Soil Soil Organic Carbon Content Research Articles

Changes in soil organic carbon and microbial community in saline soil following different forms of straw incorporation

AbstractStraw has been commonly incorporated into soil to increase soil organic carbon (SOC) content. However, the response and mechanism of organic carbon transformation following the addition of different forms of straw to saline soil to have not been fully evaluated. Thus, in our study, chopped wheat straw, granular wheat straw and wheat straw biochar were used as exogenous organic amendments. The carbon‐to‐nitrogen ratio of each form of straw was adjusted to 25:1. Under the same amount of carbon input, the three forms of straw were added to saline soil for a one‐year incubation experiment. The research focused on studying the changes in SOC transformation, enzyme activity, and microbial community structure. At the end of incubation, both granular and biochar straw treatments had significantly increased SOC relative to the unamended control, with extent of SOC increase greater in the biochar straw treatment. The biochar straw treatment also had significantly higher dissolved organic carbon than the other treatments at end of the incubation. The enzyme activity of β‐cellobiohyrolase (CBH) was the highest in the granular straw treatment, which was increased by 71.9% compared with the chopped straw treatment (p < 0.05). The cumulative carbon mineralization amount (Cum C) in the granular straw treatment was the highest, which was increased by 757.4% and 21.3% compared with the biochar straw and chopped straw treatments, respectively (p < 0.05). The Mantel test analysis further revealed significant correlations between soil bacterial community and SOC content in soils treated with different straw incorporation forms, as well as between fungal community and extracellular enzyme activities. We assumed that a possible reason for the high SOC mineralization amount in the granular straw treatment was changing the structure of the Bacteroidota in the bacterial phylum and Blastocladiomycota in the fungal phylum. Based on these findings, it is believed that adding biochar can increase the SOC content, but its utilization by microorganisms is limited. However, applying granular straw will not have a negative impact on its use as a microbial carbon source, thereby promoting carbon transformation. Therefore, in future research aimed at enhancing soil fertility in saline soil regions, the adoption of granular straw as an agricultural practice is recommended and should be promoted.

Sentinel-1 Imagery Used for Estimation of Soil Organic Carbon by Dual-Polarization SAR Vegetation Indices

Despite optical remote sensing (and the spectral vegetation indices) contributions to digital soil-mapping studies of soil organic carbon (SOC), few studies have used active radar remote sensing mission data like that from synthetic aperture radar (SAR) sensors to predict SOC. Bearing in mind the importance of SOC mapping for agricultural, ecological, and climate interests and also the recently developed methods for vegetation monitoring using Sentinel-1 SAR data, in this work, we aimed to take advantage of the high operationality of Sentinel-1 imaging to test the accuracy of SOC prediction at different soil depths using machine learning systems. Using linear, nonlinear, and tree regression-based methods, it was possible to predict the SOC content of soils from western Bahia, Brazil, a region with predominantly sandy soils, using as explanatory variables the SAR vegetation indices. The models fed with SAR sensor polarizations and vegetation indices produced more accurate results for the topsoil layers (0–5 cm and 5–10 cm in depth). In these superficial layers, the models achieved an RMSE in the order of 5.0 g kg−1 and an R2 ranging from 0.16 to 0.24, therefore explaining about 20% of SOC variability using only Sentinel-1 predictors.

Spatial variations in soil erodibility induced by rock outcropping on sloping cropland in the karst region of Southwest China

