Knowledge about the stabilization of organic matter input to soil is essential for understanding the influence of different agricultural practices on turnover characteristics in agricultural soil systems. In this study, soil samples from a long-term field experiment were separated into silt- and clay-sized particles. In 1967, 14C labeled farmyard manure was applied to three different cropping systems: crop rotation, monoculture and permanent bare fallow. Humic acids (HAs) were extracted from silt- and clay-sized fractions and characterized using photometry, mid-infrared and fluorescence spectroscopy. Remaining 14C was determined in size fractions as well as in their extracted HAs. Yields of carbon and remaining 14C in HAs from silt-sized particles and Corg in clay-sized particles decreased significantly in the order: crop rotation > monoculture ≫ bare fallow. Thus, crop rotation not only had the largest overall C-pool in the experiment, but it also best stabilized the added manure. Mid-infrared spectroscopy could distinguish between HAs from different particle size soil fractions. With spectroscopic methods significant differences between the cropping systems were detectable in fewer cases compared to quantitative results of HAs (yields, 14C, Corg and Nt). The trends, however, pointed towards increased humification of HAs from bare fallow systems compared to crop rotation and monoculture as well as of HAs from clay-sized particles compared to silt-sized particles. Our study clearly shows that the largest differences were observed between bare fallow on one hand and monoculture and crop rotation on the other.
Blackwater streams are found throughout the Coastal Plain of the southeastern United States and are characterized by a series of instream floodplain swamps that play a critical role in determining the water quality of these systems. Within the state of Georgia, many of these streams are listed in violation of the state's dissolved oxygen (DO) standard. Previous work has shown that sediment oxygen demand (SOD) is elevated in instream floodplain swamps and due to these areas of intense oxygen demand, these locations play a major role in determining the oxygen balance of the watershed as a whole. This work also showed SOD rates to be positively correlated with the concentration of total organic carbon. This study builds on previous work by using geostatistics and Sequential Gaussian Simulation to investigate the patchiness and distribution of total organic carbon (TOC) at the reach scale. This was achieved by interpolating TOC observations and simulated SOD rates based on a linear regression. Additionally, this study identifies areas within the stream system prone to high SOD at representative 3rd and 5th order locations. Results show that SOD was spatially correlated with the differences in distribution of TOC at both locations and that these differences in distribution are likely a result of the differing hydrologic regime and watershed position. Mapping of floodplain soils at the watershed scale shows that areas of organic sediment are widespread and become more prevalent in higher order streams. DO dynamics within blackwater systems are a complicated mix of natural and anthropogenic influences, but this paper illustrates the importance of instream swamps in enhancing SOD at the watershed scale. Moreover, our study illustrates the influence of instream swamps on oxygen demand while providing support that many of these systems are naturally low in DO.
In spatial sampling, once initial samples of the primary variable have been collected, it is possible to take additional measurements, an approach known as second-phase sampling. Additional samples are usually collected away from observation locations, or where the kriging variance is maximum. However, the kriging variance (also known as prediction error variance) is independent of data values and computed under the assumption of stationary spatial process, which is often violated in practice. In this paper, we weight the kriging variance with another criterion, giving greater sampling importance to locations exhibiting significant spatial roughness that is computed by a spatial moving average window. Additional samples are allocated using a simulated annealing procedure since the weighted objective function is non-linear. A case study using an exhaustive remote sensing image illustrates the procedure. Combinations of first-phase systematic and nested sampling designs (or patterns) of varying densities are generated, while the location of additional observations is guided in a way which optimizes the proposed objective function. The true pixel value at the new points is extracted, the semivariogram model updated, and the image reconstructed. Second-phase sampling patterns optimizing the proposed criterion lead to predictions closer to the true image than when using the kriging variance as the main criterion. This improvement is stronger when there is a low density of first-phase samples, and decreases however as the initial density increases.
Identifying the spatial variability and risk areas for southern root-knot nematode [Meloidogyne incognita (Kofoid & White) Chitwood] (RKN) is key for site-specific management (SSM) of cotton (Gossypium hirsutum L.) fields. The objectives of this study were to: (i) determine the soil properties that influence RKN occurrence at different scales; and (ii) delineate risk areas of RKN by indicator kriging. The study site was a cotton field located in the southeastern coastal plain region of the USA. Nested semivariograms indicated that RKN samples, collected from a 50×50 m grid, exhibited a local and regional scale of variation describing small and large clusters of RKN population density. Factorial kriging decomposed RKN and soil properties variability into different spatial components. Scale dependent correlations between RKN data showed that the areas with high RKN population remained stable though the growing season. RKN data were strongly correlated with slope (SL) at local scale and with apparent soil electrical conductivity deep (EC(a-d)) at both local and regional scales, which illustrate the potential of these soil physical properties as surrogate data for RKN population. The correlation between RKN data and soil chemical properties was soil texture mediated. Indicator kriging (IK) maps developed using either RKN, the relation between RKN and soil electrical conductivity or a combination of both, depicted the probability for RKN population to exceed the threshold of 100 second stage juveniles/100 cm(3) of soil. Incorporating EC(a-d) as soft data improved predictions favoring the reduction of the number of RKN observations required to map areas at risk for high RKN population.
We investigated the extraction of manganese (Mn) oxides and iron (Fe) oxides from samples of three Andisols and seven other major Japanese soils with acidified 0.1 M NH2OH–HCl in order to evaluate this method's applicability to selective dissolution of Mn oxide. The temporal changes in the amounts of Mn and Fe extracted revealed the following: 1. Mn oxides in non-volcanic-ash soil (low Alo + 1/2Feo) could be dissolved semi-completely upon extraction for 30 min, which is the customary extraction time. 2. Mn oxides in volcanic ash-containing soils (moderate or high Alo + 1/2Feo) continued to dissolve substantially after 30 min of extraction; 3. Fe oxides in all soils gradually dissolved throughout 4 h of extraction, although dissolution rates in volcanic ash-containing soils were much lower than in other soils. These results indicated that the solubility of Mn and Fe oxides in volcanic ash-containing soils were low, suggesting characteristic state of the oxides in these soils, and that the method is therefore unsuitable.Graphical abstractResearch Highlights► Mn oxide in soils which contain high Alo+1/2Feo was dissolved slowly in NH2OH-HCl. ► Soils which contain high Alo+1/2Feo had low solubility of Fe oxide with NH2OH-HCl. ► Selective dissolution method for Mn oxide in soils cannot apply to volcanic ash soils.
DRIFT spectroscopic experiments are described here that build on our previous time-resolved in situ experiments using dihaloethanes as molecular probes. The earlier studies produced the first molecular level evidence for differences between clay minerals, humic substances, and lysimeter soils as sorbents for these chemicals. In the current study, spectroscopic experiments with several established field soils are combined with porosity characterizations derived from both N2 and CO2 isotherms. Density functional theory is used to generate porosity distributions from both types of isotherms which, in turn, are used here to define the microstructural character of the soil. Results from the combined techniques strongly suggest that sorption/desorption behavior of volatile organic chemicals (VOCs) can be understood in the context of soil particle architecture. The porosimetry provides evidence that the microstructural array in particles of three of the soils investigated is determined primarily by the platelet character of quasicrystalline clay minerals while that of another one of the soils, an Oxisol, appears to be dominated by its high iron oxide content. With all the soils, curve-fitting of the time-resolved DRIFT spectra indicates the presence of both a sorbed liquid and a persistently sorbed species that accumulate at different rates: band areas for the persistent species continue to increase during desorption. For the soils whose microstructure is dominated by clay minerals, rates of accumulation for both species increase and desorption of the sorbed liquid is facilitated after treatment with hypochlorite to partially remove organic matter. The treatment has little effect, however, on the sorption behavior observed with the Oxisol. The changes in sorption/desorption behavior correlate well with the differences in porosity distributions determined using N2. In contrast, CO2 determined microporosity (<2 nm), while related to the organic matter content of the untreated soils, was not a good predictor of any of the observed effects. These results are entirely consistent with those obtained using lysimeter soils as sorbents and support our earlier hypothesis that the persistent species band can be attributed to vapor phase chemical entrapped within the pore network of the sorbent particles. Soil organic matter again appears to impact the retention process primarily by impeding the flow of both liquid and vapor phase chemical into and out of that pore network rather than by associating with the chemical itself. The implications of these results for other volatile organic pollutants and the potential role of surface tension forces within mesopores (2–50 nm) in promoting the formation of persistently sorbed chemical are also explored.
