Influence Of Soil Organic Carbon Research Articles

Influence of Soil Organic Carbon, Water Holding Capacity, and Moisture Content on Heavy Metals in Rice Paddy Soils of Western Ghats of India

Analysis of soil samples collected from 16 rice paddy fields located in the Western Ghats region was performed to quantify the concentration of Cu, Zn, Mn, Fe, Ni, Cr, Cd, and Pb using atomic absorption spectroscopy. High concentrations of these heavy metals were found in rice paddy fields regularly cultivated using agrochemicals. We compared this concentration with soils of rice paddy field that was not under cultivation. Cu, Zn, Mn, Fe, Cr, Ni, Pb, and Cd showed increases of 1.2, 1.3, 2.3, 2.2, 1.8, 2.8, 1.8, and 8.5 times, respectively, in the rice paddy fields cultivated with synthetic fertilizers such as NPK, urea, potash, diammonium phosphate, etc., and several categories of pesticides belonging to the class organophosphates, carbamates, and acetanilide herbicide. In contaminated sites, the heavy metals exhibited maximum correlation with soil moisture content (SMC) (Zn, Fe, Cr, Ni, and Cd), soil organic content (SOC) (Fe, Cr, Ni, and Cd), and water holding capacity (WHC) (Cu, Pb, and Cd) than those observed for the reference site. The principal component analysis (PCA) revealed a total of 77.944% variance of heavy metals contributed from WHC (40.259%), SMC (20.854%), and SOC (16.832%). This indicates the build-up of heavy metals in rice paddy soils under the strong influence of moisture content, water holding capacity, and organic carbon content of the soil.

Cover crop root-derived organic carbon influences aggregate stability through soil internal forces in a clayey red soil

Soil aggregate stability is influenced by soil organic carbon (SOC). However, the influence of SOC from different fall/winter cover crop roots (annual and perennial crops) on aggregate stability through soil internal forces (SIFs) (electrostatic repulsive force (Perf), van der Waals attractive force (Pvdw), and surface hydration repulsive force (Ph)) remains unclear. The objectives of this study were compare three fall/winter cover crops (lucerne, rapeseed, six-year-vetiver (Vet_6Y)) and two control (fallow and fallow with SOC removal (fallow-SOC-R)) to determine SOC effects red soil aggregation through SIFs. Plant root morphological and chemical traits, SOC properties, soil surface charge properties, soil internal forces and aggregate breakdown were all determined. The perennial crop (Vet_6Y) showed the highest root number density (0.21 counts cm−2), second highest root mass density (1.80 mg cm−3), and the lowest root lignin/cellulose ratio (0.69), and thus contributed to higher SOC content and SOC functional groups (mainly polysaccharide-C) than annual crops (rapeseed and lucerne). Such SOC traits in perennial vetiver resulted in a higher specific surface area (SSA), lower surface charge density and surface charge number (Qs) compared to annual crops (rapeseed and lucerne) and controls, and the relationship between the root, SOC and soil surface properties (i.e., SSA) was further confirmed by significant correlation (r ranging from 0.76 to 0.97). Ultimately, these soil surface charge properties after cover crops increased Pvdw, decreased Perf, and thus decreased repulsive net force (Pnet) (the sum of Pvdw, Perf, and Ph) and the average repulsive net force displayed in order of fallow-SOC-R (2.77 MPa) > fallow (2.11 MPa) > lucerne (2.02 MPa) > rapeseed (1.78 MPa) > Vet_6Y (1.45 MPa) at a distance of 1 nm. This resulted in the mass percentage of released small particles (<20, <10, and <2 µm) w (<d) % from aggregate being reduced after cover crops compared to controls. Generally, clayey red soil aggregate stability can be improved by decreasing the repulsive soil Pnet through cover crop root contribution to SOC and SOC-functional groups.

