Distribution of soil organic carbon across contrasting fractionation techniques - results from a long-term field trial with increasing shares of leguminous cover crops

Diversification of no-till cropping systems through the inclusion of leguminous crops can be a sustainable means for enhancing both the bioavailability as well as the persistence of soil organic carbon (SOC). Therefore a comprehensive assessment of long-term soil organic matter (SOM) dynamics is crucial to realize the potential of sequestering atmospheric carbon dioxide, while concomitantly restoring the productivity and functionality of degraded soils. In the current study, soil samples were taken from a 39 years old subtropical trial at seven soil increments until one meter depth. Treatments included five maize-based cropping systems, with increasing shares of leguminous cover crops, with or without nitrogen (N) fertilizer applications to the maize plants. Varying degrees of labile and recalcitrant SOC were distinguished by means of thermal analysis as well as through physio-chemical fractionation, yielding contrasting results in shares of carbon accumulation across conceptual pools. While thermally recalcitrant SOC (combusted at temperatures between 400˚C and 800 ˚C) represented a small percentage of total C accrual, chemically recalcitrant SOC (silt and clay fraction resisting sodium hypochlorite oxidation) reported both a large share of total C content, as well as a high C accumulation. Although limited overlap among C pools from the two fractionations was found, irrespective of C detection method, systems with higher shares of leguminous cover crops reported highest SOC stocks. Interestingly, including additional leguminous cover crops contributed in storing as much SOC as systems with N fertilization, but the depth at which SOC sequestration occurred varied between fertilized and non-fertilized systems. While SOC stocks in the 0-30 cm depth correlated positively to total C inputs from crop residues in both fertilized and unfertilized systems, SOC stocks in the subsoil (30-100 cm depth) only correlated (p<0.05) to inputs from leguminous cover crops in non-fertilized systems.

Moreover, shifts in δ¹³C signatures across the five physically separated C fractions (silt+clay, recalcitrant SOC , stable aggregates + sand, dissolved organic carbon and particulate organic matter), were used to calculate contributions of C₃ leguminous C inputs in mixed C₃/C₄ cropping systems and study pathways of SOC dynamics.  

Overall, the study reports high potential of leguminous cover crops to contribute to SOC build-up in subtropical Oxisols, especially through the association of labile organic matter to soil minerals.