Sloping croplands in karst areas of Southwest China usually contain a multitude of rock outcrops with diameters larger than 25 cm. Rock outcrops and spatial location have substantial effects on soil properties and thus likely affect soil erodibility. Moreover, rock outcropping is a indicator of rocky desertification and has an important impact on soil erosion and soil formation processes. However, the potential effects of spatial location and rock outcrops on the soil erodibility of sloping cropland in karst areas are still unclear. This study was performed to explore the variations in soil erodibility reflected by saturated hydraulic conductivity (KS), mean weight diameter (MWD), soil structural stability (SSI), soil erodibility factor (K) and soil organic carbon content (SOC) at three spatial locations in sloping cropland with rock outcrops on the karst hillside. The results showed that the soil erodibility indicators KS, MWD, SSI and SOC declined with increasing soil depth for all tested sites. SSI and SOC were predominant factors contributing to the variation in soil erodibility, which were 34.9 % and 35.1 % at the upper spatial location, 31.4 % and 30.9 % at the middle spatial location, and 32.1 % and 32.6 % at the lower spatial location, respectively. There were significant variations in the K factor between the sloping cropland with rock outcrops and the control group. The variations in soil erodibility indicators with rock outcrops and spatial locations were attributed to the changes in soil particle distribution, soil bulk density and SOC. There was a significant linear correlation between the K factor and spatial location, slope gradient, coverage and height of rock outcrops. These results indicated that rock outcrops and their characteristics can reduce soil erosion intensity by redistributing precipitation and runoff, thus further affecting infiltration and soil erodibility. These findings are helpful to comprehend the processes and mechanisms of the spatial variation in soil erosion on sloping cropland with rock outcrops.

Assessing a VisNIR penetrometer system for in-situ estimation of soil organic carbon under variable soil moisture conditions

Soils are one of the main terrestrial carbon sinks and contain more carbon than the combined total in the atmosphere and terrestrial vegetation. A major challenge is quantifying how much carbon is stored in the soil and how much it has been sequestered due to changed management practices. In this study, a visible and near-infrared (VisNIR) penetrometer system was developed to measure in-situ soil VisNIR reflectance spectra to a depth of 90 cm into the soil profile. A spectral library of VisNIR soil spectra collected in air-dry and ground states were used to calibrate Cubist regression models for estimating soil organic carbon (SOC) contents. The External Parameter Orthogonalisation (EPO) transformation was applied to the in-situ spectra to remove soil moisture and other in-situ effects from the spectra. This penetrometer system was used to estimate SOC contents at three sites on three different days under variable soil moisture states. The penetrometer EPO performed better compared to the EPO constructed from moist cores. After applying EPO transformation matrix, soil wetness did not have any influence on soil SOC content estimations. The SOC contents were estimated using the in-situ VisNIR system were validated against lab-measured SOC content with mean R 2 , RMSE, and bias of 0.88, 0.32%, and 0.15%, respectively. It is concluded that the VisNIR penetrometer can potentially be used to measure SOC content rapidly and cost-effectively in-situ with a fine depth resolution. • A penetrometer system was used to acquire in-situ soil VisNIR spectra. • A spectral library from agricultural regions of NSW was used for calibration. • External Parameter Orthogonalisation (EPO) reduced confounding soil moisture effect. • External validation was performed to measure soil organic carbon contents.

Effects of Straw Mulching Thickness on the Soil Health in a Temperate Organic Vineyard

Soil mulching is one of the common measures applied in organic agricultural production which could replace plastic films and protect the environment. In order to fully evaluate the effects of different straw mulching thicknesses on soil health, maize straw was mulched with the thicknesses of 0 cm (CK), 2 cm, 4 cm and 6 cm on soil surface to assess the effects on soil temperature (ST) and moisture (SM), soil pH, soil organic carbon (SOC), total nitrogen (TN), C/N, soil aggregates and soil bulk density (SBD) in a temperate organic vineyard. We found that straw mulching had a significant regulating effect, with soil moisture being elevated with increasing mulching thickness by 5.8%, 9.0% and 11.1% compared with CK. The soil SOC content increased by 3.0%, 2.4% and 2.3%. Although soil pH and C/N significantly (p < 0.05) increased, they fluctuated with increasing mulch thickness. Straw mulching also increased the content of >2 mm soil particle size and elevated the mean weight diameter (MWD) and geometric mean diameter (GMD). The increasing mulching thickness prolonged the effect on the stability of soil aggregates. The 4 cm maize straw mulching thickness has the best effect for ecologically and environmentally managing warm-temperate organic vineyards so it may have a great application prospect on a global scale.