Studies of isotherms and isobars of vapor adsorption by substances of large surface area show that the difference in the water content caused by the difference of the water vapor pressure in the ambient may amount to 1%.To diminish this error, due to water vapor adsorption, it is recommended to dry the mineral substances (clays, silicagel) at temperatures (> 105°C) (≈ 150°C).To diminish the error, due to water vapor adsorption, in substances which contain organic matter, it is recommended to dry the substances in vacuum over desiccant.
This paper reviews the major mechanisms proposed to explain podzolization. These include the production of organic acids that form soluble complexes with aluminium/iron thereby enhancing weathering, followed by illuviation by precipitation/adsorption processes occurring at greater depth. Precipitation of aluminium and iron is explained by decreasing solubility of increasingly metal-rich complexes, or by microbial degradation of the organic ligand. We also discuss proposed role of inorganic hydroxy-aluminium–silicate sols in podzolization. The paper is introductory to a multidisciplinary study of podzolization performed in the Nordic Countries presented in this volume.
10Be concentrations of six soils on the raised coral reef terraces of Kikai Island, southwest Japan range from 0.80 to 7.17×109 atoms g−1. The annual deposition rate of 10Be from the atmosphere to Kikai Island from 2000 to 2002 ranges from 2.0 to 3.5×106 atoms cm−2 year−1, with a mean of 2.8±0.7×106 atoms cm−2 year−1. Minimum absolute ages, calculated from the inventory of meteoric 10Be in the soils and the annual deposition rate of 10Be, are: Initial Rendzina-like soil, ≥8 ka; Rendzina-like soil, ≥18 ka; Brown Rendzina-like soil, ≥56 ka; Terra fusca-like soil, ≥127 ka; Terra rossa-like soil, ≥136 ka; and Intergrade between Terra rossa-like and Red-Yellow soil, ≥102 ka. Therefore, it was concluded that soil formation during the first ∼60,000 years would be as Rendzina-like soils and more than ∼100,000 years would be required for Red-Yellow soils to develop on raised coral reef terraces under subtropical humid climatic conditions.
A short historical review of Russian pedology is given, starting with the revolutionary ideas introduced by V. V. Dokuchaev in 1874. It was not until the beginning of the Soviet period that soil science became fully recognized as a valid discipline, and in the intervening years enormous progress has been made.Many techniques and methods developed in the U.S.S.R. have proved of inestimable value to that country and many have been widely applied in foreign countries.The present position of soil science in the U.S.S.R. is reviewed in relation to other countries. Soil approximation and soil mapping have resulted in 2000 distinct and different soil-agricultural regions for the whole Soviet Union territory.
We analyzed a highly complex soilscape of fluvial sediments by a hierarchical expert system. Using (i) inquiries, (ii) relief analysis on basis of a DEM 5, and (iii) soils' apparent electrical conductivity (EM38) as a database, we first defined zones of identical pedogenic context. Next, multi-temporal remote sensing data of winter wheat were obtained by an airborne multi-spectral scanner (Daedalus-ATM), which gives radiometric information with a geometric (ground) resolution of 1 m2 (pixel size). Leaf area index (LAI) was semi-physically modeled using red and near-infrared canopy reflectances and related to above-ground biomass. Further, the resulting spatial patterns of vegetation parameters were processed by image analysis methods, i.e. an opening–closing procedure using a circular element with a radius of 5 m. These coarser patterns of LAI and biomass, respectively, were interpreted as patterns of site quality within each zone of pedogenic context. By our multi-temporal approach we were able to distinguish between stationary and time-variant pattern. Combined with point calibration on basis of a 50-m raster we identified available water capacity (AWC) and O2 deficiency due to stagnant water as the most important soil properties constituting site quality for plant growth. Our results will be used for precision agriculture practices in future.
129Xe nuclear magnetic resonance (NMR) spectroscopy of adsorbed xenon was applied for the characterisation of soil meso- and microporosity. Model systems (Ca-montmorillonite, quartz sand) and three soil types (Luvisol Alh, Bt and Cv horizons; Gleysol Go horizon; Podzol Bvs horizon) were studied. For Ca-montmorillonite, the average intercrystallite pore size has been evaluated. In natural soils, 129Xe resonance parameters were shown to be affected by different factors: pore heterogeneity, influence of organic functional groups, possible presence of paramagnetic compounds, occurrence of xenon exchange between inter- and intraparticle void spaces. The effect of those factors on the pattern of 129Xe NMR spectra was tested. In the three soils studied, no micropores within the mineral phase available for xenon adsorption were found. The most probable reason is that such pores are occupied by small molecules of the soil organic matter (SOM). Variable extent of accessibility of mesopores within the mineral phase of the various soils was revealed. It was highest in the Podzol. Here, xenon exchange between different adsorption zones (i.e., pores of differing size, e.g., internal and external void spaces) was slow on an NMR time scale that allowed to detect separate signals, each characterising xenon behaviour in the respective adsorption zone. The pore system of the soil organic matter was shown to be not accessible for xenon, as it is accepted for N2 and other nonpolar adsorbates. Based on analysis of the spectra, a model for the possible mutual location of organic matter and iron compounds in natural soils was suggested. According to this model, a certain part of organic matter species can form the layers above iron species, thus masking them and preventing 129Xe NMR spectra from significant low-field shifts and signal broadening.
N2 adsorption (77 K) was combined with 129Xe nuclear magnetic resonance spectroscopy of adsorbed xenon to characterise soil meso- (2–50 nm) and microporosity (<2 nm). Materials from the Alh and Bt horizons of a Luvisol, the Go horizon of a Gleysol and the Bvs horizon of a Podzol were analysed. Additionally, we examined samples obtained by mixing of H2O2-treated soil fractions with organic soil material (“soil + organic matter” samples). N2- specific surface areas (SBET) and micropore volumes (Vmicro) and areas (Smicro) were markedly affected by the presence of iron oxides in soils. Their removal with dithionite-citrate-bicarbonate (DCB) treatment was accompanied by a significant decrease in SBET and almost complete disappearance of the micropores. The organic carbon (OC) content decreased by 10–35% after the DCB-procedure showing that a certain proportion of the soil organic matter was extracted together with iron oxides. This may point to a close association between carbon compounds and iron oxides, possibly by incorporation of low molecular weight organic compounds into the phase of iron oxides. Such interactions are expected to contribute to the stabilisation of organic carbon in soils. Indeed, as compared to the top horizon (Alh of Luvisol), a higher proportion of organic matter was co-extracted with iron oxides from the subsurface horizons (Bt of Luvisol, Go of Gleysol) characterised by higher amounts of organic carbon resisting oxidation with H2O2. Examination of the mixed “soil + organic matter” samples supports that after addition, organic molecules occupy micropores (evidenced by N2 adsorption) and narrower mesopores of the mineral matter (evidenced by 129Xe NMR).
The 13C/12C ratios were determined for the organic matter of all horizons of a podzol profile and of the A1 horizons of some ferrallitic soils, in some grass shoots and in a fossil root fragment from the B2h horizon of the podzol. The isotope ratio in the organic matter of the A1 horizon of the podzol matches those in grass shoots from the present savanna vegetation. The ratios in the lower horizons match those of organic matter in the A1 horizons of soils under forest and that of the fossil root fragment in the B2h horizon. The ratios thus demonstrate that the humus enrichment of the B2h horizon of the podzol occurred while it was under forest vegetation and that the present grass vegetation did not take part in the podzolization process. The differences also indicate that savanna replaced forest vegetation after the profile had been formed.
