Active Soil Organic Carbon Pools Research Articles

Active Soil Organic Carbon Pools Decrease with Increased Time since Land-Use Transition from Rice Paddy Cultivation to Areca Nut Plantations under the Long-Term Application of Inorganic Fertilizer

Many croplands in the tropics of China have been converted over the last decades into areca nut plantations due to their high economic returns. This land-use transition was accompanied by changes in agricultural practices such as soil moisture regimes and fertilizer inputs, which may affect soil organic carbon (SOC) and its fractions, especially in tropical soils with low fertility and high nitrogen loss. Yet, how the time since land-use transition from rice paddy cultivation to areca nut plantations affects soil carbon dynamics and their underlying mechanisms in the tropics of China remains elusive. Here, areca nut plantation soils with different ages (2, 5, 10, 14, and 17 years) and paddy fields in the tropical region of China were investigated. The study result indicates that the contents of dissolved organic carbon (DOC), particulate organic carbon (POC), easily oxidized organic carbon (EOC), light organic carbon (LFOC), and microbial biomass carbon (MBC) decreased significantly with increased time since land-use transition from rice paddy cultivation to areca nut plantations. Similarly, the ratios of DOC/SOC, MBC/SOC, POC/SOC, LFOC/SOC, and EOC/SOC decreased significantly with increased time since land-use transition. Compared with the paddy soil, the carbon pool management index decreased by 36.6–76.7% under the areca nut plantations, concluding that increasing the time since land-use transition from rice paddy cultivation to areca nut plantations with high application rates of chemical fertilizers resulted in reduced soil active carbon fractions and SOC supply capacity. Therefore, agricultural practices such as the use of organic fertilizers should be applied to improve the soil’s ability to supply organic carbon in managed plantation ecosystems in the tropics of China.

Spatial heterogeneity of particulate organic matter for the sorption of ciprofloxacin at the microstructure scale

Particulate organic matter (POM) is a major antibiotic sorbent of the active soil organic carbon pool. Until now, the spatial heterogeneity of POM for ciprofloxacin (CFC) sorption at the microstructure scale has not been quantified. To our knowledge, this is the first study that combines a batch sorption experiment, NanoSIMS, and a 13C isotopic tracer to expound the distribution characteristics of CFC and the dominant sorption components of POM. Four POMs separated from soils (wetland, oil waste field, and farmlands) were utilized to study the sorption mechanisms using batch experiments. POM separated from the wetland was utilized for further study at the microstructure scale. The results revealed that the POM had a great CFC sorption capacity ranging from 65.20 mg g−1 to 77.51 mg g−1. Both the kinetics and NanoSIMS results showed that the sorption mechanisms included surface distribution and intra-particle diffusion. The microstructural distribution characteristics exhibited a marginal accumulation tendency and a surface accumulation effect. A combination of the NanoSIMS with the pH and salinity results explained that CFC adsorption was dominated by the organic components of POM. Overall, the present study demonstrates that the characteristics and organic components of the POM played a decisive role in the CFC-POM interactions. This study provides the first evidence of the vector potential of CFC rereleased into the environment during the POM turnover process.

Effect of conservation agriculture on soil organic carbon dynamics and mineral nitrogen under different fertilizer management practices in maize (Zea mays)-wheat (Triticum aestivum) cropping system

Soil organic carbon (SOC) is the center of all physical, chemical and biological properties of soils, its maintenance and buildup in soils is necessary to sustain the intensive cereal-based cropping system of Indo Gangetic plains. Intensive tillage, residue removal and indiscriminate use of fertilizers led to a continuous deterioration of soil health in Indo Gangetic parts of the country. Under this context, a field experiment on conservation agriculture (CA) was conducted at Taroari, Karnal, comprising four combinations of tillage and residue management [i.e. conventional tillage (CT) with complete residue removal (CT-RR), CT with 20% residue incorporation of wheat, 50% residue incorporation of maize and incorporation of green gram residue (CT+RI+GI), permanent beds (PB) with 20% residue retention of wheat and 50% residue retention of maize (PB+RR), and PB with 20% residue retention of wheat, 50% residue retention of maize and retention of green gram residue (PB+RR+GR)] and three nutrient management options [viz. farmers fertilization practice (FFP), recommended fertilizer dose (RDF) and site-specific nutrient management (SSNM)]. The results showed that, there was an increase in SOC mainly the active SOC pools, permanganate oxidizable-C (1.70 g/kg), hot water extractable-C (0.32 g/kg) and soil microbial biomass C (310 mg/kg) under CA-based treatment (PB+RR+GR) compared to CT (0.58, 0.23 g/kg, 183 mg/kg, respectively). Also, the mineral N was invariably greater under residue treatments. Therefore, crops residue retention as well as balanced fertilization (RDF and SSNM) under CA helped in improving SOC; mineral N and soil aggregation stability which can lead to increased sustainability under cereal-based intensive cropping systems.

