOC enrichment ratio. The DOC migration flux in the surface runoff and in interflow of the rill erosion stage was 1.39–2.84, 3.22–7.78 times significantly higher than that of the sheet erosion stage at each maize growth stage, respectively (p < 0.05). The average DOC mass concentration in the surface runoff, the total DOC content of the sediment, and the SOC enrichment ratio in the sheet erosion stage increased by 100.58–146.44%, 44.44–126.15%, 141.32–191.26%, respectively, compared with the rill erosion stage. Under the experimental conditions, we found that DOC loss mainly occurred at the seedling and mature stages for maize. We also found that maize growth could promote the production of soil interflow, leading to intense soil loss occurring at the subsurface. Compared with DOC mass concentrations in the surface runoff, there was an increase of 4.90–28.29% in the soil interflow, indicating that soil interflow plays a more important role in DOC loss. The growth of maize could impact formation of surface runoff and interflow, reducing the loss of SOC caused by soil erosion. This study helps to understand the carbon loss process in agricultural production in purple soil areas.
Understanding sediment sources is essential to enable more effective targeting of in-field mitigation approaches to reduce diffuse pollution from agricultural land. In this paper we report on the application of rare earth element oxides to arable soils at hillslope scale in order to determine sediment source areas and their relative importance, using a non-intrusive method of surface spraying. Runoff, sediments and rare earth elements lost from four arable hillslope lengths at a site in the UK with clay soils were monitored from three rainfall events after tracer application. Measured erosion rates were low, reflecting the typical event conditions occurring at the site, and less than 1% of the applied REO tracers were recovered, which is consistent with the results of comparable studies. Tracer recovery at the base of the hillslope was able to indicate the relative importance of different hillslope sediment source areas, which were found to be consistent between events. The principal source of eroded sediments was the upslope area, implying that the wheel tracks were principally conduits for sediment transport, and not highly active sites of erosion. Mitigation treatments for sediment losses from arable hillslopes should therefore focus on methodologies for trapping mobile sediments within wheel track areas through increasing surface roughness or reducing the connectivity of sediment transport processes.
We use a soil carbon (C) model (RothC), driven by a range of climate models for a range of climate scenarios to examine the impacts of future climate on global soil organic carbon (SOC) stocks. The results suggest an overall global increase in SOC stocks by 2100 under all scenarios, but with a different extent of increase among the climate model and emissions scenarios. The impacts of projected land use changes are also simulated, but have relatively minor impacts at the global scale. Whether soils gain or lose SOC depends upon the balance between C inputs and decomposition. Changes in net primary production (NPP) change C inputs to the soil, whilst decomposition usually increases under warmer temperatures, but can also be slowed by decreased soil moisture. Underlying the global trend of increasing SOC under future climate is a complex pattern of regional SOC change. SOC losses are projected to occur in northern latitudes where higher SOC decomposition rates due to higher temperatures are not balanced by increased NPP, whereas in tropical regions, NPP increases override losses due to higher SOC decomposition. The spatial heterogeneity in the response of SOC to changing climate shows how delicately balanced the competing gain and loss processes are, with subtle changes in temperature, moisture, soil type and land use, interacting to determine whether SOC increases or decreases in the future. Our results suggest that we should stop looking for a single answer regarding whether SOC stocks will increase or decrease under future climate, since there is no single answer. Instead, we should focus on improving our prediction of the factors that determine the size and direction of change, and the land management practices that can be implemented to protect and enhance SOC stocks.