Plant residues incorporated into soils are subjected to contrasted stabilization and biodegradation processes and may contribute to pools of soil organic matter (SOM) displaying different turnover times. Little is known about the relationship between the chemical structure of plant macromolecules and their long-term turnover in soils. Our research objective was to quantify the in situ turnover of phenols derived from lignin, which is a major component of plant tissues often considered slowly biodegradable relative to total plant organic matter. In this study, we used natural 13C labeling of SOM generated by a 9-year chronosequence of maize C4 crop (δ13C around −12‰) replacing the previous wheat C3 crop (δ13C around −27‰) at the Closeaux experimental field, in France.
The impact of forest fire on soil quality is not well appreciated. This study investigated the influences of forest wild fires that occurred 12, 8, 2 years and 2 weeks before the time of sampling on the composition of the forest floor organic matter by comparing total carbon (C) and total nitrogen (TN), composition of organic functional groups as determined by 13C CP/MAS-NMR and soil aggregate stability of unburned and burned forest floor in Çanakkale, Turkey. Fire altered soil organic matter composition and reduced organic C content of surface (0–5 cm) soil. The 13C CP/MAS-NMR analyses confirmed that the forest soils exposed to fires 12, 10 and 2 years before the time of sampling had higher potential for humification than unburned control soils. However, soils exposed to the fire 2 weeks before the time of sampling became more humified than unburned control soils. Carbohydrate contents of the recently burned soils were distinctly lower than those of the control soils. This is expected, as burning would reduce fresh litter which contains labile and easily decomposable materials. There was 20% decrease in 1990, 52% in 1994, 43% in 2000 and 11% in 2002 of soil organic carbon values of burned soils, compared to unburned forest floor. Fire also reduced the stability of soil aggregates by 1–16%. It was found that carbohydrate content of soil organic matter was directly related (r2=0.92) to the stability of soil aggregates but not to the total amount of organic matter. The very high correlation coefficient suggests that carbohydrate C functional group plays an important role in the stabilization of soil aggregates.
Litter decomposition and humification in different horizons of two subalpine Rendzinas of the Bavarian Alps, differing mainly in morphology and soil climate, were studied by using chemical degradation methods, IR and 13C NMR spectroscopy, as well as pyrolysis-field ionization mass spectrometry. The Tangelrendzina, classified as a Lithic Borofolist, has a thick, peat-like organic surface layer directly overlying the parent material. The specific soil climate is cold and wet due to northerly aspect. The Moderrendzina, classified as a Lithic Rendoll, has a southerly exposure, a warmer soil climate, and a pronounced humic A horizon. Both soils are derived from dolomite debris.In both soils (L to Oh/Ah horizons) similar mechanisms are responsible for litter decomposition and humification, including loss of polysaccharides, increase of alkyl and carboxyl C proportions, and degradation of lignin. The aromatic C proportion remains nearly constant in the Tangelrendzina, indicating similar decomposition rates for aromatics and total organic C or an equilibrium between decomposition and inputs (roots). In the Moderrendzina, it decreases due to higher mineralization rates. The decrease in lignin subunits with increasing soil depth is more pronounced than the decrease in total aromatics. Water-soluble organic substances containing decomposed lignin fragments are leached from the acid surface horizons and precipitated in deeper layers by Ca ions. Intermediate substances of lignin biodegradation like demethylated lignin dimers dominate the pyrolysis products from the Oh, ca horizon of the Tangelrendzina. Products typical of advanced lignin degradation, producing furanoid structures during pyrolysis, accumulate in the Ah layer of the Moderrendzina. This shows that the humification process in the two profiles differs only in intensity and not in the specific pathways of transformation.
Human-induced soil erosion has severe economic and environmental impacts throughout the world. It is more severe in the tropics than elsewhere and results in diminished food production and security. Kenya has limited arable land and 30% of the country experiences severe to very severe human-induced soil degradation. The purpose of this research was to test visible near infrared diffuse reflectance spectroscopy (VNIR) as a tool for rapid assessment and benchmarking of soil condition and erosion severity class. The study was conducted in the Saiwa River watershed in the northern Rift Valley Province of western Kenya, a tropical highland area. Soil 137Cs concentration was measured to validate spectrally derived erosion classes and establish the background levels for different land use types. Results indicate VNIR could be used to accurately evaluate a large and diverse soil data set and predict soil erosion characteristics. Soil condition was spectrally assessed and modeled. Analysis of mean raw spectra indicated significant reflectance differences between soil erosion classes. The largest differences occurred between 1350 and 1950 nm with the largest separation occurring at 1920 nm. Classification and Regression Tree (CART) analysis indicated that the spectral model had practical predictive success (72%) with Receiver Operating Characteristic (ROC) of 0.74. The change in 137Cs concentrations supported the premise that VNIR is an effective tool for rapid screening of soil erosion condition.
Soil erosion rates are quantified using the fallout radionuclide (caesium-137) approach and models (empirical RUSLE and physically based SIBERIA) for a small catchment in south-eastern Australia. Two hillslope transects (under native grass) were sampled for 137Cs activity and soil redistribution rates were determined using empirical and theoretical conversion methods. These soil redistribution rates were compared with RUSLE predictions for the two transects and SIBERIA model predictions for the entire catchment. The net soil loss rates established in this study were also compared with the results of other studies in the region obtained with a range of different methods. Estimates based on 137Cs using an empirical conversion method compared well with published regional rates derived using rainfall-runoff plots, sediment yields and 137Cs, whereas theoretical 137Cs conversion models were found to over-estimate soil redistribution rates. Similarly, the RUSLE model significantly overestimated soil erosion rates in this study as was the case in other studies in the region. The agreement between SIBERIA and 137Cs, and erosion rates obtained elsewhere in the region, provides confidence in SIBERIA for catchment scale erosion assessments. The results of this study demonstrate the limitations associated with using theoretical 137Cs conversion models in environments for which they are not suited. This study also highlights the need for caution when quantifying soil erosion using both field methods and modelling approaches. The results demonstrate that DEM based erosion models are reliable tools for the prediction of soil erosion on the hillslope and catchment scale.
Although the excess of lead-210 (210Pbex) has been described as an alternative or complementary nuclide to cesium-137 (137Cs) for soil erosion assessment, the potential use of 210Pbex in forested environments has not been examined sufficiently. In this study, to investigate the potential use of 210Pbex for the assessment of soil erosion on various forested hillslopes, we compared soil erosion rates derived from 210Pbex to those from 137Cs obtained in forest stands. The study was conducted in four types of forest: a Japanese cypress stand with a bare forest floor, a Japanese cypress stand with a fern understory, a Japanese cedar stand with sufficient litter cover, and a natural broadleaf forested stand in the Tsuzura River basin of southeastern Japan. 137Cs and 210Pbex inventories for seven potential reference sites on flat ridge tops and along transects in each stand were measured using gamma-ray spectrometry to calculate soil erosion rates. To describe the behavior of organic matter in each stand, the amounts of eroded sediment and organic matter were observed for approximately 2 years in erosion plots (1.7–2.0 m long, 0.5 m wide) in the Japanese cypress stand (bare), Japanese cedar stand, and broadleaf forest stand. From the preliminary assessment of potential reference sites, spatial variability in 210Pbex was comparable to that of 137Cs. Mean erosion rates estimated from both Cs and Pb were highest in the Japanese cypress stand (bare), followed by the Japanese cypress stand (fern), the broadleaf forest stand, and the Japanese cedar stand. This magnitude relationship was consistent with that of observation results of eroded sediment in erosion plots. In the Japanese cypress stand (bare), a significant correlation was found between Cs- and Pb-based soil erosion rates in the Japanese cypress stand (bare) while no significance was found in the other three stands. These results provide evidence of the feasibility of 210Pbex for soil erosion assessment in forest stands with bare forest floors. In the other three stands, the preferential loss of 210Pbex associated with organic matter may explain the poor correlation between the two erosion rates.