A quantitative study of the influence of soil organic carbon and pore characteristics on the stability of aggregates of the karst peak-cluster depression area in Southwest China

A quantitative study of the influence of soil organic carbon and pore characteristics on the stability of aggregates of the karst peak-cluster depression area in Southwest China

Changes in Soil Aggregate Fractions, Stability, and Associated Organic Carbon and Nitrogen in Different Land Use Types in the Loess Plateau, China

Rational land use can enhance soil nutrient sequestration and control erosion, but the mechanisms of the ecological restoration of soil-aggregate-associated carbon and nitrogen are still not well understood. A large-scale ecological restoration program was launched in the Loess Plateau during the 1990s. The ecological restoration programs involved converting slope farmland to woodland, grassland, shrub land, and terrace. We studied their effects in relation to cultivated land as control on soil aggregate structure and stability and their associated organic carbon and total nitrogen contents to 60 cm soil depth in the Loess Plateau. Our results indicate that the restoration practices reduced soil aggregate fragmentation, increased soil structure stability, and transformed micro-aggregates into small and large aggregates. Comparing with the soil aggregate &gt;0.25 mm in cultivated land, the amount in woodland, grassland, shrub land and terrace increased by 71%, 66%, 46%, and 35%, respectively, which improved soil health overall. The mean weight diameter of aggregate indicates that soil aggregate stability (SAS) increased and soil hydraulic erosion resistance improved. In conclusion, ecological restoration directly or indirectly affected SAS through the influence of soil organic carbon and total nitrogen in different soil layers. Results of this study provide a scientific reference for understanding stabilization of soil aggregate and regional restoration.

The Influence of Soil Organic Carbon and Climate Variability on Crop Yields in Kongwa District, Tanzania.

This study assessed the influence of soil organic carbon (SOC) accumulation and climate variability on crop yields in Kongwa District, central Tanzania. In doing so, climate data and soil samples were collected from Mnyakongo and Ugogoni villages through soil sampling, interviews and surveys. Walkley-Black method, Mann-Kendall test, and MS Excel were used to analyze SOC, climate, crop yields respectively. The results exhibited that the accumulation of SOC was significantly greater in soils under organic fertilization (1.15 and 0.80 MgC ha-1 at soil 0-20 cm and 20-30 cm depth) than under no-fertilization (0.35 and 0.30 MgC ha-1 at 0-20 cm and 20-30 cm) and decreased with increasing soil depths. Under these two soil treatments, the average yields for maize, sorghum and millet were almost 1.8 tn ha-1 under organic fertilization and 0.6 tn ha-1 under no-fertilization. Specifically, maize yields ranged from 1.5 to 2.2 tn ha-1, while both sorghum and millet had 1.1-1.7 tn ha-1. Therefore, yields were significantly higher under organic fertilizations than under no-fertilizations. Besides, the mean annual rainfall or temperature (1980‒2020) fluctuated at a decreasing (R2 = 0.21) or an increasing trend (R2 = 0.30). Comparatively, the yields for maize, sorghum or millet fluctuated at a decreasing trend at R2 = 0.07, 0.05, or 0.85, respectively. Correspondingly, it was found that the temporal increase in rainfall and temperature had positive (R2 ~0.5) and negative (R2 ~0.3) correlations with crop yields, respectively. In contrast, the decline in rain's intensity and frequency had negative impacts on crop yields. Thus, both SOC and climate correlated with crop yields.

Long-term straw return with N addition alters reactive nitrogen runoff loss and the bacterial community during rice growth stages

Crop residue return is an effective, eco-friendly tillage method for decreasing reactive nitrogen (Nr) losses via surface runoff. However, the associated variation in Nr characteristics and its prospective mechanisms are not well understood. We systematically evaluated the response of Nr runoff loss and N variation in standing water to the abiotic and biotic parameters of soil in a paddy field after 6 years of straw return. Five experimental treatments of different fertilization strategies in combination with straw return were tested during the rice growth season. The results indicated that under equivalent fertilizer input, long-term straw return significantly reduced Nr runoff loss by 11.5% (P < 0.05), even though the loss increased with N fertilizer addition. We report that variations in abiotic soil properties (P < 0.05) and bacterial communities (P < 0.01) were both responsible for Nr loss differences between the rice growth stages and among the tested fertilizing patterns. Soil inorganic nitrogen (r = 0.18) had a significant positive influence on Nr runoff loss, but this effect was surpassed by the overall negative influence of soil organic carbon (r = −0.43), soil pH, (r = −0.40), and bacterial community composition (r = −0.14), which was especially apparent during the tillering stage. Our results emphasize the importance of jointly considering biotic and abiotic factors in soil and standing water when characterizing the effects of long-term straw return and N addition on Nr runoff loss, which will aid in mitigating N-based agricultural non-point pollution.