Soil Structural Quality and Relationships With Root Properties in Single and Integrated Farming Systems

Single farming systems (SFS) such as monocultures may negatively affect soil structural quality. This study tested the hypothesis that integrated farming systems (IFS), i.e., the combination of cropping and forestry and/or livestock farming, improves soil structural quality, root development and soil organic carbon. An experimental area was set up in 2012 at the Canguiri experimental farm belonging to the Federal University of Paraná, Southern Brazil. The soils are predominantly Ferralsols. The experimental treatments representing different farming systems, organized in a random block design with three replicates, were: Forestry (F), Conventional Crop Production (C), Livestock (L), and integrated Crop-Forestry (CF), Crop-Livestock (CL), Livestock-Forestry (LF), and Crop-Livestock-Forestry (CLF). In situ measurements and sampling were carried out in the 0–0.3 m layer during summer 2019/20, and included soil penetration resistance (PR), soil structural quality based on visual evaluation of soil structure (SqVESS scores), root length (RL), root volume (RV) and soil organic carbon content (SOC). Soil structural quality, penetration resistance, root length and volume, and SOC varied between farming systems, but no significant differences were found between single (C, L, F) and integrated farming systems (CF, CL, LF, CLF). The single system Forestry (F) and the integrated systems including forestry (LF, CF, CLF) tended to have higher SqVESS scores, i.e. poorer soil structural quality, and higher PR, which we associate with the generally drier soil conditions that are due to higher soil water uptake and higher interception and reduce the frequency of wetting-drying cycles. Roots were concentrated in the shallow soil layer (0–0.1 m depth), and this was especially pronounced in the Crop (C) single farming system. Based on the measured values, our results suggest an acceptable soil structural quality in all farming systems. Our data revealed strong, significant relationships between soil structural quality, penetration resistance, root growth and SOC, demonstrating that improvements in soil structure results in lower soil penetration resistance, higher root volumes and higher SOC, and vice versa. Soil PR was positively correlated with SqVESS (R2 = 0.84), indicating that better soil structural quality resulted in lower soil mechanical resistance. This, in turn, increased root length and volume, which increases carbon input to soil and therefore increases SOC in the long run.

Distribution and storage of soil organic and inorganic carbon in steppe riparian wetlands under human activity pressure

Soil organic carbon (SOC) and soil inorganic carbon (SIC) are key components of the global wetland soil carbon pool, which plays a crucial role in carbon cycling. However, research on carbon storage in riparian wetland soils, especially in inland steppe river environments impacted by human activities, is relatively scarce. Thus, we evaluated the SOC and SIC distributions and storage in riparian wetland soils under the pressure of human activities in the Xilin River Basin (XRB). We collected surface and profile soil samples, determined the SOC and SIC contents, aboveground biomass, and soil physicochemical properties, and calculated the SOC and SIC storage (SOCs and SICs) values. The surface soil SOC content decreased substantially from the upstream to the downstream zones (mean value range: 76.30–3.18 g/kg), and the SIC content showed the opposite trend (mean value range: 0.08–34.38 g/kg). The SIC content of the riparian wetlands along the permanently flowing stretch of river was much lower than that in the wetlands along the intermittently flowing stretch. In the XRB, the SOCs was primarily affected by vegetation coverage, soil water content, and soil pH, whereas the SICs was greatly affected by soil texture. A dry lake zone was markedly affected by wetland degradation, indicating a potential increase in the decomposition rate of the surface soil SOC; however, the SOC in the deep soils was relatively stable. The high SIC content most likely resulted from weathering, resuspension, and carbonate rock reprecipitation. Compared to different steppe grassland types, riparian wetlands appear to be potential hotspots of SOCs and SICs in the Inner Mongolian region. Riparian wetland SOCs in the upstream zones was substantially greater than in adjacent terrestrial locations. However, the SOCs in the downstream wetlands was similar to that in the grasslands. Under the impact of human activities, the water and soil environments in the downstream zone of the Xilin River were substantially modified, leading to severe wetland degradation, which most likely caused the carbon storage function of the riparian wetlands to weaken and carbon loss to accelerate. The mean SOCs in the riparian wetland soils decreased from 33.3 kg/m2 in the upstream area to 16.2 kg/m2 downstream, and the mean SICs increased from 2.3 kg/m2 to 34.6 kg/m2. Therefore, urgent measures are needed to restore the carbon sink function of river wetlands for long-term climate change mitigation.