Although soil erosion is a serious environmental problem in many African countries, its assessment using traditional techniques is hampered by a range of problems. Reliable information on soil erosion rates is, nevertheless, an essential prerequisite for the design of targeted erosion and sediment control strategies. This contribution reports the use of 137Cs measurements to quantify medium-term (∼40 years) soil erosion and redistribution rates in both cultivated and uncultivated areas within the Upper Kaleya River basin in southern Zambia. Typical net soil erosion rates are estimated to be 4.3 t ha−1 year−1 for areas under commercial cultivation, 2.9 t ha−1 year−1 for bush grazing areas and 2.5 t ha−1 year−1 for areas under communal cultivation. Although these erosion rates reflect land use in these broad areas over the past 40 years, rather than present land use, they are nevertheless thought to also be representative of current conditions. The findings indicate that any attempt to develop effective erosion and sediment control strategies in the study area should involve all land use types and should aim to reduce both on-site erosion and sediment delivery from the slopes to the stream channel.
Measurable quantities of 137Cs were found in soil samples from profiles on a toposequence in southwest Niger. Erosional and depositional sites were identified by comparing 137Cs inventories with a reference site, although, because of the addition of dust, the identification of such a site was difficult. A first approximation of the reference inventory (2066±125 Bq m−2) was achieved by modelling. Marked disparities between the 137Cs profiles and the reference profile at some sites may be due to the eluviation of 137Cs fixed to clay, where soils have been exposed to cycles of wetting and drying. This hypothesis is supported by principal components analysis and non-hierarchical multivariate classification. Other divergences from the reference profile were interpreted as the results of the redeposition of material with small 137Cs content, derived from subsoil and gully walls. These complications created the need for additional modelling to estimate the net soil flux. At some sites the net soil flux was calculated using models that related 137Cs movement to soil redistribution. It appears that a vegetation canopy protects accumulated dust from water erosion on steep slopes and from wind erosion on gentle slopes. The net soil flux was found to be −16±2 t ha−1 yr−1.
Erosion is the main cause of soil degradation in tropical regions, where the lack of methods for long-term studies is the principal constraint to addressing soil erosion problems. Recently, the analysis of 137Cs redistribution within the landscape has been used for assessing long-term soil erosion and net deposition. In the present study, measurements of 137Cs distribution were used to calculate long-term soil erosion in a Mexican tropical deciduous ecosystem under undisturbed forest and pasture conditions.Sheet erosion processes caused 137Cs redistribution within the landscape. The crests had significantly higher 137Cs activity than midslopes and lower concentration than the footslopes. There was no clear relationship between 137Cs redistribution and local topographic variables in our study (that is, slope). Soils in a gentle midslope had lower 137Cs activity than those in a steeper midslope. However, hill morphology explained 137Cs redistribution within landscape, that is, high 137Cs activity was associated with sites at the base of hillslopes. Thus, net erosion was found to be strongly influenced by hill morphology. Calculated erosion and deposition rates for the undisturbed watershed were 13.2 Mg ha−1 yr−1 and 4.9 Mg ha−1 yr−1, respectively.Net erosion within the pasture conversion plots was strongly influenced by rainfall erosivity during the year following perturbation. High net erosion was associated with plots with high annual erosivity immediately after forest burn. This suggests that the first year after slash and burn is critical for susceptibility to soil erosion. Based on erosion rates calculated in the present study, the top 5 cm of soil could be removed in only 25 years. This represents soil productivity loss, as this top layer represents the principal soil nutrient pool for the Chamela region.
The relationship between soil redistribution processes (i.e. soil erosion and deposition), using the caesium-137 (137Cs) method, and the movement, storage and loss of soil organic carbon (SOC) are examined for a small catchment in south-eastern Australia. While recent studies have found strong and statistically significant relationships between 137Cs and SOC within heavily cultivated (i.e. highly disturbed) landscapes, there has been a dearth of studies in uncultivated conditions. The site used in this study is characterized by different land use histories and soil types and therefore offers a unique opportunity to investigate the relationship between 137Cs and SOC for both cultivated and uncultivated conditions. Depth distribution profiles and hillslope transects were sampled for 137Cs and SOC to examine the relationship between the redistribution of soil particles and SOC at the point and hillslope scale. It was noted that the distribution of 137Cs and SOC with depth in the soil profile differs among land use and soil types. The relationship between 137Cs and SOC was also investigated, with results indicating that there was no relationship between 137Cs and SOC for uncultivated hillslopes. In contrast, strong and statistically significant relationships were found for the previously cropped transects. The lack of a relationship within uncultivated hillslope areas in the current study appears to indicate that SOC and 137Cs are not moving along the same physical pathways or by the same mechanisms, rather it is suggested that the spatial distribution of SOC is a result of biological factors (e.g. biological oxidation, mineralization). The results of this study suggest that the use of 137Cs data to predict SOC redistribution patterns in grazing, largely undisturbed landscapes is problematic. Caution is thus required before using 137Cs to predict the spatial distribution of SOC within uncultivated landscapes in this region of Australia, and within similar dry climatic regions.
In Canada, the risk of soil erosion is expected to be greatest in regions where intensive cropping and tillage systems are used on highly erodible landscapes — such as the potato (Solanum tuberosum L.) growing regions of Atlantic Canada. However, no previous studies have looked at the combined impacts of tillage and water erosion within a region where potato production is the predominant cropping system. Therefore, the objective of this project was to estimate and model the relative contributions of tillage and water erosion in the eastern Canadian province of New Brunswick using the fallout radionuclide cesium-137 (137Cs) and two established models: (i) the water erosion component of the Water and Tillage Erosion Model (WaTEM); and (ii) the Tillage Erosion Model (TillEM). Depth incremental soil samples were collected at an Agriculture and Agri-Food Canada benchmark site (20NB) located near the town of Grand Falls, New Brunswick. Site 20NB is 4.3 ha in size and under conventional up-and-down slope potato production (slope gradients: 2 to 17%). Soil samples were collected in the fall of 2005 and were analyzed for 137Cs. Current inventories of 137Cs were compared to those taken from a nearby reference site and also to those previously taken at this site in 1990.
Irrigation water can have an adverse effect on the chemical and physical properties of the soil. In order to detect changes caused by long-term irrigation, the soils from an irrigated area on the Spanish–Portuguese border were monitored, and 1428 georeferenced topsoil samples were analyzed. The results confirm a generalized acidification of the irrigated soils when compared with rain-feed soils. There was also an increase in the electrical conductivity and exchangeable sodium percentage. The irrigated soils showed a significant decrease in organic matter content. All these results evolved significantly over at least a 30-year period. The information was implemented in a Geographical Information System, which allowed the detection of the areas of greatest irrigation impact, and the delimitation of particularly vulnerable zones where special attention is recommendable for soil conservation
Charcoal fragments have been reported frequently in the light fraction (LF) and this suggests that charcoal C is an important constituent of LF of North American soils. The LF is considered to have a rapid turnover but charcoal is highly resistant to biological degradation, hence it will have significant implications for studies of LF composition, dynamics and modeling exercises. This study aimed to quantify the contribution of charcoal to free LF (density ≤ 1.8 g cm− 3) C, and its effect on the turnover of C in that fraction using the δ¹³C technique. Duplicate free LF samples were obtained from the 0–20 cm depth of no-tilled and conventionally tilled soils, each under corn and tobacco/rye cropping. Based on morphological properties, charcoal and plant fragments were handpicked under a light microscope from one set of free LF samples and their δ¹³C were measured. The δ¹³C of whole free LF samples were also measured. The chemical properties of charcoal were characterized using solid ¹³C NMR spectroscopy technique, and these were compared with those documented for thermally generated charcoal. A two end-member mixing model was used to estimate the proportion of free LF C derived from non-charcoal and charcoal residues in continuous corn plots.