The effect of soil erosion on global carbon cycling, especially as
a source or sink for greenhouse gases, has been the subject of intense
debate. The controversy arises mostly from the lack of information on
the fate of eroded soil organic carbon (SOC) whilst in-transit from
the site of erosion to the site of longer-term deposition. Solving
this controversy requires an improved understanding of the transport
distance of eroded SOC, which is principally related to the settling
velocity of sediment fractions that carry the eroded SOC. Although
settling velocity has already been included in some erosion models, it
is often based on mineral particle size distribution. For aggregated
soils, settling velocities are affected by their actual aggregate size
rather than by mineral particle size distribution. Aggregate stability
is, in turn, strongly influenced by SOC. In order to identify the
effect of aggregation of source soil on the transport distance of
eroded SOC, and its susceptibility to mineralization after transport
and temporary deposition, a rainfall simulation was carried out on
a silty loam. Both the eroded sediments and undisturbed soil were
fractionated into six different size classes using a settling tube
apparatus according to their settling velocities. Weight, SOC
concentration and instantaneous respiration rates were measured for
each of the six class fractions. Our results indicate that:
(1) 41% of the eroded SOC was transported with coarse aggregates
that would be likely re-deposited down eroding hillslopes, rather
than with fine particles likely transferred to water courses;
(2) erosion was prone to accelerate the mineralization of eroded SOC,
and thus might contribute more CO2 to the atmosphere than
current estimates which often ignore potential effects of aggregation;
(3) preferential deposition of SOC-rich coarse aggregates potentially
causes an increase of SOC remaining in the colluvial system and
a reduction of SOC flux to the alluvial or aquatic system. These
findings identify a potential error of overestimating net
erosion-induced carbon sink effects, and thus add an additional factor
to consider when improving our current understanding of SOC erosion
and deposition on hillslopes.
Soil carbon (C) is a critical component of Earth system models (ESMs) and its diverse representations are a major source of the large spread across models in the terrestrial C sink from the 3rd to 5th assessment reports of the Intergovernmental Panel on Climate Change (IPCC). Improving soil C projections is of a high priority for Earth system modeling in the future IPCC and other assessments. To achieve this goal, we suggest that (1) model structures should reflect real-world processes, (2) parameters should be calibrated to match model outputs with observations, and (3) external forcing variables should accurately prescribe the environmental conditions that soils experience. Firstly, most soil C cycle models simulate C input from litter production and C release through decomposition. The latter process has traditionally been represented by 1st-order decay functions, regulated primarily by temperature, moisture, litter quality, and soil texture. While this formulation well captures macroscopic SOC dynamics, better understanding is needed of their underlying mechanisms as related to microbial processes, depth-dependent environmental controls, and other processes that strongly affect soil C dynamics. Secondly, incomplete use of observations in model parameterization is a major cause of bias in soil C projections from ESMs. Optimal parameter calibration with both pool- and flux-based datasets through data assimilation is among the highest priorities for near-term research to reduce biases among ESMs. Thirdly, external variables are represented inconsistently among ESMs, leading to differences in modeled soil C dynamics. We recommend the implementation of traceability analyses to identify how external variables and model parameterizations influence SOC dynamics in different ESMs. Overall, projections of the terrestrial C sink can be substantially improved when reliable datasets are available to select the most representative model structure, constrain parameters, and prescribe forcing fields.
We examined whether it is possible to relate concentrations of trace elements in alluvial sediments with records concerning the intensity of mining, and use them as a means of dating. We conducted our research in the medieval mining district of the town of Kutná Hora in the Czech Republic. Samples were collected under the pond dam and analysed for clay, silt and sand content and for As, Be, Cd, Co, Cr, Cu, Mo, Ni, Pb, V and Zn concentrations. We observed two main peaks of element concentrations (Be, Co, Cr, Cu, Hg, Pb, V and Zn), independent of grain fractions. The peaks were interpreted as a result of human activity. The concentration curves, stratigraphy and location of the dam/alluvium boundary were compared with historical records of mining production. This means of dam dating into the 16th century agreed with historical dating from written sources. Trace elements were also successfully used as stratigraphic markers. The comparison between concentration patterns of V and other well interpreted elements (Be, Co, Cr, Cu, Hg, Pb and Zn) enabled to recognize a material directly originating from the mines. The elements thus helped to interpret local sedimentation history.