Accuracy and Reproducibility of Laboratory Diffuse Reflectance Measurements with Portable VNIR and MIR Spectrometers for Predictive Soil Organic Carbon Modeling.

Soil spectroscopy in the visible-to-near infrared (VNIR) and mid-infrared (MIR) is a cost-effective method to determine the soil organic carbon content (SOC) based on predictive spectral models calibrated to analytical-determined SOC reference data. The degree to which uncertainty in reference data and spectral measurements contributes to the estimated accuracy of VNIR and MIR predictions, however, is rarely addressed and remains unclear, in particular for current handheld MIR spectrometers. We thus evaluated the reproducibility of both the spectral reflectance measurements with portable VNIR and MIR spectrometers and the analytical dry combustion SOC reference method, with the aim to assess how varying spectral inputs and reference values impact the calibration and validation of predictive VNIR and MIR models. Soil reflectance spectra and SOC were measured in triplicate, the latter by different laboratories, for a set of 75 finely ground soil samples covering a wide range of parent materials and SOC contents. Predictive partial least-squares regression (PLSR) models were evaluated in a repeated, nested cross-validation approach with systematically varied spectral inputs and reference data, respectively. We found that SOC predictions from both VNIR and MIR spectra were equally highly reproducible on average and similar to the dry combustion method, but MIR spectra were more robust to calibration sample variation. The contributions of spectral variation (ΔRMSE < 0.4 g·kg−1) and reference SOC uncertainty (ΔRMSE < 0.3 g·kg−1) to spectral modeling errors were small compared to the difference between the VNIR and MIR spectral ranges (ΔRMSE ~1.4 g·kg−1 in favor of MIR). For reference SOC, uncertainty was limited to the case of biased reference data appearing in either the calibration or validation. Given better predictive accuracy, comparable spectral reproducibility and greater robustness against calibration sample selection, the portable MIR spectrometer was considered overall superior to the VNIR instrument for SOC analysis. Our results further indicate that random errors in SOC reference values are effectively compensated for during model calibration, while biased SOC calibration data propagates errors into model predictions. Reference data uncertainty is thus more likely to negatively impact the estimated validation accuracy in soil spectroscopy studies where archived data, e.g., from soil spectral libraries, are used for model building, but it should be negligible otherwise.

Combination of soil texture with Nix color sensor can improve soil organic carbon prediction

The optimum ecosystem functioning is reliant on soil organic carbon (SOC) content which is traditionally measured in the laboratory via a cumbersome wet-chemistry method. This preliminary research tested whether a combination of Nix color sensor and portable X-ray fluorescence (PXRF) spectrometer data with soil texture data can improve soil SOC prediction accuracy relative to using them independently. A total of 371 samples representing diverse soil texture and SOC content were collected from three different ecoregions of eastern India: coastal saline zone, red and lateritic zone, and Gangetic alluvial zone. All dried, ground, and sieved samples were scanned via Nix and PXRF and random forest (RF) regression was used to predict soil SOC with different combinations of data. Soils were grouped into nine textural classes while soil SOC content exhibited substantial variability (0.08–2.26%). Comparing soil SOC with texture (sand + silt + clay), satisfactory prediction accuracy was observed (validation R2 = 0.70). Combining Nix extracted color parameters with texture substantially improved the model performance, producing the validation determination coefficient of 0.81. In contrast, PXRF-Rb, as a proxy of soil clay content was unable to achieve satisfactory prediction performance (R2 = 0.24), indicating the heterogeneity in soil mineralogical composition. The RF variable importance plot using Nix alone identified redness (a*) and yellowness to blueness (b*) as influential predictors, manifesting the impact of red color from Fe and Al-oxides and their significant negative correlation with soil SOC (r = −0.62 and −0.57 for a* and b*, respectively). These color parameters were again identified by the RF variable importance plot of (Texture + Nix)-model, implying that the SOC prediction improvement may be linked with the Nix sensor’s capability of extracting useful information in the visible range. Summarily, a combination of Nix color variables and texture data was adept at predicting soil SOC in lieu of traditional laboratory analysis. The robustness of the (Texture + Nix)-based SOC prediction model can be augmented by incorporating more soil samples representing all 12 soil textural classes and variable SOC content, showing all possible soil colors in the pedogenic environment.