Effect of the combined application of fungal residue and chemical fertilizers on the mineralization of soil organic carbon in paddy fields.

Improving soil quality, reducing waste, and mitigating climate change require an understanding of the balance between soil organic carbon (SOC) accumulation and depletion after the application of different quantities of fungal residue and chemical fertilizers. We evaluated the mineralized carbon (MC) content and mineralization rate (MR) after nine applications of chemical fertilizers (C) and fungal residue (F) in paddy fields, at rates of 0, 50, and 100%. A double exponential model was used to calculate the potential rates of MC and SOC turnover. The combined application of fungal residue and chemical fertilizers led to significantly higher MC and MR, by 24.97-100.05 and 24.36-98.07%, respectively, during 57days of incubation than that of the control. The MC and MR values were highest with the C50F100 treatment. Simulations with the double exponential model showed that both the active SOC pools (C1) and potential SOC mineralization flux C1 + C2 were highest with C50F100, and the MR constants, k1 and k2, were highest with C100F100. The potential SOC MR [(C1 + C2) / SOC] was highest with C50F100. The application of fungal residue and chemical fertilizers to paddy fields effectively alleviated soil acidification caused by chemical fertilizers and increased the nutrient content, MC, MR, C1, and C1 + C2 of soils. However, the over-use of fungal residue or chemical fertilizers produces the reverse effects. Therefore, appropriate quantities of chemical fertilizers and fungal residue need to be applied to enhance the carbon sequestration capacity of soils while improving the MC and MR.

Effects of Steel Slag and Biochar Incorporation on Active Soil Organic Carbon Pools in a Subtropical Paddy Field

Industrial wastes and agricultural byproducts are increasingly used in crop production as fertilizers, but their impacts on soil carbon (C) sequestration remain poorly understood. The aim of this study was to examine the effects of applying steel slag (SS), biochar (B), and a combination of these two materials (SS + B) on total soil organic C (SOC), active SOC fractions, and C pool management index (CPMI) in a subtropical paddy field in China. The treatments were applied at a rate of 8 t ha−1 to rice at the two (early and late) crop seasons in 2015. The SOC concentrations in the top 30 cm soils in the SS + B treatments were 28.7% and 42.2% higher in the early and late crops, respectively, as compared to the controls (p < 0.05). SOC was positively correlated with soil C:N ratio across the two crop seasons (r = 0.92–0.97, p < 0.01). As compared to the control, SS + B treatment had significantly higher carbon pool index (CPI) in both early (22.4%) and late (40.1%) crops. In the early crop, the C pool activity index (CPAI) was significantly lower in B and SS + B treatments by over 50% than in the control, while the soil C pool management index (CPMI) in the SS, B, and SS + B treatments was lower than that in the control by 36.7%, 41.6%, and 45.4%, respectively. In contrast, in the late crop, no significant differences in CPAI and CPMI were observed among the treatments. Our findings suggest that the addition of steel slag and biochar in subtropical paddy fields could decrease active SOC pools and enhance soil C sequestration only in the early crop, but not the late crop.