Both modelling and experimental approaches have been applied to assess C exchange fluxes at large spatial scales. Yet, these approaches are subjected to substantial limitations and uncertainties. Here, we aim to highlight two key mechanisms able to improve the estimation of the hillslope aggregated CO2 fluxes: (i) the persistence of soil organic carbon (OC) in deep colluvium deposits; and (ii) the physical controls on CO2 fluxes along soil profiles. This study focuses on a sloping cropland in the central loess belt of Belgium. On two contrasted soil types along the studied hillslope, we recorded time-series of CO2 concentration, water content and temperature along 1 m long soil profiles during two periods of 6 months. Then, we calculated profiles of CO2 fluxes using the gradient method. To extrapolate these fluxes to entire yearly periods (2011–2013), we performed simulation using the SOILCO2RothC model.
The vertical partitioning of the soil CO2 fluxes shows that ca. 90 to ca. 95% of the surface CO2 fluxes originates from the 10 first centimeters of the soil profile at the footslope. We show that high water filled pore space at this slope position disables the transfer of biotic CO2 along the soil profile. However, the total annual flux averaged along 3 years of simulation show that the top soil layer (0–10 cm) of the footslope generates CO2 fluxes (870 ± 64 CO2-C m−2 year−1 which exceed those observed at the summit position (583 ± 61 CO2-C m−2 year−1. Hence, our results reconcile two seemingly contradictory hypotheses, i.e. (i) these support that soil OC at such a footslope is stored along the main part of the soil profile and submitted to a long-term stabilization, and (ii) at the same time these support that the depositional footslope profile emits more CO2 than the summit, due to its high amount and quality of OC. Our results support the need to consider slopes when modeling soil-atmosphere C exchanges. If landscapes dynamic processes are not accounted for, we pointed out a risk to under-estimate annual soil-atmosphere CO2 exchanges by ca. 20%.
Lateral movement of organic matter (OM) due to erosion is now considered an important flux term in terrestrial carbon (C) and nitrogen (N) budgets, yet most published studies on the role of erosion focus on agricultural or grassland ecosystems. To date, little information is available on the rate and nature of OM eroded from forest ecosystems. We present annual sediment composition and yield, for water years 2005–2011, from eight catchments in the southern part of the Sierra Nevada, California. Sediment was compared to soil at three different landform positions from the source slopes to determine if there is selective transport of organic matter or different mineral particle size classes. Sediment export varied from 0.4 to 177 kg ha−1, while export of C in sediment was between 0.025 and 4.2 kg C ha−1 and export of N in sediment was between 0.001 and 0.04 kg N ha−1. Sediment yield and composition showed high interannual variation. In our study catchments, erosion laterally mobilized OM-rich litter material and topsoil, some of which enters streams owing to the catchment topography where steep slopes border stream channels. Annual lateral sediment export was positively and strongly correlated with stream discharge, while C and N concentrations were both negatively correlated with stream discharge; hence, C : N ratios were not strongly correlated to sediment yield. Our results suggest that stream discharge, more than sediment source, is a primary factor controlling the magnitude of C and N export from upland forest catchments. The OM-rich nature of eroded sediment raises important questions about the fate of the eroded OM. If a large fraction of the soil organic matter (SOM) eroded from forest ecosystems is lost during transport or after deposition, the contribution of forest ecosystems to the erosion-induced C sink is likely to be small (compared to croplands and grasslands).
