How the chemical composition and heterogeneity of crop residue mixtures decomposing at the soil surface affects C and N mineralization

The effects of plant litter characteristics on its decomposition in soil or at the soil surface is of primary importance for adequate management of nutrients and carbon (C) in agro-ecosystems. However, understanding the influence of mixtures of residues, which is actually the most common situation encountered in agriculture, is still poorly known in cultivated soils. Therefore we analyzed the effect of mixing leaf and stem litters from 25 species of plants (main crops and cover crops), representative of agricultural systems in subtropical conditions, on subsequent C and nitrogen (N) mineralization. We characterized the chemistry of leaves, stems and mixtures and determined the heterogeneity of the mixtures using Gowerʹs similarity coefficient. We incubated crop residues at the surface of a sandy loam soil at 25 °C over 120 days and continuously measured the mineralization of C and N. The 25 mixtures represented a wide range of chemical qualities and heterogeneity. Significant differences in C mineralization and soil N accumulation clearly differentiated crop families (notably Poaceae species vs. Fabaceae species), and plant parts (stems vs. leaves). The differences between observed and expected values for C mineralization were low or nil, indicating mostly an additive effect of mixing. Significant synergetic effects existed for only 7 species and resulted in an average additional 9% C mineralized. For N, an antagonistic effect was observed only with Fabaceae mixtures having high average N content and high chemical heterogeneity. We concluded that the decomposition of mixtures appeared mainly controlled by their average chemical composition and to a less degree by their chemical heterogeneity. In these cases, the availability of N in the mixtures appeared to increase the microbial N immobilization, reducing the net accumulation of mineral N in the soil.