Changes in soil carbon fractions and enzyme activities under different vegetation types of the northern Loess Plateau.

Knowledge of the soil organic carbon components and enzyme activities during long‐term natural vegetation restoration is essential for managing the restoration of vegetation. In this study, the variations of soil organic carbon components (i.e., soil organic carbon (SOC), microbial biomass carbon (MBC), easily oxidized carbon (EOC), particulate organic carbon (POC)) and enzyme activities (i.e., amylase, catalase, urease, and sucrase) were measured in four vegetation types: control (grasslands, GL), forest (Xanthoceras sorbifolia, XS), and shrublands (Hippophae rhamnoides, HR; Caragana korshinskii, CK). We found that vegetation types significantly affect soil organic carbon components and enzyme activities. The SOC content of the XS plot is higher than HR, CK, and GL by 88.43%, 117.09%, and 37.53% at the 0–20 cm layer; the soil SOC content of the XS plot is higher than HR and CK by 27.04% and 26.87%, and lower than GL 12.90% at the 20–40 cm layer. The highest POC and urease were observed in the XS plot at a depth of 0–20 cm, that is, 1.32 g/kg and 98.51 mg/kg, respectively. The highest EOC, amylase, and sucrase were observed in GL at a depth of 0–20 cm, that is, 5.44 g/kg, 39.23, and 607.62 mg/g. On the vertical section of the soil, the SOC fractions and the enzyme activities were greater in the upper layer than in the lower layer for each vegetation type except for MBC and catalase activity. Correlation analysis demonstrated that the SOC and POC content significantly influenced urease and sucrase activities and that MBC significantly influenced catalase activity. These results provide important information about SOC fractions and enzyme activities resulting from vegetation types in the Loess Plateau and also supplement our understanding of soil C sequestration in vegetation restoration.

Local adaptation strategies to increase or maintain soil organic carbon content under arable farming in Europe: Inspirational ideas for setting operational groups within the European innovation partnership

Abstract In the European Union, the setting of Operational Groups (OG) is supported by the European Innovation Partnership to tackle specific problems and favor innovation in agriculture. They constitute an important aspect of the current Common Agricultural Policy. Increasing or maintaining soil organic carbon (SOC) content under arable farming has been acknowledged as a primary target of European agriculture. SOC-preserving agriculture needs its techniques to be tailored to local conditions, namely, the combination of factors related to the environment (climate and soil characteristics), to the farming system (land use type, farm specialization, crop management), but also to the social and cultural context (market and availability of production means, subsidies, farmers’ education, propensity for innovation and change). In this paper we present inspirational ideas and show success examples of local adaptations strategies to increase or maintain SOC content in soils under arable farming in Europe. They include: · Adoption of soil management strategies to improve SOC storage in irrigated systems. · Precision farming and other high-tech solutions able to generate local diagnosis and adaptive strategies for increasing SOC and reducing greenhouse gasses emissions. · Innovative strategies for extending soil cover periods and introducing cover crops in rotations in areas with limited water availability or prone to harsh weather conditions. · Management of rainfed and low input crops to maintain and increase SOC in dry climates and erosive prone soils. These case studies could facilitate the setting up of OGs and the application of innovative practices in different European countries.

Effects of Biochar Application on Soil Organic Carbon Composition and Enzyme Activity in Paddy Soil under Water-Saving Irrigation.

