The Relationship Between Carbon Input, Aggregation, and Soil Organic Carbon Stabilization in Sustainable Cropping Systems

Approximately 10% of the earth's soil C is stored within agricultural soil ecosystems. Because farming systems hold promise for sequestering C, their sustainability, environmental impact, and potential role in mitigating rising atmospheric CO₂ concentrations must be addressed. Our current challenges are to provide credible evidence that agricultural practices can sequester significant amounts of C and to quantify the mechanisms, capacity, and longevity of agricultural lands as C sinks. Agronomic practices that influence yield and, therefore, affect the proportion of crop residues returned to the soil (e.g. cover cropping, irrigation, fertilizer addition, and compost application) are likely to influence soil organic carbon (SOC). The objectives of this study were (1) to determine the influence of C input on C sequestration in SOC fractions and (2) to evaluate how aggregation (MWD) relates to SOC and cumulative C input, across 10 different cropping systems. Using SOM fractionation techniques, soil samples from 10 cropping systems at LTRAS (Long-term Research on Agricultural Systems, Davis, CA) were separated into four aggregate size classes (LM: >2000μ m, sM: 250-2000μ m, m: 53-250μ m, and silt&clay: <53μ m) and into three SOM fractions within LM and sM (cPOM:250-2000μ m, mM: 53-250μ m, and silt&clay: <53μ m). All fractions were analyzed for their C content. Empirically derived relationships between yield and aboveground biomass-C plus yield and belowground biomass-C were used to quantify C input from corn, wheat, and tomato residues as well as for legume cover crops and compost for the different cropping systems. We found a positive correlation between cumulative C input and SOC (R²=0.45, P<0.0001). After 9 years, MWD increased linearly with greater C input (R²=0.64, P<0.0001) and SOC (R²=0.61, P<0.0001), respectively. We observed that aggregate-C shifts from the microaggregate fraction (53-250μ m) in low C input systems to macroaggregate fractions (>2000μ m and 250-2000μ m) in high C input systems. Our findings indicate that management practices directed towards improving annual production, thereby, increasing residue C input would result in greater aggregate stability and aggregate associated SOC levels and have the capability of long-term C stabilization.