Spectra of two whole soils have been recorded by cross polarisation n.m.r. spectroscopy with magic angle spinning. Magic angle spinning allows detailed structural comparisons between the types of organic carbon in whole soils to be made. Sufficient resolution is achieved to show that the two soils differ considerably in polysaccharide content.
Cross-Polarisation Magic Angle Spinning Carbon-13 Nuclear Magnetic Resonance spectroscopy (CPMAS 13C-NMR) represents one of the most powerful tools to investigate soil organic matter (SOM) mainly because of its inherent capacity to provide a semi-quantitative evaluation of carbon distribution. A critical parameter during acquisition of CPMAS 13C-NMR spectra is the contact time required to obtain the cross-polarisation between proton and carbon nuclei. The procedure to evaluate the best contact time for the acquisition of a quantitative CPMAS 13C-NMR spectrum is to perform Variable Contact Time (VCT) experiments. In this work the structural features of a number of purified humic substances from Italian and Costarican volcanic soils were investigated by CPMAS 13C-NMR spectroscopy after having performed preliminary VCT experiments. The VCT experiments showed that the average contact times vary according to the origin and chemical structure of the humic material. The optimal contact times (OCT) for nine humic samples were between 250 and 800 μs These values were different from the time of 1000 μs that is commonly applied as the best average contact time for humic materials. Moreover, by comparing the NMR data to those obtained by elemental analysis (C/H ratio), it appeared that the efficiency of the cross-polarisation between protons and carbons, and hence the contact time, is affected not only by the number of protons, but also by their distribution over the molecules. The evaluation of errors in quantitative estimation of the different carbons revealed that the use of OCT generally reduced by half the loss of signals occurring when the average contact time of 1000 μs is used in CPMAS 13C-NMR spectra of humic substances.
To reduce soil destruction by urban sprawl, land use planning has to promote the use of soils within cities. As soil functions are now protected by law in Germany, urban soil quality has to be evaluated before soil management. We studied contributions from elemental carbon (EC) and soil organic matter (SOM) quality in topsoil horizons at seven sites in Stuttgart, Germany, differing in impurities by technogenic substrates. The most disturbed site was found at a disused railway area while high-density areas, public parks and garden areas showed varying degrees of disturbance by anthropogenic activities. For most soils, compounds derived from plant litter dominated organic matter (OM) quality characterized by nuclear magnetic resonance (NMR) spectroscopy. Although high contents of EC (up to 70% of soil organic carbon) were indicated by thermal oxidation, this was not confirmed by aromatic C intensities in NMR spectra. Only for the highly aromatic railway soil were results for elemental carbon by thermal oxidation and NMR similar. As other technogenic substrates beside EC like plastics may also contribute in the long-term to OM in urban soils, new analytical techniques are therefore required. This knowledge will promote the evaluation of urban soil properties and their sustainable use.
In soils of the eastern Amazonian forest, modifications in soil organic matter (SOM) contents as a consequence of deforestation and pasture installation were investigated. Profile distribution of total organic carbon (C) and nitrogen (N), and of ¹³C isotope abundance (expressed in δ¹³C%o units) were compared. The two soils, one under native forest and the other one after ten years under pasture of Pennisetum purpureum had similar C/N values, which slightly decreased with increasing depth, from 13.6 to 11.9–12.7 within the first 40 cm. In the pasture soil, the C content was slightly lower than in the forest soil, and reached 29 t ha⁻¹ compared with 31 t ha⁻¹, and 15 t ha⁻¹ compared with 16 t ha⁻¹, in the 0–20 and 20–40 cm layers, respectively.
We compared the quantitative responses of liquid-state (LS) and solid-state (CPMAS) 13C-NMR spectroscopy of four different soil humic substances. The intensities of signals for the alkyl carbons (0–40 ppm) were significantly larger in CPMAS than in LS spectra. This difference is in agreement with the pseudo-micellar model of the conformational nature of humic substances. By this view, the hydrophobic interactions holding together the heterogeneous molecules of humic micelles inhibit the molecular motions of the alkyl carbons, thereby enhancing the spin-lattice relaxation times and consequently lowering the sensitivity of liquid-state NMR. Conversely, regardless of their position in the humic conformation, a better estimation of the number of alkyl carbons can be obtained by CPMAS-NMR because of the cross-polarization of hydrogen nuclei in CH2 and CH3 groups. The intensity of the 40–110 ppm region is also slightly lower in LS than in CPMAS-NMR spectra, despite the hydrophilicity of the oxidized and peptidic carbons resonating in this chemical shift interval. Their molecular motion may also be reduced by either the formation of intra- and inter-molecular hydrogen bondings due to poorly acidic hydroxyl groups of saccharides, or the degree of conformational rigidity that a pseudo-micellar arrangement confers even to hydrophilic domains. The higher content of aromatic carbons (110–160 ppm) found in the LS spectra was attributed partly to the high degree of substitution of the aromatic ring that slows down cross-polarization in CPMAS experiments and partly to the relative overestimation of this region by LS-NMR due to a lack of signal in the aliphatic interval. The slightly lower content of carboxyl carbons estimated in CPMAS spectra as compared to LS spectra was also attributed to slow cross-polarization. This work shows that the combined use of both NMR techniques is profitable in conformational analysis of humic substances and of dissolved organic matter in general.
In calcareous parent material, pedogenic carbonate formation mostly involves dissolution and recrystallization of lithogenic carbonates with CO2 of soil air, leading to a complete exchange of lithogenic carbon with soil-derived carbon. Interest in pedogenic carbonates has increased in recent decades because they are a useful tool for reconstructing paleoclimatic conditions (δ13C and δ18O) and past atmospheric CO2 concentrations as well as for radiocarbon dating of soils. For such investigations, the recrystallization rate of primary CaCO3 by pedogenic carbonate formation and the dependence of the recrystallization rate on environmental factors are essential, but still unquantified factors.The recrystallization rate of primary CaCO3 of loess at three CO2 concentrations was estimated by isotopic exchange between primary CaCO3 and the 14C of artificially labeled CO2. Loess was used for the study as a parent substrate for soil formation to simulate initial rates of CaCO3 recrystallization. CO2 concentrations of 380 ppm, 5000 ppm and 50,000 ppm lead to recrystallization rates of 4.1 · 10− 7 day− 1, 8.1 · 10− 7 day− 1 and 16.9 · 10− 7 day− 1, respectively. The relation between CO2 concentrations and recrystallization rates was described by a saturation curve. Under the tested experimental conditions, complete (95%) recrystallization of loess carbonate and formation of pedogenic carbonate would take 4.9–20.0 · 103 years, strongly depending on CO2 concentration. We expect faster recrystallization rates under field conditions because of permanent CO2 supply by root and rhizomicrobial respiration. This impedes the equilibrium between the inorganic C pools in solid, liquid and gaseous phases.
Chemically and physically fractionated samples extracted from the surface horizon of a soil developed under a mix of coniferous and deciduous vegetation in southwestern Colorado were studied. 13C NMR data on this soil's organic matter and its HF(aq)-washed residue, as well as the classic acid/base-separated humic fractions (humic acid, fulvic acid, humin), were examined for chemical–structural detail, e.g., the various structural functionalities present (especially lipids, carbohydrates, aromatics, polypeptides and carbonyl/carboxyls). Among the humic fractions, it was found that the lipid concentrations are in the order humic acid>fulvic acid= humin; for carbohydrates the order is fulvic acid>humin>humic acid; for aromatic carbons the order is humic acid>humin>fulvic acid; for polypeptides it is humic acid>fulvic acid>humin and for carbonyl/carboxyl species it is humin>humic acid>fulvic acid, but the differences are small. 13C spin–lattice relaxation times indicate that at least two types of “domains” exist in each, corresponding to “higher” and “lower” concentrations of paramagnetic centers, e.g., Fe3+.
