Altered soil organic matter composition and degradation after a decade of nitrogen fertilization in a temperate agroecosystem

Abstract

Nitrogen (N) fertilization has been found to alter soil carbon storage in agroecosystem as various soil organic matter (OM) components respond differently to environmental change. However, how the direction and magnitude of soil carbon storage changes in response to N fertilization remains unclear, and a lack of deep understanding on soil OM composition particularly under various fertilization rates constrains our ability to reveal soil OM dynamics. In this study, we investigated soil OM composition and degradation at the molecular-level in response to various rates of N fertilization, and the potential mechanisms that govern these alterations. Soil samples (0−10 cm) were collected from a 10-year N fertilization experiment in Southern Ontario, Canada with increasing rates (non-fertilized control, 30, 87, 145 and 260 kg ha−1 yr−1). The samples were analyzed for soil organic carbon and total N concentrations, soil OM composition (i.e. plant-derived steroids, cutin-, suberin-, lignin-derived compounds), and microbial biomass and community structure. Despite similar soil organic carbon (21.7–23.1 g/kg) and total N contents (2.0–2.2 g/kg) across all treatments, decreased concentrations of plant-derived steroids, cutin- and suberin-derived compounds under some N addition treatments were observed. This was consistent with lower alkyl carbon contents from solid-state 13C nuclear magnetic resonance (NMR) spectroscopy analysis. The lower plant-derived steroids, cutin- and suberin-derived lipids may be explained by the enhanced degradation of steroids and cutin likely associated with higher microbial biomass under N fertilization. Lignin-derived phenols were elevated under N fertilization, which is consistent with the aromatic/phenolic carbon contents (mainly from lignin) from NMR analysis. The enhanced lignin degradation, which is consistent with higher fungal biomass (lignin degraders), did not result in lower lignin-derived compound or aromatic/phenolic carbon content under N addition. As other studies reported higher lignin contents in corn residues under N fertilization, we suggest that the turnover of lignin-derived phenols may be governed not only by degradation processes but also by the quantity and quality of crop residues. Furthermore, we observed that different N rates exert various controls on soil OM cycling. For example, the increase in degradation of steroids, cutin and lignin was smaller above the threshold N rates 145 kg ha−1 yr−1, implying that the degradation of these compounds was fertilization rate-dependent. These results highlight that various N fertilization levels alter the allocation of carbon and soil OM dynamics likely through changing biodegradation and the quantity and quality of crop residues.