Influence of soil organic carbon, fertiliser management, and weeding regime on weed dynamics and maize productivity on sandy soils in eastern Zimbabwe

Labour bottlenecks and multiple operations at the start of a cropping season often result in inadequate early weed control and subsequent poor crop performance. Therefore, there is a need to establish management practices that provide the best opportunities for the gains associated with weeding and nutrient management across farms. We investigated the influence of soil organic carbon (SOC), fertiliser management, and weeding regimes on weed dynamics and maize productivity on smallholder farms with contrasting SOC in eastern Zimbabwe. On each site, and for two seasons, a 2 × 5 factorial experiment laid in a randomised complete block design was used. Fertiliser management was NPK or NPK + cattle manure (CM); weeding regimes were herbicide + hoe weeding, hoe weeding thrice/twice/once, or weedy check. Principal component analysis was used to evaluate weed density. The grain yield of maize increased by 13% on the sites with higher SOC. Integrating NPK + CM increased weed density and maize grain yield by 1.32 and 1.46-times, respectively, compared with NPK application only. The increased maize yield from fertiliser-managed treatments occurred only in early frequently weeded treatments. However, fertiliser application had little effect when weeding was delayed, as maize yield instead declined by 40–80%. We concluded that higher SOC increased weed density and weed biomass. Smallholder farmers are encouraged to combine herbicide application combined with hoe-weeding options for sustainable maize production.

Influence of Soil Organic Carbon on the Aroma of Tobacco Leaves and the Structure of Microbial Communities.

The soil organic carbon is associated with the plant quality and the microbial community structure. In the present study, carbon fertilizers were applied to paddy soil to elucidate the relationship between soil carbon and neutral aroma substances in both tobacco and soil microbiome by transcriptome sequencing and 16S rDNA-based analysis, respectively. Our results showed that (1) the increase in soil carbon content was closely correlated with the abundance of microorganisms belonging to two classes (which could potentially affect tobacco plants), namely Gammaproteobacteria and Chloroflexia, (2) soil carbon apparently affected tobacco neutral aroma substances, and (3) soil carbon improved neutral aroma substances by affecting the transcriptional processes of sesquiterpenoid and chlorophyll biosyntheses. These results suggest that increased soil carbon-especially active organic carbon-resulted in desirable improvements in aroma substances in tobacco leaves.

Relating soil C and organic matter fractions to soil structural stability

Soil organic matter (SOM) is important for maintaining soil structural stability (SSS). This study quantified the influence of soil organic carbon (SOC) and different organic matter components on various SSS measures. We used a silt loam soil with a wide range of SOC (8.0–42.7 g kg−1 minerals) sampled in spring 2015 from the Highfield Ley-Arable Long-Term Experiment at Rothamsted Research. Four treatments were sampled: Bare fallow, continuous arable rotation, ley-arable rotation, and grass. Soils were tested for clay dispersibility (DispClay), clay-SOM disintegration (DI, the ratio between clay content without and with SOM removal) and dispersion of particles <20 μm. The SSS tests were related to SOC, permanganate oxidizable carbon (POXC), hot water-extractable carbon (HWC), mid-infrared photoacoustic spectroscopy (FTIR-PAS) and mineral fines/SOC ratio. SSS increased with increasing content of SOM components. The relationships between SOM components and SSS followed a broken-stick regression with a change point at ~23.0 g SOC kg−1 minerals (clay/SOC~10) coinciding with a change from the tilled treatments to the grass treatment. We found a greater influence of SOC, POXC and HWC on SSS at contents below the change point than above. A stronger linear relation between POXC and DispClay compared to SOC and HWC suggests that POXC was a better predictor of the variation in DispClay. POXC and HWC were less related to DI than SOC. The grass treatment had a very stable structure, shown in all SSS tests, probably due to the absence of tillage and large annual inputs of stabilizing agents. This suggests that a change in management from arable rotation to permanent grass is one effective tool for improving SSS.