The importance of soil organic matter functions is well known, but structural information, chemical composition and changes induced by anthropogenic factors such as tillage practices are still being researched. In the present paper were characterized Brazilian humic acids (HAs) from an Oxisol under different treatments: conventional tillage/maize-bare fallow (CT1); conventional tillage/maize rotation with soybean-bare fallow (CT2); no-till/maize-bare fallow (NT1); no-till/maize rotation with soybean-bare fallow (NT2); no-till/maize-cajanus (NT3) and no cultivated soil under natural vegetation (NC). Soil HA samples were analyzed by electron paramagnetic resonance (EPR), solid-state 13C nuclear magnetic resonance (13C NMR), Fourier transform infra-red (FTIR) and UV-Vis fluorescence spectroscopies and elemental analysis (CHNS). The FTIR spectra of the HAs were similar for all treatments. The level of semiquinone-type free radical determined from the EPR spectra was lower for treatments no-till/maize-cajanus (NT3) and noncultivated soil (1.74×1017 and 1.02×1017 spins g−1 HA, respectively), compared with 2.3×1017 spins g−1 HA for other soils under cultivation. The percentage of aromatic carbons determined by 13C NMR also decreases for noncultivated soil to 24%, being around 30% for samples of the other treatments. The solid-state 13C NMR and EPR spectroscopies showed small differences in chemical composition of the HA from soils where incorporation of vegetal residues was higher, showing that organic matter (OM) formed in this cases is less aromatic. The fluorescence intensities were in agreement with the percentage of aromatic carbons, determined by NMR (r=0.97 P<0.01) and with semiquinone content, determined by EPR (r=0.97 P<0.01). No important effect due to tillage system was observed in these areas after 5 years of cultivation. Probably, the studied Oxisol has a high clay content that offers protection to the clay–Fe–OM complex against strong structural alterations.
Nuclear magnetic resonance (NMR) is a valuable tool for the characterization of soil organic matter and humification processes in soils. This review highlights soil organic matter studies based mainly on solid-state 13C and 15N NMR spectroscopy and some emerging applications, that may provide significant progress in our knowledge on soil organic matter. A major advantage of Nmr spectroscopy is that it can be used as a non-invasive method for solid soil samples or soil fractions. Although resolution is limited, one can obtain an overview on the organic matter structures present in the soil sample. Application of 13C and 15N NMR to soils has, for a long time, been confined to the study of bulk soils or humic extracts for structural characterization. The transformations of soil organic C and N are now being investigated after addition of 13C- and 15N-labelled parent materials to the soil and following their evolution in different C and N pools. With labelling techniques it is also possible to study the interaction of organic pollutants with soil organic matter. Contamination of a soil with man-made additives, such as soot or brown coal dust, can also be detected in soils or individual soil fractions.
Soil samples were collected from the surface mineral horizon (Ah horizon) of four adjacent soils (sites I, II, III, IV) and one remote soil (site V) derived from volcanic ash in Japan. The four adjacent sites were managed as Miscanthus sinensis grassland for several hundred years by the use of annual burning to prevent the regrowth of native forest species. At site I, annual burning was still being practiced when soil samples were collected; however, at sites II, III and IV annual burning to maintain grassland vegetation ceased about 20–30, 40–50 and more than 100 years ago, respectively, and the sites were left to return to forest. At site V, a mature, broad-leaf, deciduous forest established by natural regeneration existed. The influence of annual burning and vegetative cover on the chemistry of the organic materials contained in the whole soil, the < 53 μm soil fraction, the residues remaining after photo-oxidation of the < 53 μm soil fraction, and the humic acid fraction present at each of the five sites was examined using solid-state 13C NMR. On site I, where grasses were still burned annually, SOM and humic acid fraction contained a greater proportion of aromatic and carbonyl carbons compared to the other sites. Alkyl carbon made a relatively small (19%) contribution to the composition of SOM on site I. When grassland was invaded by forest, the chemical nature of the SOM and humic acid fraction changed. The greatest changes occurred during the first 20–30 years, after which changes in the chemistry of SOM and humic acid fraction were of a smaller magnitude. The changes in SOM chemistry included a decrease in aromatic and an increase in alkyl carbon contents, indicating that SOM produced under forest was richer in alkyl carbon than that produced under grasses managed with annual burning. The SOM at the remote site, under deciduous forest (site V), was highly aliphatic in nature with alkyl carbon contributing 35% of the total soil carbon. Application of a proton-spin relaxation editing (PSRE) procedure to the SOM of each site indicated heterogeneity within the SOM structures, and subspectra of carbon associated with slower- and faster-relaxing protons were derived. Subspectra of the slowly-relaxing fractions from sites I, II and III were similar and resembled spectra of partly decomposed plant materials. The fast-relaxing subspectrum from site I contained a strong central resonance at 130 ppm and a small peak at 176 ppm, and was very similar to spectra obtained for charcoal and charred residues. The fast relaxing fractions from other sites included less aromatic carbon and had some O-alkyl materials. The Bloch decay spectrum of SOM from site I showed more aromatic and carbonyl carbons than the CP/MAS spectrum and highlighted an important limitation of the CP/MAS technique when it is applied to SOM containing charcoals or charred plant residues.
Soil organic matter plays an important role in soil properties and influences ecosystem cycles of C, N, Al, Fe, and other major and trace elements. We examined spatial variations in the structure and chemistry of soil organic matter at the Hubbard Brook Experimental Forest in New Hampshire, USA. Humic substances were extracted and isolated chromatographically into humic acid, fulvic acid, and polysaccharide fractions. Chemical methods and solid-state 13C NMR spectroscopy were used to determine structural chemistry. On average, extractable humic substances accounted for nearly 50% of soil organic matter, with alkyl and O-alkyl C (carbohydrate) being the largest C fractions in whole soils and isolated humic substances. Alkyl C ranged from 33% to 56% of C, while O-alkyl C comprised 20–45% of C. Alkyl C increased, while O-alkyl C decreased with soil depth in whole soils, humin, and humic acid. Aromatic C increased with soil depth in whole soils and humin, while carbonyl C increased with depth in whole soils and fulvic acids. Fulvic acids were more acidic than humic acids, and were less phenolic and aliphatic than humic acids. Carboxylic acidity accounted for about 80% and 50% of total acidity in fulvic acid and humic acid, respectively. Soil from higher-elevation sites exhibited greater alkyl C and lower O-alkyl and aromatic C in the Oa horizon, suggesting a greater degree of decomposition of the organic matter in the Oa horizon of these conifer-rich sites. Mineral soils in conifer-rich sites contained organic matter that was more aromatic than in hardwood sites. Variations in humification processes, source materials, and transport of organic matter could account for variations in the structure and chemistry of organic matter in these forest soils.
Differences in 13C inputs between two adjacent Canadian mixed grass prairie communities were used to estimate turnover of two soil organic matter fractions with increasing depth below the surface. Partial cultivation and abandonment of a C4-dominated grassland ca. 1930, and natural recovery to a native C3-dominated grassland changed the 13C/12C ratio (reported in units of δ13C‰) of plant residue inputs to soil organic matter. After 70 years, macro-organic matter carbon (MOMC) differed up to 5.6‰, and fine soil organic carbon (FSOC) by 2.9‰ near the surface; differences declined to a depth of 1 m. Mean residence time estimates differed among fractions from a short 35 years for MOMC 0–5 cm, to a longer 413 years for FSOC 5–10 cm. The background carbon pool on this site was substantial, and the minimum change in soil carbon of 10.29 Mg C ha−1 over 70 years represented less than 7% of the carbon pool to a 1-m depth. Considerable vertical and horizontal variation in soil organic carbon mass and 13C content were observed. Variation due to small sample size was a more substantial limitation than cumulative analytical variation for calculating turnover estimates >500 years or at depths >20 cm. Corrections for bias were also necessary since imperfect physical separation of soil organic matter fractions, or CO32− contamination, were issues.
