Priming of soil organic carbon decomposition induced by corn compared to soybean crops

The rate of soil organic carbon (C_S) loss via microbial respiration (decomposition rate k, y^-1), and the rate of stabilization of vegetation inputs (C_V) into C_S (humification rate h, y^-1) are usually considered independent of C_V. However, short-term laboratory studies suggest that the quality and quantity of C_V controls k, which is often referred to as a priming effect. We investigated how the chemical composition of different residues, (corn and soybean) controls k and h under field conditions in a no-till ecosystem. Using C_V-driven shifts in δ¹³C, we estimated changes in carbon (C) stocks, k and h of both the labile particulate organic matter fraction (C_POM) and the stabilized mineral associated organic matter fraction (C_MAOM). After two years of high C inputs (corn: 4.4Mgha^-1y^-1 aboveground and C:N=78; soybean: 3.5Mgha^-1y^-1, C:N=17), we found no changes in C_POM and C_MAOM stocks in the top 5-cm of soil or in deeper layers. However, C_MAOM in corn had higher k (0.06y^-1) and C output fluxes (0.67Mgha^-1y^-1) than in soybean (0.03y^-1 and 0.32Mgha^-1y^-1), but similar rates and fluxes in C_POM in the top 5-cm of soil. In addition, while C inputs to C_POM were also similar for both crops, C inputs from C_V to C_MAOM were higher in corn (0.51Mgha^-1y^-1) than in soybean (0.19Mgha^-1y^-1). Overall, corn plots had higher k and C inputs into C_MAOM and therefore higher C cycling in this fraction. Our data suggests that the type of crop residue strongly influences C cycling in the topsoil of no-till cropping systems by affecting both the stabilization and the decomposition of soil organic matter.