Rice water-saving irrigation technology can remarkably reduce irrigation water input and maintain high yield; however, this technology can also accelerate the decomposition of soil organic matter in paddy fields. The spatial and temporal distributions of soil organic carbon (SOC), water-soluble organic carbon (WSOC), and soil microbial biomass carbon (SMBC) under different water-carbon regulation scenarios were analyzed on the basis of field experiments in the Taihu Lake region in China to explore the effects of biochar application on SOC and its components in water-saving irrigation paddy fields. The response of soil catalase (CAT) and invertase (INV) to biochar application in water-saving irrigated rice fields was clarified. The results showed that water-saving irrigation reduced the SOC content by 5.7% to 13.3% but increased WSOC and SMBC contents by 13.8% to 26.1% and 0.9% to 11.1%, respectively, as compared with flooding irrigation. Nonflooding management promoted the oxidative decomposition of soil organic matter. Two years after straw biochar was added, paddy soil SOC content under water-saving irrigation was increased by 4.0% to 26.7%. The WSOC and SMBC contents were also increased by 4.0% to 52.4% and 7.0% to 40.8%, respectively. The high straw biochar addition rate exhibited great impact on SOC. Remarkable correlations among SOC, WSOC, and SMBC were observed, indicating that the addition of straw biochar improved soil labile C, such as WSOC and SMBC, which promoted SOC transformation and stability in paddy soil under water-saving irrigation. Soil CAT and INV were related to SOC conversion. In conclusion, the combination of water-saving irrigation and straw biochar addition was beneficial to the improvement of soil properties and fertility of paddy fields.

How to measure, report and verify soil carbon change to realize the potential of soil carbon sequestration for atmospheric greenhouse gas removal.

There is growing international interest in better managing soils to increase soil organic carbon (SOC) content to contribute to climate change mitigation, to enhance resilience to climate change and to underpin food security, through initiatives such as international ‘4p1000’ initiative and the FAO's Global assessment of SOC sequestration potential (GSOCseq) programme. Since SOC content of soils cannot be easily measured, a key barrier to implementing programmes to increase SOC at large scale, is the need for credible and reliable measurement/monitoring, reporting and verification (MRV) platforms, both for national reporting and for emissions trading. Without such platforms, investments could be considered risky. In this paper, we review methods and challenges of measuring SOC change directly in soils, before examining some recent novel developments that show promise for quantifying SOC. We describe how repeat soil surveys are used to estimate changes in SOC over time, and how long‐term experiments and space‐for‐time substitution sites can serve as sources of knowledge and can be used to test models, and as potential benchmark sites in global frameworks to estimate SOC change. We briefly consider models that can be used to simulate and project change in SOC and examine the MRV platforms for SOC change already in use in various countries/regions. In the final section, we bring together the various components described in this review, to describe a new vision for a global framework for MRV of SOC change, to support national and international initiatives seeking to effect change in the way we manage our soils.

Lithology and coarse fraction adjusted bulk density estimates for determining total organic carbon stocks in dryland soils

Pedotransfer functions (PTFs) have been developed to estimate soil fine fraction bulk density (BDFF) using the relationships with soil organic carbon content (SOC) and particle size distribution. Current PTF's overlook the influence of lithology on BDFF estimations. In this study, we examine this influence by developing two PTF's for total bulk density (BDT), which includes both fine (FF) and coarse fragments (CF), using measured percent SOC in soils derived from felsic and mafic lithologies (90 felsic and 52 mafic, 142 total). Our results show that SOC is highly correlated with BDT in soils derived from felsic (R2 value of 0.79, p < 0.0001) and mafic (R2 value of 0.84, p < 0.0001) material and that the relationships for these two lithologies are highly distinctive. In addition, we adjust estimated BDT with soil pedon CF content (>2 mm) to determine estimated BDFF. We use an independently collected validation subset of 70 samples to compare our model against 23 published PTFs. When accounting for lithological differences in BDT and adjusting for site-specific CF content, which is highly variable vertically and horizontally within the watershed, we observe substantial improvements (average of 10.05 ± 4.89%) in BDFF estimation and associated errors compared to other PTFs. Findings from our study demonstrate that incorporating both lithology and CF content into site-specific empirical relationships can provide reliable BDFF and consequently soil carbon stock estimates.

Spatial distribution characteristics of soil organic carbon in subtropical forests of mountain Lushan, China.