SOC Effects on Diethyl Phthalate Sorption to Four Soils in China

Purpose: Phthalate esters (PAEs) are commonly detected in agricultural soils of China, which can post potential threats to human health. Understanding their sorption to soils is important in assessing their transport and bioavailability in environment. Limited research focuses on the influence of soil organic carbon (SOC) on PAEs adsorption to soils at different depths and increased dissolved organic carbon (DOC) after adsorption. Materials and Methods: Batch sorption experiments of diethyl phthalate (DEP) to four types of soils were conducted in this study, including black soil, fluvo-aquic soil, paddy soil, and red soil. Total concentration of solute DEP and the concentration of free DEP not sorbed to DOC were measured with HPLC. The UV absorbance of supernatant was measured and specific UV absorbance at 254 nm was calculated for DOC aromaticity. Sorption isotherms of DEP to soil particles were fitted to the Langmuir model and the Freundlich model. Results and Discussion: Partial SOC can dissolve to DOC in solution, and 47.4- 89.4% of total DEP can be sorbed to DOC. Increasing DEP in solution can enhance aliphatic SOC dissolving. The sorption coefficients of DEP are higher to fluvo-aquic soils (Koc for surface soils 1820 L kg-1 and subsurface soils 1388 L kg-1) than to other soils (<520 L kg-1), which indicates that SOC fractions of different soils have varied affinity to DEP. Conclusions: SOC plays an important role in DEP sorption to soil particles. Soil samples from surface layers have higher affinity than those from subsurface layers. DOC from SOC in solution is important for DEP transport in soil, and organic pollution can accelerate SOC dissolving.

Evaluation and enhancement of permafrost modeling with the NASA Catchment Land Surface Model.

Besides soil hydrology and snow processes, the NASA Catchment Land Surface Model (CLSM) simulates soil temperature in six layers from the surface down to 13m depth. In this study, to examine CLSM's treatment of subsurface thermodynamics, a baseline simulation produced subsurface temperatures for 1980-2014 across Alaska at 9-km resolution. The results were evaluated using in situ observations from permafrost sites across Alaska. The baseline simulation was found to capture the broad features of inter- and intra-annual variations in soil temperature. Additional model experiments revealed that: (i) the representativeness of local meteorological forcing limits the model's ability to accurately reproduce soil temperature, and (ii) vegetation heterogeneity has a profound influence on subsurface thermodynamics via impacts on the snow physics and energy exchange at surface. Specifically, the profile-average RMSE for soil temperature was reduced from 2.96°C to 2.10°C at one site and from 2.38°C to 2.25°C at another by using local forcing and land cover, respectively. Moreover, accounting for the influence of soil organic carbon on the soil thermal properties in CLSM leads to further improvements in profile-average soil temperature RMSE, with reductions of 16% to 56% across the different study sites. The mean bias of climatological ALT is reduced by 36% to 89%, and the RMSE is reduced by 11% to 47%. Finally, results reveal that at some sites it may be essential to include a purely organic soil layer to obtain, in conjunction with vegetation and snow effects, a realistic "buffer zone" between the atmospheric forcing and soil thermal processes.

Concentration and spatial distribution of organophosphate esters in the soil-sediment profile of Kathmandu Valley, Nepal: Implication for risk assessment

