Contrasting Tillage Practices and Fertilization Rates Alter Microbial Community and Soil Organic Matter Molecular Composition

Different tillage and nitrogen (N) fertilization practices markedly alter soil carbon dynamics yet the underlying mechanisms and their interactive controls on soil organic matter (OM) biogeochemistry are still poorly understood. Soil samples were collected from a 24 year experiment comprising of conventional tillage (CONVT) and conservation tillage (CONST) under either low or high N fertilization rates (12 vs. 132 kg N ha-1 yr-1) in Southern Ontario, Canada. Soil organic carbon, molecular-level OM characterization using targeted compound analyses and solid-state 13C nuclear magnetic resonance (NMR) were used to assess differences. Bacterial and fungal gene abundance using qPCR (16S and 18S rRNA) and community composition using DNA high-throughput sequencing (16S rRNA and ITS) were also analyzed. Despite similar soil organic carbon across treatments, higher degradation of steroids and cutin-derived compounds was observed under CONST than CONVT, likely related to higher bacterial and fungal gene abundances and changes in community composition. Particularly, cyclic lipids, cutin- and suberin-derived compounds were either similar or higher under CONST than CONVT, suggesting that the contents of these compounds were likely controlled by both OM inputs and microbial decomposition. Lignin-derived compounds were lower under CONST than CONVT, likely due to enhanced degradation associated with higher fungal gene abundance. Compared with low N rates, high N fertilization increased the degradation of steroids, cutin-, and lignin-derived compounds and decreased their concentrations under CONVT but not CONST. However, the relative abundance of cellulose (O-alkyl carbon) increased with high N fertilization, which is consistent with the lower soil OM decomposition (alkyl/O-alkyl carbon ratio). Compared with CONVT, CONST results in smaller differences between low and high N levels for several soil OM compounds and microbial community composition, highlighting the interactive controls of CONST and N fertilization on soil carbon dynamics. This study shows that CONST may enhance soil OM decomposition likely associated with changes in microbial communities and increase the resilience of soil carbon compositional change to disturbance induced by N fertilization.