Cultivation reduces soil C content and changes the distribution and stability of soil aggregates. We investigated the effect of cultivation intensity on aggregate distribution and aggregate C in three soils dominated by 2:1 clay mineralogy and one soil characterized by a mixed (2:1 and 1:1) mineralogy. Each site had native vegetation (NV), no-tillage (NT), and conventional tillage (CT) treatments. Slaked (i.e., air-dried and fast-rewetted) and capillary rewetted soils were separated into four aggregate-size classes (<53, 53-250, 250-2000, and >2000 μm) by wet sieving. In rewetted soils, the proportion of macroaggregates accounted for 85% of the dry soil weight and was similar across management treatments. In contrast, aggregate distribution from slaked soils increasingly shifted toward more microaggregates and fewer macroaggregates with increasing cultivation intensity. In soils dominated by 2:1 clay mineralogy, the C content of macroaggregates was 1.65 times greater compared to microaggregates. These observations support an aggregate hierarchy in which microaggregates are bound together into macroaggregates by organic binding agents in 2:1 clay-dominated soils. In the soil with mixed mineralogy, aggregate C did not increase with increasing aggregate size. At all sites, rewetted macro- and microaggregate C and slaked microaggregate C differed in the order NV > NT > CT. In contrast, slaked macroaggregate C concentration was similar across management treatments, except in the soil with mixed clay mineralogy. We conclude that increasing cultivation intensity leads to a loss of C-rich macroaggregates and an increase of C-depleted microaggregates in soils that express aggregate hierarchy.
Experiments investigating the spatial variability of soil moisture conducted in the 10.5 ha Tarrawarra catchment, southeastern Australia, are described. The resulting data include high-resolution soil moisture maps (over 10,000 point measurements at up to 2060 sites), information from 125 soil cores, over 1000 soil moisture profiles from 20 sites, 2500 water level measurements from 74 piezometers, surface roughness and vegetation measurements, meteorological and hydrological flux measurements, and topographic survey data. These experiments required a major commitment of resources including 250 person days in the field, with a further 100 person days in the laboratory preparing for field trips and checking and collating data. These data are available on the World Wide Web (http://www.civag.unimelb.edu.au/data/).
Riverine export of particulate organic carbon (POC) to the ocean affects the atmospheric carbon inventory over a broad range of timescales. On geological timescales, the balance between sequestration of POC from the terrestrial biosphere and oxidation of rock-derived (petrogenic) organic carbon sets the magnitude of the atmospheric carbon and oxygen reservoirs. Over shorter timescales, variations in the rate of exchange between carbon reservoirs, such as soils and marine sediments, also modulate atmospheric carbon dioxide levels. The respective fluxes of biospheric and petrogenic organic carbon are poorly constrained, however, and mechanisms controlling POC export have remained elusive, limiting our ability to predict POC fluxes quantitatively as a result of climatic or tectonic changes. Here we estimate biospheric and petrogenic POC fluxes for a suite of river systems representative of the natural variability in catchment properties. We show that export yields of both biospheric and petrogenic POC are positively related to the yield of suspended sediment, revealing that POC export is mostly controlled by physical erosion. Using a global compilation of gauged suspended sediment flux, we derive separate estimates of global biospheric and petrogenic POC fluxes of 157(+74)(-50) and 43(+61)(-25) megatonnes of carbon per year, respectively. We find that biospheric POC export is primarily controlled by the capacity of rivers to mobilize and transport POC, and is largely insensitive to the magnitude of terrestrial primary production. Globally, physical erosion rates affect the rate of biospheric POC burial in marine sediments more strongly than carbon sequestration through silicate weathering. We conclude that burial of biospheric POC in marine sediments becomes the dominant long-term atmospheric carbon dioxide sink under enhanced physical erosion.