Despite the fact that soil and sediments, which act as a sink or potential source of organic pollutants, have been polluted with organophosphate esters (OPEs) around the globe, extremely constrained data is accessible on environmental concentration and fate of OPEs in solid matrices in whole of the South Asia particularly if there should be an occurrence in Nepal. In this study, surface soil (N=19) and sediments samples (N=20) were analyzed for eight different OPE in Kathmandu Valley during October 2014. The concentration of ∑8OPE measured in sediments samples was 12 times higher than soil and ranged 983–7460ng/g dw (median 2210ng/g dw) and 65–27,500ng/g dw (186ng/g dw), respectively. TMPP was most abundant in soil followed by TCIPP, TEHP and EHDPHP and ranged 17–25,300ng/g dw (41.3ng/g dw), 11.2–911ng/g dw (31.7ng/g dw), 8.52–858ng/g dw (26.1ng/g dw) and 10.2–114ng/g dw (25.6ng/g dw), respectively. TEHP was most prevalent in sediments followed by TMPP and EHDPHP and were in the range of 657–3020ng/g dw (median 1140ng/g dw), 267–2630ng/g dw (median 815g/g dw), 34–418ng/g (median 131ng/g dw), respectively. The sources of the high level of OPEs in soil was related to the end point use of consumer materials, traffic emission, and close proximity to commercial and industrial areas; while domestic sewage discharges and effluents from carpet industry were identified as the possible entry of OPE in sediments. Total organic carbon (TOC) and black carbon (BC) content in soil were moderately and positively correlated with ∑8OPE indicating more or less influence of soil organic carbon. The health risk assessment suggested dermal absorption of OPEs via soil is the primary pathway of human exposure to the general population. The significantly high-risk quotient (RQ) estimated for ∑8OPEs especially TMPP and TPHP suggested significant potential adverse risk for aquatic organisms.

Spatio-Temporal Organization and Biomass Dynamics of Plant Communities in a Dry Tropical Peri-Urban Region:Deterministic Role of Alien Flora in Anthropo-Ecosystems

The species nativity, growth form, habit, invasion status, aboveground biomass (AGB) and belowground biomass (BGB) distribution and soil characteristics across six diverse habitat conditions were studied in a peri-urban region in Indian dry tropics to understand their deterministic impact on vegetation structure. Eighty-seven plant species, predominantly annuals (67%), belonging to 28 angiosperm families were recorded. Among them, 89% were exotics (largest of American and Asian origins) and 48% of the exotics were invasives while 16% were naturalized. AGB of annuals was higher than perennials, but the difference in their BGB was insignificant. Compared to natives, the exotics had higher AGB and BGB. Among them, the AGB species of South America, Asia and the Indian subcontinent had higher AGB, but the order was reversed in case of BGB. Similarly, AGB of invasives was higher than natives, but they showed comparable BGB. 'Non-native annual forbs' were the most dominant functional groups in terms of both AGB and BGB. The ordination results varied with plant BGB and AGB. Canonical correspondence analysis indicated dominant role of exotic invasives (mainly of American and European origin) and significant influence of soil organic carbon (SOC) and total nitrogen on vegetation organization. At lower SOC, AGB and BGB were comparable for both native and exotics. However, with the increase in SOC, native AGB and BGB declined, whereas AGB of nonnative species increased. In conclusion, the study revealed large intrusion of alien floras into anthropoecosystems in Indian dry tropics, which significantly impacted structure and ecological processes both aboveground and belowground, as against better adaptation potential of the natives belowground.

The influence of soil organic carbon on interactions between microbial parameters and metal concentrations at a long-term contaminated site

The effects of lead, zinc, cadmium, arsenic and copper deposits on soil microbial parameters were investigated at a site exposed to contamination for over 200years. Soil samples were collected in triplicates at 121 sites differing in contamination and soil organic carbon (SOC). Microbial biomass, respiration, dehydrogenase activity and metabolic quotient were determined and correlated with total and extractable metal concentrations in soil. The goal was to analyze complex interactions between toxic metals and microbial parameters by assessing the effect of soil organic carbon in the relationships. The effect of SOC was significant in all interactions and changed the correlations between microbial parameters and metal fractions from negative to positive. In some cases, the effect of SOC was combined with that of clay and soil pH. In the final analysis, dehydrogenase activity was negatively correlated to total metal concentrations and acetic acid extractable metals, respiration and metabolic quotient were to ammonium nitrate extractable metals. Dehydrogenase activity was the most sensitive microbial parameter correlating most frequently with contamination. Total and extractable zinc was most often correlated with microbial parameters. The large data set enabled robust explanation of discrepancies in organic matter functioning occurring frequently in analyzing of contaminated soil processes.