Soil samples from the surface mineral horizons (Ah) of two adjacent sites (sites I and III) and one remote site (site V), derived from volcanic ash in Japan, were collected and separated into fractions with densities < 1.0 free, < 1.6 free, < 1.6 occluded, 1.6–1.8, 1.8–2.0 and > 2.0 Mg m−3. The terms free and occluded were used to indicate density fractions in which organic materials weakly associated with soil mineral particles resided external to or within soil aggregates, respectively. The studied sites were under different vegetative covers and had different burning histories. Sites I and III were managed as grassland for several hundred years by the use of annual burning to prevent the regrowth of native forest. At site I, annual burning of Japanese pampa grass (Miscanthus sinensis) was still occurring. However at site III vegetation burning was stopped more than 100 years ago and the site was left to return to forest. At site V a mature, broad leaf deciduous forest maintained by natural regeneration existed.
The investigation of the chemical composition of soil organic matter (SOM) in Ferralsols by means of solid-state cross-polarization magic angle spinning (CPMAS) 13C nuclear magnetic resonance (NMR) spectroscopy is limited by their high iron oxide concentration and their low organic carbon content. In order to circumvent those limitations, such samples are often treated with hydrofluoric acid (HF) to remove paramagnetic material and to concentrate the amount of SOM. The main objectives of this study were to elucidate the impact of this approach on the resolution of the CPMAS 13C NMR spectra and on the chemical composition of the SOM in the A and B horizons of four Brazilian Ferralsols. Therefore, those soils were subjected to up to eight successive treatments with 10% (w/w) HF. Each of those extractions resulted in an enhancement of the C content of the samples. A relationship between mass loss caused by the treatment and texture and mineralogy was observed. However, high losses of carbon occurred during the HF treatments, particularly in the B horizons, but no consistent alterations in the distribution of carbon functional groups were determined by CPMAS 13C NMR, suggesting that preferential loss of specific carbon groups was not induced. The concentration of total and dithionite extractable Fe increased after two treatments for most of the samples. This may be best explained by the preferential dissolution of silicate leading to a selective enrichment of iron containing minerals. After four treatments, the Fe concentration declined considerably. After the second treatment, the spectral resolution improved. Considering the obtained results, it can be confirmed that the efficiency of the HF treatment is rather controlled by the number than by the duration of the extraction. We suggest that, for A horizons of Ferralsols, four repetitions of HF treatment are sufficient to yield well-defined spectra. For their B horizons, on the other hand, the CPMAS 13C NMR spectra obtained after four HF extractions were of poor quality, indicating that for these soils at least eight HF treatments have to be performed to acquire reasonable spectra.
The particle-size separation of subalpine soils was used for delineating the different functional C pools of soil organic matter (SOM) by solid-state cross-polarization/magic-angle-spinning (CP/MAS) 13C NMR spectroscopy. The soils collected from hemlock forest and dwarfed bamboo grassland along the slope positions at elevations from 2550 to 3200 m in central Taiwan, received high precipitation (>3000 mm). The obtained data show that organic matter fractions differ according to particle-size distribution and vegetations. Being more aliphatic, clay-size fractions were significantly different from bulk soils. Sand-size fractions generally gave very similar results to bulk soils. Alkyl-C (mainly polymethylene) increased from coarse sand (>250 μm) to clay (<2 μm) particles size in all sites, suggesting an accumulation of recalcitrant material in fine particle-size fraction. O-Alkyl-C (mainly carbohydrate) content decreased from coarse to fine particle size under the hemlock forest, whereas no consistent trend was found in dwarfed bamboo grassland soils. High amounts (>35%) of O-alkyl-C in all particle-size fractions in dwarfed bamboo grassland soils were attributed to that of the linkage of phenolic acids to cell-wall polysaccharides in bamboo litter. Based on a humification index of alkyl/O-alkyl-C, the humification degrees in particle-size fractions are in the order: clay>fine silt>coarse silt>fine sand>coarse sand in both of hemlock forest and dwarfed bamboo grassland soil. The alkyl/O-alkyl-C ratios in hemlock forest soil were consistently higher than those in dwarfed bamboo grassland soils, suggesting that humification of SOM is higher in the forest soil than in the dwarfed bamboo grassland soils. The 13C NMR analysis and the change of C/N ratio of the particle-size fractions indicate that the accumulation of recalcitrant material in fine particle size favors alkyl-C rather than aromatic-C in these subalpine soils. The low content of aromatic-C in soil might be due to the high precipitation in this subalpine area.
14C activity measurements have been used in soil science for studying soil organic matter (SOM) dynamics as well as to separate carbon pools of different age. In this study, we used 14C activity measurements to quantify the contribution of organic carbon derived from lignite to the total carbon content of soils containing lignite and recently formed organic carbon in mixture. Samples were taken from the forest floor and mineral soils of a chronosequence of rehabilitated mine soils under pine forests (planted in 1966, 1981 and 1987). They were analyzed for their carbon content and 14C activity.All mineral soils show very old 14C ages. Old radiocarbon age is generated by contribution of lignite, which does not contain 14C. The lignite content of a sample was calculated by correcting the measured 14C activity for the 14C activity level of the recent plant-derived material contributing to SOM. The 14C activity of the recent carbon component was estimated in each stand by conducting a time series model using the Rothamsted model of soil carbon dynamics. The 14C activity of recent SOM was rather insensitive (±4% relative) to the tested hypotheses on carbon dynamics in these young forest stands and allowed a precise estimate of the proportion of recent carbon. Lignite accounted for 80% to 93% of total carbon in the mineral topsoil (0–5 cm). The stock of recent soil carbon had a maximum increase between 20 and 32 years, and reached 36±9 t/ha after 32 years, a value close to the current stock under ancient forests in the area.
14C-isoproturon residues were incorporated in wheat plants by growing seedlings for 18 days in quartz sand with nutrient solution which was treated with ring-labeled 14C-isoproturon, resulting in 14C-concentration equivalent to 15.4 nmol isoproturon per g dry shoot mass. The residues were characterized by extraction and HPLC-analysis, and were shown to consist of unchanged isoproturon, soluble metabolites (monodemethyl-isoproturon, didemethyl-isoproturon, 1-OH-isoproturon, 2-OH-isoproturon, 2-OH-monodemethyl-isoproturon, 2-OH-didemethyl-isoproturon, isopropenyl-isoproturon and unidentified metabolites), as well as nonextractable residues. Dried plant samples containing these residues were mixed with soil samples originating from different farming systems, and mineralization to 14CO2 was determined in a closed aerated laboratory system. In addition, the microbial biomass and bioactivity of soils were estimated by determination of substrate-induced heat output, basal heat output, metabolic heat quotient, total adenylate content and adenylate energy charge. Significant positive correlations between 14CO2 production or adenylate content and microbial biomass were observed in three soils; 14CO2 production and total microbial biomass were highest in soil samples from organic farming. Soil samples from a former hops plantation contaminated with copper from previous fungicide applications did not fit this correlation, but exhibited a higher mineralization capacity per unit of microbial biomass. Our results indicate that general soil microbial parameters in many cases are insufficient to describe the influence of biotic factors on the fate of pesticides in soil.