Terrestrial sedimentation buries large amounts of organic carbon (OC) annually, contributing to the terrestrial carbon sink. The temporal significance of this sink will strongly depend on the attributes of the depositional environment , but also on the characteristics of the OC reaching these sites and its stability upon deposition. The goal of this study was to characterise the OC during transport and stored in the depositional settings of a medium-sized catchment (111 km 2) in SE Spain, to better understand how soil erosion and sediment transport processes determine catchment-scale OC redistribution. Total organic carbon (TOC), mineral-associated organic carbon (MOC), particulate organic carbon (POC), total nitrogen (N) and particle size distributions were determined for soils (i), suspended sediments (ii) and sediments stored in a variety of sinks such as sediment wedges behind check dams (iii), channel bars (iv), a small delta in the conjunction of the channel and a reservoir downstream (v), and the reservoir at the outlet of the catchment (vi). The data show that the OC content of sediments was approximately half of that in soils (9.42 ± 9.01 g kg −1 versus 20.45 ± 7.71 g kg −1 , respectively) with important variation between sediment deposits. Selectivity of mineral and organic material during transport and deposition increased in a downstream direction. The mineralisation, burial or in situ incorporation of OC in deposited sediments depended on their transport processes and on their post-sedimentary conditions. Upstream sediments (alluvial wedges) showed low OC contents because they were partially mobilised by non-selective erosion processes affecting deeper soil layers and with low selectivity of grain sizes (e.g. gully and bank erosion). We hypothesise that the relatively short transport distances, the effective preservation of OC in microaggregates and the burial of sediments in the alluvial wedges gave rise to low OC mineralisation, as is arguably indicated by C : N ratios similar to those in soils. Deposits in middle stream areas (fluvial bars) were enriched in sand, selected upon deposition and had low OC concentrations. Downstream, sediment transported over longer distances was more selected, poorly microaggregated, and with a prevalence of silt and clay fractions and MOC pool. Overall , the study shows that OC redistribution in the studied catchment is highly complex, and that the results obtained at finer scales cannot be extrapolated at catchment scale. Se-lectivity of particles during detachment and transport, and protection of OC during transport and deposition are key for the concentration and quality of OC found at different de-positional settings. Hence, eco-geomorphological processes during the different phases of the erosion cycle have important consequences for the temporal stability and preservation of the buried OC and in turn for the OC budget.
Laboratory-based aggregate stability (AS) tests should be applied to material wetted to a moisture content comparable with that of a field soil. We have improved our original laser granulometer (LG)-based AS test published in this journal by including a pre-wetting stage. Our method estimates disaggregation reduction (DR; µm) for a soil sample (1–2-mm diameter aggregates). Soils with more stable aggregates have larger DR values. We apply the new technique to soils from 60 cultivated sites across eastern England, with ten samples from each of six different parent material (PM) types encompassing a wide range of soil organic carbon (SOC) concentrations (1.2–7.0%). There are large differences between the median DR values (rescaled to < 500 µm) for soils over the PM types, which when used as a predictor (in combination with SOC concentration) accounted for 53% of the variation in DR. There was no evidence for including an interaction term between PM class and SOC concentration for the prediction of DR. After applying the aggregate stability tests with the 60 regional soil samples, they were stored for 9 months and the tests were repeated, resulting in a small but statistically significant increase in DR for samples from some, but not all, PM types. We show how a palaeosol excavated from a site in southern England can be used as an aggregate reference material (RM) to monitor the reproducibility of our technique. It has been suggested that soil quality, measured by critical soil physical properties, may decline if the organic carbon concentration is less than a critical threshold. Our results show that, for aggregate stability, any such thresholds are specific to the PM.
Agriculture in the hummocky ground moraine landscape of NE-Germany is characterized by an increase in
energy crop cultivation, like maize or sorghum. Both enhance lateral C fluxes by erosion and induce feedbacks
on C dynamics of agroecosystems as a result of reduced wintertime plant cover and vigorous crop growth
during summer. However, the actual impact of these phenomena on the CO2-sink/-source function of agricultural
landscapes, is still not clear.
Therefore, the interdisciplinary project “CarboZALF” was established in Dedelow/Prenzlau (NE-Germany)
in 2009. Within the field experiment CarboZALF-D, CO2 fluxes for the soil-plant systems were monitored,
covering typical landscape relevant soil states in respect to erosion and deposition, like Calcic Cutanic Luvisol
and Endogleyic Colluvic Regosol.