Importance of soil organic carbon on surface soil water content variability among agricultural fields

Summary Improvements to the downscaling estimates of soil water content (SWC) from passive microwave retrievals require detailed knowledge of field scale influences on SWC variability. The Soil Moisture Active Passive Validation Experiment (SMAPVEX-12) field campaign provided SWC and physical properties from 50 cropland fields to assess the influence of soil organic carbon (SOC) on SOC variability in a range of SOC, SWC and soil textural class over a 6 week period. Field average SWC over the duration of the experiment was optimally predicted by combination of soil texture and SOC in all soil wetness conditions, although either %Sand or SOC separately also expressed 82% of variance in SWC over all fields covering three soil textural groups. Soil OC explained greater variance in SWC than texture in dry conditions, while texture predominated in moist conditions. The high correlation between SOC and SWC suggests soil OC may contribute to the initiatives to downscale SWC estimates from satellite to field scale where SWC data are sparse or inaccurate.

Solubility of Lead and Copper in Biochar-Amended Small Arms Range Soils: Influence of Soil Organic Carbon and pH

Biochar is often considered a strong heavy metal stabilizing agent. However, biochar in some cases had no effects on, or increased the soluble concentrations of, heavy metals in soil. The objective of this study was to determine the factors causing some biochars to stabilize and others to dissolve heavy metals in soil. Seven small arms range soils with known total organic carbon (TOC), cation exchange capacity, pH, and total Pb and Cu contents were first screened for soluble Pb and Cu concentrations. Over 2 weeks successive equilibrations using weak acid (pH 4.5 sulfuric acid) and acetate buffer (0.1 M at pH 4.9), Alaska soil containing disproportionately high (31.6%) TOC had nearly 100% residual (insoluble) Pb and Cu. This soil was then compared with sandy soils from Maryland containing significantly lower (0.5-2.0%) TOC in the presence of 10 wt % (i) plant biochar activated to increase the surface-bound carboxyl and phosphate ligands (PS450A), (ii) manure biochar enriched with soluble P (BL700), and (iii) unactivated plant biochars produced at 350 °C (CH350) and 700 °C (CH500) and by flash carbonization (corn). In weak acid, the pH was set by soil and biochar, and the biochars increasingly stabilized Pb with repeated extractions. In pH 4.9 acetate buffer, PS450A and BL700 stabilized Pb, and only PS450A stabilized Cu. Surface ligands of PS450A likely complexed and stabilized Pb and Cu even under acidic pH in the presence of competing acetate ligand. Oppositely, unactivated plant biochars (CH350, CH500, and corn) mobilized Pb and Cu in sandy soils; the putative mechanism is the formation of soluble complexes with biochar-borne dissolved organic carbon. In summary, unactivated plant biochars can inadvertently increase dissolved Pb and Cu concentrations of sandy, low TOC soils when used to stabilize other contaminants.

Influence of Soil Organic Carbon and Total Nitrogen of Soil under Different Plant Communities in Central Yunnan

By the field investigation and laboratory analysis methods, through research the characteristics of SOC, TN in soil of mixed Pine and Quercus, Nepalensis, Bayberry bushes, Pinus Yunnanensis and Abandoned Land in central Yunnan. It resulted that different plant communities took little impact on the soil physical properties, besides the soil moisture was remarkable, the bulk density and soil porosity effect is not prominent. The content of nutrients in different plant communities shows that vegetation abundant soil is more rich than vegetation sparse soils, and in 0~10 ㎝ SOC and TN content is higher than in 10~20 ㎝. Then by the study the C: N ratio in the soil, it showed that relative to the accumulation of TN, different communities in favour of the process SOC accumulation. The C: N ratio as follows: Abandoned land &gt; Pinus Yunnanensis &gt; mixed Pine and Quercus &gt; Bayberry bushes &gt; Nepalensis, so that revealed the ratio of C and N take negative correlation with their decomposition rate.