The role of DOC for the build-up of soil organic carbon pools is still not well known, but it is thought to play a role in the transport of carbon to a greater depth where it becomes more stable. The aim of this study was to elucidate within-year dynamics of carbon transport from litter to the O (Oe and Oa) and A horizons. Mesocosms with constructed soil profiles were used to study dynamics of C transport from 14C-enriched (about 1000‰) leaf litter to the Oe/Oa and A horizons as well as the mineralization of leaf litter. The mesocosms were placed in the field for 17 months during which time fluxes and 14C content of DOC and CO2 were measured. Changes in 14C in leaf litter and bulk soil C pools were also recorded. Significant simultaneous release and immobilization of DOC occurring in both the O and A horizons was hypothesized. Contrary to our hypothesis, DOC released from the labeled Oi horizon was not retained within the Oe/Oa layer. DOC originating in the unlabeled Oe/Oa layer was also released for transport. Extensive retention of DOC occurred in the A horizon. DOC leaching from A horizon consisted of a mix of DOC from different sources, with a main fraction originating in the A horizon and a smaller fraction leached from the overlaying horizons. The C and 14C budget for the litter layer also indicated a surprisingly large amount of carbon with ambient Δ14C-signature to be respired from this layer. Data for this site also suggested significant contributions from throughfall to dissolved organic carbon (DOC) transport into and respiration from the litter layer. The results from this study showed that DOC retention was low in the O horizon and therefore not important for the O horizon carbon budget. In the A horizon DOC retention was extensive, but annual DOC input was small compared to C stocks and therefore not important for changes in soil C on an annual timescale.
The analyses of stable isotope ratios of carbon (δ¹³C) and nitrogen (δ¹⁵N) of soil organic matter (SOM) is an increasingly used tool to estimate soil carbon turnover, to assess degree of soil development, and to study historical C3/C4 vegetation changes. However, the exact processes that control ¹³C- and ¹⁵N-enrichment of SOM within a soil profile are still not clearly identified. To better understand the isotopic processes associated with decomposition of SOM, we studied two Vertisol profiles and one Oxisol profile from southern Queensland by radiogenic (¹⁴C), stable isotopic (δ¹³C, δ¹⁵N), and spectroscopic (¹³C-NMR and FTIR) methods. The findings of this study demonstrate that fundamental differences exist in δ¹³C and δ¹⁵N fractionation dynamics in different soil types and that isotopic fractionation is highly influenced by soil chemistry, mineralogy, and type of organic matter input. Stable isotopic analyses of the Oxisol show the typically observed increase in δ¹³C and δ¹⁵N in the subsurface horizon whereas the Vertisols show consistently decreasing values with depth. The high degree of ¹³C-enrichment in the Oxisol compared with the Vertisols cannot be simply explained with increased fractionation due to soil age, as the ¹⁴C age of the Vertisols is greater and increases more rapidly with depth, compared with that of the Oxisol.
Information on the mean residence time (MRT) of soil organic carbon (SOC) on different soil types and management regimes is required for pedo-geological, agronomic, ecological and global change interpretations. This is best determined by carbon dating the total soil together with acid hydrolysis and carbon dating of the non-hydrolyzable residue (NHC). Midwestern US soils in a transect from Lamberton, MN to Kutztown, PA were found to contain from 33% to 65% of their SOC in the non-hydrolyzable fraction. Soils on lacustrine deposits had the most NHC; glacial till and shale soils, the least. The MRTs of the SOC of surface horizons of soil ranged from modern to 1100 years with an average of 560 years. The MRT increased to an average of 1700 years in the 25–50-cm depth increment and 2757 years at 50–100 cm. The NHC was 1340 years greater at the surface and 5584 years at depth. The MRTs of the total SOC were inversely correlated to sand and directly related to clay content. Silt did not have a significant effect on the MRT of total SOC, but was significantly correlated with the MRT of the NHC. A four-parameter model described the relationship between the SOC content and MRT. The complexity of this equation reflected the strong effect of depth, which greatly decreased SOC while increasing the MRT. The MRT of these soils, as determined with carbon dating of the naturally occurring 14C, was compared to that measured with the 13C signal produced by approximately 30 years of continuous corn (Zea mays L.) (C4) on soils with a known plant history of C3–C4 cropping. The equation of 14C MRT=176(13CMRT)0.54 with an R2 of 0.70 showed that although short-term 13C studies correlate well with the total MRT, they reflect the dynamics of the active and slow pools, not the total SOC.
In Early Holocene, Chernozems were assumed to have covered the entire loess landscape of the Lower Rhine basin—today mirrored by the distribution of Luvic Phaeozems. These Luvic Phaeozems have characteristic dark brown (Bht) horizons accumulating clay and humus, inherited and translocated from their precursors Chernozem black humic A horizons. We examined Luvic Phaeozems along a 33-km-long and 2.0–2.5-m-deep gas pipeline trench in the Lower Rhine Basin, west of Cologne. Along this transect we discovered clusters of hundreds of regularly shaped pits. These pits were always connected to the Bht horizons of adjacent Luvic Phaeozems. The Luvic Phaeozem horizons and the pits were investigated by combining methods from (geo-) archaeology (geographical distribution within the landscape, shape of the pits, soil texture), geochemistry (content of carbon, nitrogen and black carbon), palaeobotany (species determination of charcoals) and AMS 14C measurements.
Deamination as well as decarboxylation of amino acids and their polycondensation with polyphenols catalyzed by Mn(IV) oxide have been reported. However, the mechanisms of cleavage of amino acids and their polycondensation with polyphenols by soil inorganic components are not well understood. The objective of this study was, thus, to investigate the enhanced abiotic decarboxylation of carboxyl group and dealkylation of alkyl group of glycine by birnessite (δ-MnO2, 0.2–2 μm) both in the presence and absence of pyrogallol and the resultant formation of humic polycondensates in systems free of microbial activity. The degradation of 14C-labeled glycine through either carboxyl or alkyl group on the birnessite surface was demonstrated. Birnessite enhanced the dealkylation of alkyl C and especially decarboxylation of carboxyl C of glycine. Furthermore, it promoted the interaction of glycine and pyrogallol and the resulting simultaneous decarboxylation and dealkylation of glycine, ring cleavage of pyrogallol, and polycondensation. Carboxyl C and especially alkyl C of glycine were incorporated into the humic polycondensates. This study indicates that structural configuration and functionality of amino acids and polyphenols merits closer attention in understanding mechanisms of humification processes catalyzed by soil mineral colloids.
A high CO2 concentration in soil resulting from microbial and root respiration is the main factor controlling the dissolution of primary (lithogenic, geogenic) carbonates and the formation of secondary (pedogenic) carbonates. Although several estimations of soil age and many paleo-environmental reconstructions are based on the radiocarbon age and/or δ13C of secondary carbonates, many assumptions are difficult to check experimentally because of long-term CaCO3 re-crystallization processes. In the present study we used the isotopic exchange between primary carbonates of loess and 14C respired from the rhizosphere of wheat that was artificially labeled in a 14CO2 atmosphere under controlled conditions. An ascending number of 14CO2 pulses (1 … 4) showed a linear increase of rhizosphere 14C recovered in the CaCO3 of loess. Based on this connection, the initial re-crystallization rates of loess carbonate were calculated by linear regression: for loess containing 27% CaCO3, the initial rate of carbonate re-crystallization was 0.000029 day−1. Subsequently, using linear and exponential approaches with different lengths of growing season, we extrapolate the observed CaCO3 re-crystallization on longer time periods. The calculations show that at least 100 years, but probably between 400 and 2000 years, are necessary for full (99%) re-crystallization of the CaCO3 of loess. We suggest a general equation for calculating the remaining not re-crystallized CaCO3 depending on time of soil formation (t): %CaCO3 (t) = 100·exp(−t·0.00078·Growing-Season-Length/365/initial-CaCO3-percentage). Different approaches for calculating the period of secondary carbonate re-crystallization are discussed and compared with literature data. We conclude that despite the high analytical precision of radiocarbon dating and δ13C mass spectrometry of secondary carbonates (used, e.g. for paleo-environmental reconstructions), the methodological resolution cannot be better than the periods necessary for CaCO3 re-crystallization.