Automated chamber systems, each consisting of four transparent chambers (2.5 m height, basal area 2.25
m2), were placed along gradients at both measurement sites. Monitored CO2 fluxes were gap-filled on a
high-temporal resolution by modelling ecosystem respiration (Reco), gross primary productivity (GPP) and net
ecosystem exchange (NEE) based on parallel and continuous measurements of the CO2 exchange, soil and air
temperatures as well as photosynthetic active radiation (PAR). Gap-filling was e.g. needed in case of chamber
malfunctions and abrupt disturbances by farming practice. The monitored crop rotation was corn-winter wheat (2
a), sorghum-winter triticale and alfalfa (1.5 a).
In our presentation we would like to show insights from a 4 years observation period, with prounounced
differences between the eroded and the colluvial soil: The Endogleyic Colluvic Regosol showed higher flux
rates for Reco, GPP and NEE compared to the Calcic Cutanic Luvisol. Site-specific NEE and C-balances were
positively related to soil C-stocks as well as biomass production, and generated a minor C-sink in case of the
Calcic Cutanic Luvisol and a highly variable C-source in case of the Endogleyic Colluvic Regosol. Moreover,
obtained high local variability in CO2 fluxes and C-balances at both sites, can be interpreted in terms of relevant
drivers.
Dissolved organic matter (DOM) plays a crucial role in many important processes that take place in terrestrial and aquatic systems. These include carbon and nutrient cycling, pedogenesis, and microbial metabolism. Here we highlight the results of studies that demonstrate the role of DOM in linking terrestrial and aquatic systems. We emphasize three fundamental aspects of the research, which together show the importance of DOM in linking terrestrial and aquatic systems: First, tracing DOM properties during its transport through terrestrial and aquatic systems is a powerful tool for improving our conceptual understanding of the mechanistic drivers of DOM dynamics. Second, linking DOM dynamics to important physical processes such as hydrology provides important insights into the nature of terrestrial-aquatic links. Third, interrelations between DOM dynamics and human impacts on ecosystems highlight how the role of DOM in coupled terrestrial-aquatic systems may change in the future. New measurement and modeling approaches have enabled a more thorough assessment of all three aspects of DOM dynamics. They show that both natural and anthropogenic drivers not only greatly influence DOM dynamics in soils, but owing to the mobility of DOM, also have substantial influence on aquatic systems. This physical connection of soils and surface waters demonstrates the importance of understanding fundamental processes such as nutrient cycling, pedogenesis, and microbial metabolism at a whole-landscape scale.
Minerals and organic matter (OM) may form intricate associations via myriad interactions. In soils, the associations of OM with mineral surfaces are mainly investigated because of their role in determining the long-term retention of OM. OM "must decay in order to release the energy and nutrients that drive live processes all over the planet" (Janzen, 2006). Thus, the processes and mechanisms that retain OM in soil are a central concern to very different branches of environmental research. An agronomist may want to synchronize periods of high nutrient and energy release with the growth stages of a crop. An environmental chemist may wish to either immobilize an organic soil contaminant or enhance its decomposition into less harmful metabolites, while climate scientists need to understand the processes that mediate the production of potent greenhouse gases from decomposing OM. Associations of OM with pedogenic minerals (henceforth termed mineral-organic associations (MOAs)) are known to be key controls in these and many other processes. Here we strive to present an overview of the current knowledge on MOAs and identify key questions and future research needs.
The assessment of the net effect of soil erosion on the global carbon budget is still incomplete because of lack of enough focused studies and field data. Two of the major gaps on our understanding of the erosion induced terrestrial carbon sink issue include rate of eroded soil organic carbon (OC) replacement by production of new photosynthate and stability of eroded OC post deposition. Here we examine the effect of erosion processes and land use change on the stock, type and stability of OC in two medium-sized subcatchments (18 and 50 ha in size) in SE Spain. We analysed soil samples from drainage areas and depositional settings for stock and isotopic composition of OC (<sup>14</sup>C and <sup>13</sup>C) and particle size distribution. In addition, we conducted land use change analysis for the period 1956–2008 and a geomorphological survey of the current erosion processes taking place in the slope-streambed connections. Our findings demonstrate how land use change influenced the dominating erosion processes and, thus, the source of eroding sediments. Carbon isotopes used as tracers revealed that in one of the subcatchments the deposited sediments derived from deep soil (average Δ<sup>14</sup>C of −271.5 ‰) through non-selective erosion processes. In the other subcatchment, topsoil material was predominantly eroded and the average Δ<sup>14</sup>C in sediments was −64.2 ‰. Replacement of eroded soil OC was positive (4 and 11 times-fold losses by erosion) for the analyzed soil profiles in the slopes suggesting that erosion processes do not necessarily provoke a decrease in soil OC stock.