Contribution of Soil Organic Carbon to the Ion Exchange Capacity of Tropical Soils

ABSTRACT Highly weathered soils represent about 3 billion ha of the tropical region. Oxisols represent about 60% of the Brazilian territory (more than 5 million km2), in areas of great agricultural importance. Soil organic carbon (SOC) can be responsible for more than 80% of the cation exchange capacity (CEC) of highly weathered soils, such as Oxisols and Ultisols. The objective of this study was to estimate the contribution of the SOC to the CEC of Brazilian soils from different orders. Surface samples (0.0 to 0.2 m) of 30 uncultivated soils (13 Oxisols, 6 Ultisols, 5 Alfisols, 3 Entisols, 1 Histosol, 1 Inceptisol, and 1 Molisol), under native forests and from reforestation sites from São Paulo State, Brazil, were collected in order to obtain a large variation of (electro)chemical, physical, and mineralogical soil attributes. Total content of SOC was quantified by titulometric and colorimetric methods. Effective cation exchange capacity (ECEC) was obtained by two methods: the indirect method—summation—estimated the ECECi from the sum of basic cations (Ca+ Mg+ K+ Na) and exchangeable Al; and the direct ECECd obtained by the compulsive exchange method, using unbuffered BaCl2 solution. The contribution of SOC to the soil CEC was estimated by the Bennema statistical method. The amount of SOC varied from 6.6 g kg−1 to 213.4 g kg−1, while clay contents varied from 40 g kg−1 to 716 g kg−1. Soil organic carbon contents were strongly associated to the clay contents, suggesting that clay content was the primary variable in controling the variability of SOC contents in the samples. Cation exchange capacity varied from 7.0 mmolc kg−1 to 137.8 mmolc kg−1 and had a positive correlation with SOC. The mean contribution (per gram) of the SOC (1.64 mmolc) for the soil CEC was more than 44 times higher than the contribution of the clay fraction (0.04 mmolc). A regression model that considered the SOC content as the only significant variable explained 60% of the variation in the soil total CEC. The importance of SOC was related to soil pedogenetic process, since its contribution to the soil CEC was more evident in Oxisols with predominance of Fe and Al (oxihydr)oxides in the mineral fraction or in Ultisols, that presented illuviated clay. The influence of SOC in the sign and in the magnitude of the net charge of soils reinforce the importance of agricultural management systems that preserve high levels of SOC, in order to improve their sustainability.

Influence of Soil Organic Carbon on Acute and Chronic Toxicity of Plant Protection Products to Poecilus cupreus (Coleoptera, Carabidae) Larvae (4 pp)

Background, Aims and Scope Variability of results from terrestrial ecotoxicological tests with arthropods demonstrate the importance of understanding the impact of soil composition has on the bioavailability of pollutants. Beside other soil parameters, the organic C content is known to be relevant to bioavailability. The aim of the present paper was to detect lethal and sublethal effects of plant protection products on soil-dwelling larvae of the carabid beetle Poecilus cupreus under different concentrations of soil organic C. The study was based on a familiar laboratory test method.

Influence of soil organic carbon on the interpretation of soil test P for wheat grown on alkaline soils

Organic carbon is known to alter crop response to applied phosphorus (P) but that fact has not been incorporated in soil test interpretations. To achieve this objective, field experiments with wheat were conducted for four years on alkaline soils of Punjab, India. The experimental soils ranged from loamy sand to loam in texture, 7.4 to 9.6 in pH, 0.16 to 0.75% in organic carbon (OC) and 2 to 40 mg Olsen extractable P kg−1 soil. Response of wheat to fertilizer phosphorus application was related to the combined effect of Olsen P and soil OC content. At a given Olsen P level, wheat yield was a function of soil OC content. Multiple regression analysis of the data showed that OC content 0.6%, OC along with Olsen P accounted for 97% of the variation in yield and there was no response to applied fertilizer P. Yield isoquants for 4 and 5 tons grains ha−1 showed that for a given Olsen P level, as OC content increased the amount of fertilizer P required to achieve a yield target decreased. It was shown that OC may be used to approximate the contribution of organic P mineralization to plant available soil P during a growing season. The reliability of fertilizer recommendations based on Olsen P may be improved on some alkaline soils by consideration of soil OC content.