Litter quality drives nitrogen release, and agricultural management (organic vs. conventional) drives carbon loss during litter decomposition in agro-ecosystems

Litter decomposition and nutrient mineralization are crucial in agricultural systems to maintain soil fertility and plant growth. Given that these processes are governed by soil microbial activity, agricultural management that affects soil microbial communities may significantly alter rates of decomposition and N release of the same litter. We hypothesized that organic compared to conventional management enhances litter decomposition and litter N release, and that this effect is stronger for litter of low quality (high C:N ratio). We tested these hypotheses using litter from 4 maize cultivars with varying initial litter quality (different C:N ratios and lignocellulose index). These litters were left to decompose in soil with different management history, yet in the same experimental field site. The field experiment consisted of randomized plots with 11 years of organic or conventional agricultural management (organic vs. mineral fertilization). During the 11 years, in year 3 and 4, two specific organic amendments were applied as soil health treatments (SHT: chitin or compost, and a control without SHT). The maize litter was contained in litter bags, buried in the top 10–15 cm soil and collected after 1, 2 and 3 months. We quantified the litter carbon (C) and nitrogen (N) loss, and soil dissolved organic carbon (DOC), mineral and dissolved organic nitrogen (DON) at each sampling time. We also determined the fungal biomass in the decomposing litter after 3 months of decomposition. Litter C loss was higher in soil under organic compared to conventional management, irrespective of litter quality. In contrast, the rate of N release from the litter was determined by initial litter quality (higher N release from low C:N litter) and not by agricultural management. In soil under organic management the concentrations of DOC, mineral N and DON were larger than in conventional managed soil, which may have stimulated microbial activity and therefore, litter decomposition. Fungal biomass in the decomposing litter negatively correlated with the amount of N in the decomposing litter, but was not affected by management system or litter cultivar. Overall, we found that in agroecosystems initial litter quality (C:N) is a main driver of litter N release, whereas soil management is a main driver of decomposing litter C loss. Our results show the importance of integrating both litter quality and soil management to enhance our understanding of litter decomposition and N release, and to harness the ecosystem services provided by crop litter in agricultural fields.