To accurately estimate past terrestrial carbon pools is the key to understand the global carbon cycle and its relationship with the climate system. SoilGen2 is a useful tool to obtain aspects of soil properties (including carbon content) by simulating soil formation processes; thus it offers an opportunity for past soil carbon pool reconstruction. In order to apply it to various environmental conditions, parameters related to carbon cycle process in SoilGen2 are calibrated based on 6 soil pedons from two typical loess deposition regions (Belgium and China). Sensitivity analysis using Morris' method shows that decomposition rate of humus ( k <sub>HUM</sub>), fraction of incoming plant material as leaf litter (fr<sub>ecto</sub>) and decomposition rate of resistant plant material ( k <sub>RPM</sub>) are 3 most sensitive parameters that would cause the greatest uncertainty in simulated change of soil organic carbon in both regions. According to the principle of minimizing the difference between simulated and measured organic carbon by comparing quality indices, the suited values of k <sub>HUM</sub>, fr<sub>ecto</sub> and k <sub>RPM</sub> in the model are deduced step by step. The difference of calibrated parameters between Belgium and China may be attributed to their different vegetation types and climate conditions. This calibrated model is improved for better simulation of carbon change in the whole pedon and has potential for future modeling of carbon cycle in paleosols.
It has been suggested that eroding landscapes can form C sinks or
sources, but the underlying mechanisms are not fully understood. Our
analysis aims to clarify the effects of soil redistribution on
physical and biogeochemical soil organic carbon (SOC) stabilization
mechanisms along a hillslope transect. The observed mineralogical
differences seem partly responsible for the effectiveness of
geochemical and physical SOC stabilization mechanisms as the mineral
environment along the transect is highly variable and dynamic. The
abundance of primary and secondary minerals and the weathering
status of the investigated soils differ drastically along this
transect. Extractable iron and aluminum components are largely
abundant in aggregates, but show no strong correlation to SOC,
indicating their importance for aggregate stability but not for SOC
retention. We further show that pyrophosphate extractable soil
components, especially manganese, play a role in stabilizing SOC
within non-aggregated mineral fractions. The abundance of microbial
residues and measured 14C ages for aggregated and
non-aggregated SOC fractions demonstrate the importance of the
combined effect of geochemical and physical protection to stabilize
SOC after burial at the depositional site. Mineral alteration and
the breakdown of aggregates limit the protection of C by minerals
and within aggregates temporally. The 14C ages of buried
soil indicate that C in aggregated fractions seem to be preserved
more efficiently while C in non-aggregated fractions is released,
allowing a re-sequestration of younger C with this fraction. Old
14C ages and at the same time high contents of microbial
residues in aggregates suggest that microorganisms either feed on
old carbon to build up microbial biomass, or that these environments
consisting of considerable amounts of old C are proper habitats for
microorganisms and preserve their residues. Due to continuous soil
weathering and, hence, weakening of protection mechanisms,
a potential C sink through soil burial is finally temporally
limited.