The study on the spatial distribution of forest soil organic carbon (SOC) is of great significance for accurate assessment of carbon storage in forest ecosystems. In the present study, by taking eight kinds of forest soils of Mountain Lushan in the subtropical area as the research object, we studied the spatial distribution characteristics of SOC in this mountainous area. The results showed that the SOC content and SOC density (SOCD) of main forest types in the Mountain Lushan were lower than the national and the world average. The soil layer of Lushan forest was thinner, and the SOC and active SOC (ASOC) contents of different forest types and SOCDs are the highest in the surface soil. SOCD of the topsoil accounts for 32.64-54.03% of the total SOCD in the whole soil profile. Surface litter is an important source of SOC, and the different vegetation types are the important reason for the different spatial distribution of SOC in this area. Soil SOC contents in the high-altitude forest (bamboo forest, deciduous broadleaf forest, Pinus taiwanensis forest, evergreen-deciduous forest, and coniferous-broadleaved mixed forest) were higher than those in the low-altitude forest (evergreen broadleaf forest, shrub, and Pinus massoniana forest). However, the difference in SOC content exhibited at the altitude gradient is significantly lower than that in SOC in the soil profile. This indicates that both soil depth and elevation are the important factors that affected SOC distribution. However, the influence of soil depth on spatial distribution of SOC may be more complex than that of altitude. Vegetation types and soil properties are the main reasons for the large differences of reduction rate in the contents of SOC and ASOC.

Occurrence and geographic distribution of polycyclic aromatic hydrocarbons in agricultural soils in eastern China.

Polycyclic aromatic hydrocarbons (PAHs) in agricultural soils, eastern China, were studied through a collection of surface samples at 109 sites. The study covered US EPA priority pollutants but also several non-priority isomers including some dibenzopyrenes which are expected to have higher toxic potentials. The total PAH concentrations varied extensively from 8.8 to 3880μg/kg, with a geometric mean of 158μg/kg. There were significant differences in both the concentrations and composition profiles between the south and the north regions. The overall geographical distribution pattern was strongly influenced by both the per capita gross domestic product and soil organic carbon (SOC) content. The former is an indicator of anthropogenic emissions while soil SOC content is associated with the accumulative capacities of the soils. The sequestration mechanism is an important process controlling fates of PAHs in agricultural soil.

Using an inexpensive color sensor for rapid assessment of soil organic carbon

Quantifying soil organic carbon (SOC) is important for soil management, precision agriculture, soil mapping and carbon dynamics research. Inexpensive sensor technologies offer the potential for rapid quantification of SOC in laboratory samples as well as in the field. The objective of this study was to use a commercially-available color sensor to develop SOC prediction models for both dry and moist soils from the Piedmont region of South Carolina. Thirty-one soil samples were analyzed for lightness to darkness, redness to greenness, and yellowness to blueness (CIEL*a*b*) color using a Nix Pro™ color sensor. Soil color was measured under both dry and moist soil conditions and the depth of each soil sample was also recorded. Using L*, a*, b* and soil depth for each sample as initial predictors, regression analyses were conducted to develop SOC prediction models for dry and moist soils. The resulting residual plots, root mean squared errors (RMSE), and coefficients of determination (R2) were used to assess model fits for predicting the SOC content of soil. Cross validation was conducted to determine the efficiency of the predictive models and the mean squared prediction error (MSPE) was calculated. The final models included soil depth, L*, and a* as independent variables (dry soils R2=0.7978 and MSPE=0.0819, moist soils R2=0.7254 and MSPE=0.1536). The results suggest that soil color sensors have potential for rapid SOC determination, and soil depth and color are useful in predicting SOC content in soils.

Impact of no-till and reduced tillage on aggregation and aggregate-associated carbon in Northern European agroecosystems

Minimum tillage practices have been shown to enhance soil aggregation and soil organic carbon (SOC) stabilization. Carbon turnover rate slows down when soil aggregation increases and SOC is protected within stable microaggregates (53–250μm). However, this has not been investigated in boreal soils. Therefore, the objective of this study was to quantify the long-term effects of no-till (NT) and reduced tillage (RT) on SOC stabilization in four soils typical for the boreal region. Distribution of SOC in different soil fractions in a 0–20cm soil layer was analyzed by wet sieving and further isolation of microaggregates (mM) from large (>2000μm, LM) and small (250–2000μm, sM) macroaggregates. Aggregate size decreased in the order of NT>RT>CT at all study sites. In addition to increased mean weight diameter (MWD) under NT, a general trend of redistribution of SOC into these formed macroaggregates was found at all study sites, i.e., the LM fraction gained SOC. However SOC was lost in other fractions under NT compared to CT at some sites and none of the sites showed any significant changes in bulk soil SOC content under NT or RT. Also our hypothesis that there would be more SOC incorporated in mM fraction in NT and RT compared to CT was corroborated only at site 4 under NT. Thus, although the potential to accumulate SOC under NT or RT compared to CT seems to be limited in boreal agroecosystems, the redistribution of SOC to the more stable conditions within the aggregates indicates positive impacts of no-till practice.

