Mineral-organic associations are enriched in both microbial metabolites and plant residues in a subtropical soil profile under no-tillage and legume cover cropping

Context Knowledge of the impacts of no-tillage and cover cropping on carbon accumulation and stabilisation in highly weathered agricultural soils of subtropical regions is scant. We hypothesised that implementation of no-tillage coupled with high-quality legume residues in highly weathered agricultural soils would result in high carbon accumulation rates, mainly as microbe- and plant-derived materials in fine mineral-organic complexes. Aims and methods We sampled soil profiles down to 100 cm in a long-term field experiment and used density and particle size fractionation in combination with carbohydrate analyses to compare the effect of conventional tillage vs no-tillage, combined or not with legume cover cropping, and combined or not with mineral nitrogen fertilisation. Key results Both no-tillage and legume cover crops favoured the accumulation and enrichment in plant-derived carbohydrates in the surface soil layer, due to the accumulation of plant residues. The ratio of microbe- to plant-derived carbohydrates increased with soil depth indicating that the soil carbon (C) was more microbially processed than at the surface. Conservation management systems also increased soil C at depth and this was most visible in the clay fraction. The additional clay-size C accumulating at depth under conservation treatments was of both microbial and plant origin. Conclusions Our results support the hypothesis that mineral-associated C is composed of both plant and microbial residues and is positively influenced by conservation management practices. Implications Our results demonstrate that no-till and legume cover cropping are efficient practises to foster C accumulation and stabilisation in heavily weathered agricultural soil profiles in a subtropical climate.