The use of 137Cs measurements has been demonstrated in the application of the technique to a study of soil erosion on two contrasting soil types under arable cultivation in the UK. Measurement of the spatial distribution of 137Cs has permitted interpretation of the pattern of soil erosion and deposition within the fields studied. These detailed patterns may be related to topography in order to aid the modelling of soil erosion. The 137Cs data have also been used to estimate the rates of soil erosion and deposition within the fields and the net loss from them. The rates of net loss compare well with data obtained by MAFF/SSEW through a combination of aerial survey and field measurement. -from Authors
Patterns of soil organic carbon (SOC) vary widely across the landscape leading to large uncertainties in the SOC budgets for agricultural systems, especially for landscapes where water, tillage, and wind erosion redistributes soil and SOC across the landscape. It is often assumed that soil erosion results in a loss of SOC from agricultural ecosystems, but recent studies indicate that soil erosion on agricultural landscapes is not a source of carbon dioxide to the atmosphere, but that soil erosion and its subsequent redistribution within agricultural fields and watersheds can lead to limited carbon sequestration on agricultural landscapes. This study investigates the relationship between SOC and soil redistribution patterns on agricultural landscapes using fallout 137caesium ( l37Cs) to determine patterns of soil redistribution. SOC and 137Cs concentrations of soils were significantly correlated in our study areas. Soils in eroding areas have significantly less SOC than soils in depositional areas. SOC decreased as gradient slope increases and soils on concave slopes had higher SOC than soils on convex slopes. These data suggest that soil redistribution and topographic patterns may be used to help understand SOC dynamics on the landscape. The strong significant relationships between soil redistribution and SOC concentrations in the soils suggest that soil and soil organic matter are transported along similar physical pathways in agricultural systems. These transport processes move soils and SOC to sites of deposition within agricultural fields, riparian zones, and water bodies in the watershed, where SOC is buried, leading to more carbon being removed from the atmosphere than is emitted, creating a sink of atmospheric carbon. Our study indicates the importance of understanding soil movement and redistribution patterns within a field or watershed for understanding soil carbon cycles in agricultural ecosystems.
Recent years have seen increasing interest in the potential for using caesium-137 as a tracer in soil erosion investigations and caesium-137 measurements are now accepted as an effective and valuable means of producing retrospective information on medium-term rates and patterns of soil redistribution. There are, however, two important limitations to the general application of the approach. These involve the complications introduced by inputs of Chernobyl-derived radiocaesium in many areas of Europe and the relatively low caesium-137 inventories encountered in some parts of the globe where fallout rates were low. Unsupported lead-210, a natural fallout radionuclide, can offer a viable alternative to caesium-137 as a sediment tracer in many environments. In addition, when used in combination with caesium-137, it offers potential as a complementary tracer, which can provide additional information on the erosional history of a site. The results of a study of soil redistribution within a small cultivated field at Butsford Barton, Colebrooke, Devon, UK, aimed at comparing the estimates provided by caesium-137 and unsupported lead-210 measurements, are presented. These results confirm the potential for using unsupported lead-210, both as an alternative to, and in combination with, caesium-137 in soil erosion investigations.
The exchange of nutrients, energy and carbon between soil organic matter, the soil environment, aquatic systems and the atmosphere is important for agricultural productivity, water quality and climate. Long-standing theory suggests that soil organic matter is composed of inherently stable and chemically unique compounds. Here we argue that the available evidence does not support the formation of large-molecular-size and persistent 'humic substances' in soils. Instead, soil organic matter is a continuum of progressively decomposing organic compounds. We discuss implications of this view of the nature of soil organic matter for aquatic health, soil carbon-climate interactions and land management.
During slope-scale erosion events, re-distribution of eroded soil and the associated soil organic carbon (SOC) is not always uniform, but very often affected by preferential transport and deposition. Under given flow conditions, the site of SOC deposition depends on the transport distances of sediment particles containing the SOC. Very often, soil and SOC erosion risk are assessed by applying mineral particle specific SOC distributions to erosion models. However, soil is not always eroded as individual mineral particles, but mostly in a form of aggregates. Aggregates are likely to increase settling velocities of individual mineral particles, which may considerably reduce the transport distance of sediment fractions and the associated SOC, skewing SOC redistribution and its subsequent fate. Yet, little is known about the potential effects of aggregation on the movement and fate of eroded SOC.