Effects of rice straw incorporation on active soil organic carbon pools in a subtropical paddy field

Understanding the dynamics of soil organic carbon (SOC) in response to changing environment and management practices is crucial for minimizing the climatic impacts of rice agriculture. This study examined the effects of rice straw incorporation on total SOC, active SOC fractions, and carbon pool management index (CPMI) in a subtropical paddy field in China. The straw collected after harvesting the early and late paddy was incorporated immediately into the top 10cm soil at a rate of 3.3Mgha−1 and the top 40cm soil samples in the triplicate treatment, control and reference plots were analyzed for the concentrations of total SOC and four other active SOC fractions at the end of the early and late seasons. The concentrations of total soil organic carbon (SOC), light fraction organic carbon (LFOC), dissolved organic carbon (DOC), and microbial biomass carbon (MBC) were significantly (P<0.05) higher in the straw application plots than in the controls by 7.1–128.6% for both the early and late paddy season. In general, significant and positive correlations were observed between SOC, DOC, LFOC, MBC and labile organic carbon (LOC) in the treatment and control plots for both early and late paddy (r=0.60–0.96, P<0.05). Moreover, there were significant and positive correlations of total SOC and active SOC fractions with both soil water content (r=0.69–0.94, P<0.01) as well as total soil nitrogen content (r=0.69–0.99, P<0.05). Straw incorporation significantly increased CPMI and other active soil organic carbon pools in the early paddy field (P<0.05), but had no significant effects on CPMI in the late paddy field. It is therefore better to incorporate straw residues in the aerobic condition during the early paddy season in order for rice to benefit from an increased CPMI.

Effect of repeated drying-wetting-freezing-thawing cycles on the active soil organic carbon pool

Samples of soddy-podzolic soil (long-term overgrown fallow and continuous bare fallow), gray forest soil (forest, farming agrocenosis), and a typical chernozem (virgin steppe, forest area, farming agrocenosis, continuous bare fallow) have been incubated under stable conditions; other samples of these soils have been subjected to six drying-wetting-incubation-freezing-thawing-incubation cycles during 136 days. The wetting of dried soils and the thawing of frozen soils result in an abrupt but short increase in the emission rate of C-CO2 by 2.7–12.4 and 1.6–2.7 times, respectively, compared to the stable incubation conditions. As the soil is depleted in potentially mineralizable organic matter, the rate of the C-CO2 emission pulses initiated by disturbing impacts decreases. The cumulative extra production of C-CO2 by soils of natural lands for six cycles makes up 21–40% of that in the treatments with stable incubation conditions; the corresponding value for cultivated soils, including continuous clean fallow, is in the range of 45–82%. The content of potentially mineralizable organic matter in the soils subjected to recurrent drying-wetting-freezingthawing cycles decreased compared to the soils without disturbing impacts by 1.6–4.4 times, and the mineralization constants decreased by 1.9–3.6 times. It has been emphasized that the cumulative effect of drying-wetting-freezing-thawing cycles is manifested not only in the decrease in the total Corg from the soil but also in the reduction of the mineralization potential of the soil organic matter.

Monitoring changes in soil organic carbon pools, nitrogen, phosphorus, and sulfur under different agricultural management practices in the tropics

Soil organic matter not only affects sustainability of agricultural ecosystems, but also extremely important in maintaining overall quality of environment as soil contains a significant part of global carbon stock. Hence, we attempted to assess the influence of different tillage and nutrient management practices on various stabilized and active soil organic carbon pools, and their contribution to the extractable nitrogen phosphorus and sulfur. Our study confined to the assessment of impact of agricultural management practices on the soil organic carbon pools and extractable nutrients under three important cropping systems, viz. soybean-wheat, maize-wheat, and rice-wheat. Results indicated that there was marginal improvement in Walkley and Black content in soil under integrated and organic nutrient management treatments in soybean-wheat, maize-wheat, and rice-wheat after completion of four cropping cycles. Improvement in stabilized pools of soil organic carbon (SOC) was not proportional to the applied amount of organic manures. While, labile pools of SOC were increased with the increase in amount of added manures. Apparently, green manure (Sesbania) was more effective in enhancing the lability of SOC as compared to farmyard manure and crop residues. The KMnO(4)-oxidizable SOC proved to be more sensitive and consistent as an index of labile pool of SOC compared to microbial biomass carbon. Under different cropping sequences, labile fractions of soil organic carbon exerted consistent positive effect on the extractable nitrogen, phosphorus, and sulfur in soil.