Effects of long-term repeated mineral and organic fertilizer applications on soil organic carbon and total nitrogen in a semi-arid cropland

To meet and the ever increasing need for food and mitigation of global climate changes, plenty of fertilizers have been used to increase crop yield in China, especially in semi-arid regions. In this study, we investigated the impacts of long-term fertilization on wheat yields, soil organic carbon (SOC) and soil nitrogen (N) in the semi-arid Loess Plateau, China. One fallow and eight winter wheat-wheat (Triticum aestivum L.) rotation cropping plots were selected for the field experiment from 1984 to 2010 in the semi-arid Loess Plateau, China. In total we conducted eight fertilization treatments including no fertilizer, mineral nitrogen fertilizer (N), mineral phosphate (P), cattle manure (M), N+P, N+M, P+M and N+P+M. In 2010, we collected three replicate soil samples from each plot to the depth of 100cm from soil surface. Meanwhile, soil bulk density, SOC, total N, and mineral N (ammonium and nitrate), wheat grain and aboveground biomass yields in each plot were measured. We found that mineral fertilizers, especially those applied together with cattle manure, increased winter wheat grain and aboveground biomass yields dramatically. Moreover, wheat biomass was found to have significant correlation with SOC and soil total N in the 0–20cm soil layer. We also found that SOC and soil N were highest in the topsoil layers (0–30cm) than other layers and declined to the depth of 50cm with insignificant changes from 50 to 100cm in all treatments. Compared to the data in 1984, fertilizer application increased surface soil SOC content, especially for the N+P+M treatment after 26 years cropping and fertilization. However, changes in soil total N and mineral N differed from SOC with decreasing N in mineral-fertilized and fallow plots but increasing N in the M-fertilized plots.

Soil organic carbon contents of agricultural land in the Netherlands between 1984 and 2004

There is some debate about the likelihood that soil organic carbon (SOC) contents of agricultural land decreases because of global warming and governmental restrictions on animal manure application rates in some countries. Here, we report on changes in the mean SOC contents of the top soils (0–5 cm) of grassland and the top soil (0–25 cm) of arable land in the Netherlands during the period 1984–2004, using a data base with ~ 2 million results of SOC determinations from farmers' fields. The analyses were made for all agricultural land on mineral soils and for agricultural land in 9 regions with distinct differences in mean soil textures and SOC contents (marine and riverine clay, peaty clays, reclaimed peat soils, and Aeolian sand and loess), and land uses (arable land and permanent grassland). Except for the regions with peaty clay and reclaimed peat soils, samples with SOC > 125 g/kg were designated as peat and peaty soils and excluded from the analyses. Mean SOC content of soils under arable land in 2003 ranged from 13 to 22 g/kg for sand, loess and clay soils to 59 g/kg for reclaimed peat soils. Mean SOC content of soils under permanent grassland in 2003 ranged from 22 to 56 g/kg for sand and clay soils. The difference in mean SOC contents between grassland and arable land is in part related to the difference in sampling depth. Mean SOC contents of all mineral soils under grasslands and arable land tended to increase annually by 0.10 and 0.08 g/kg, respectively. We observed large differences in mean trends between regions. Regions with relatively low SOC contents tended to accrue C by up to 0.37 g/kg/year, while regions with relatively high SOC contents (e.g., peaty clays) tended to lose C by up to 0.98 g/kg/year. In conclusion, mean SOC contents of the top part of mineral soils of agricultural land in most regions in the Netherlands tended to increase slightly during the period 1984–2004. This result contrasts with reports from e.g., United Kingdom and Belgium that suggest decreasing C stocks in arable land possibly due to changes in land use and climate.