Microbial Mechanisms of the Priming Effect over 12 Years of Different Amounts of Nitrogen Management

The return of crop residues and application of chemical nitrogen (N) can influence the soil organic carbon (SOC) turnover. However, the changes in the response of the priming effect (PE) to N management in real farming systems are not fully understood. In this research, we launched a 270-day in situ experiment in three N management plots (N0, no N; N1, 300 kg hm(-2); and N2, 360 kg hm(-2)) on a long-term maize farm in order to examine the microbial mechanisms that trigger the PE in the presence of C-13-labeled maize residues. We found that N1 decreased SOC mineralization and the positive PE, but increased the residual C mineralization and microbial C use efficiency in comparison with N0 and N2, respectively. The positive PE can be explained by the microbial nutrient mining theory for N0 and by the microbial stoichiometry decomposition theory for N1 and N2, as reflected by the increased abundance of oligotrophic phyla in N0 and the increased abundance of copiotrophic phyla in N1 and N2. The microbial biomass C (MBC), residue-derived MBC, and the communities' complexity were decreased in N2 due to the acidification of the soil environment, but N1 enhanced the MBC, residue-derived MBC, and bacterial communities' complexity. The keystone bacterial taxa of Vicinamibacteraceae and Gemmatimonas preferred the recalcitrant C of SOC in N0 and N2, respectively. However, Acidibacter favored the labile residual C in N1. The keystone fungal taxa of Penicillium, Sarocladium, and Cladophialophora exhibited wide substrate-use abilities in N0, N1, and N2, respectively. Our research depicts the mechanisms of how microbial communities' structures are reshaped through N management and emphasizes the functions of the keystone microbial taxa in C turnover and the PE in farming systems.