Tillage management exerts stronger controls on soil microbial community structure and organic matter molecular composition than N fertilization

Abstract

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 not well defined. Soil samples were collected from a 24-year field trial comprising of two tillage practices (conventional and conservation) and two N fertilization rates (low: 12, moderate: 132–172 kg N ha-1 yr-1) in Southern Ontario, Canada. Soil organic carbon, molecular-level OM characterization using targeted compound and solid-state 13C nuclear magnetic resonance (NMR) analyses, bacterial and fungal abundance using quantitative PCR and community composition using DNA sequencing were used to assess differences in soil carbon processes. Despite similar soil organic carbon concentrations across treatments (16.7–31.2 g/kg), conservation tillage increased specific OM components (i.e., long-chain acyclic lipids, cyclic lipids and simple sugars) than conventional tillage for both N rates. Cutin- and suberin-derived compounds were also higher under conservation than conventional tillage with both N levels, suggesting the preservation of cutin- and suberin-derived compounds with conservation tillage. In contrast, conservation tillage resulted in lower lignin-derived compounds relative to conventional tillage for both N rates (5.4–5.8 vs. 6.3–9.6 mg/g soil OC), likely due to higher decomposition of lignin associated with altered microbial community composition. Under conventional tillage, moderate N fertilization resulted in lower lignin-derived compounds than low N addition (6.3 vs. 9.6 mg/g soil OC). Interestingly, the significant differences between the two N rates for several soil OM compounds and fungal community composition were only observed with conventional tillage but not conservation tillage, suggesting that the control of N fertilization on soil OM dynamics may depend on the type of tillage practices. Overall, tillage management is a more important driver of soil carbon cycling than N fertilization, and conservation tillage may enhance the decomposition of specific soil OM components (i.e., lignin-derived compounds) via changes in microbial communities.