Cropping systems including legume cover crops favour mineral–organic associations enriched with microbial metabolites in no-till soil

Abstract

Long-term carbon (C) stabilisation in tropical and subtropical soils under no-tillage (NT) rests on the formation of mineral–organic associations (MOAs) that can be enriched with microbial metabolites. In this work, we assessed the role of long-term tillage and cropping systems and mineral N fertilisation in enriching MOAs with microbial metabolites in a subtropical soil. For this purpose, we sampled a sandy clay loam Acrisol up to 1 m depth involved in an ongoing 30-year-old experiment under two different tillage systems (conventional tillage and NT) in the presence and absence of legume cover crops and mineral nitrogen (N) fertilisation. The soil samples were subjected to particle size fractionation and n-alkane analysis. The NT and the presence of legume cover crops in the surface soil layer (0−5 cm) increased the abundance of plant-derived lipids (i.e. compounds with n-alkane chains of 25−33 C atoms) in the whole soil. Microbial-derived lipids (i.e. compounds with shorter n-alkane chains (15−24 C atoms)) were more abundant in the clay fraction of the surface (0−5 cm) and sub-surface soil layers (20−30 and 75−100 cm) in NT soil receiving high-quality residues of legume cover crops. However, N fertilisation decreased the abundance of microbial-derived lipids in the clay fraction of the 0−5 and 20−30 cm soil layers. Our findings highlight the role of N-rich residues of legume cover crops, but not of mineral N fertilisation, in the long-term stabilisation of C in MOAs in NT soils through the action of microbial residues.