Carbon accumulation and aggregation are mediated by fungi in a subtropical soil under conservation agriculture

Abstract No-tillage (NT) and legume cover crops generally improve the quality of tropical and subtropical soils, but the mechanisms underlying these effects are not well known. We performed a study to investigate the influence of NT and legume cover cropping on microbial cell-wall constituents [glucosamine (GlcN), taken as indicator of fungal cell-wall; muramic acid (MurN), taken as indicator of bacterial cell-wall], and on their relationships with soil aggregation and soil organic carbon (SOC) accumulation in different fractions (light fraction, and sand-, silt- and clay-sized fractions) of a subtropical Acrisol in Southern Brazil. The GlcN concentration ranged from 450.5 mg kg−1 in the 0–5 cm soil layer to 20.5 in the 75–100 cm soil layer, approximately 10 times greater than MurN concentrations (53.1–2.7 mg kg−1 for the same soil layers). No-tillage and legume cover crops favoured the accumulation of fungal and bacterial cell-wall constituents in whole soil, especially in the top 5 cm, with a preferential enrichment in GlcN. Legume cover cropping and NT resulted in greater accumulation of C in the light fraction in surface soil, which favoured the fungal community that, in turn, mediated an improvement in soil aggregation. Fungal-derived glucosamine also preferentially accumulated down to 100 cm depth, and more specifically in the clay-sized fraction of soil, suggesting a specific role of fungi in SOC accumulation at depth. Overall, our study provides field-based evidence that the accrual of fungal cell-wall constituents under NT and legume cover cropping is a key process leading to aggregation and SOC accumulation in subtropical soil profiles.