The Effects of Localized Plant–Soil–Microbe Interactions on Soil Nitrogen Cycle in Maize Rhizosphere Soil under Long-Term Fertilizers

Long-term fertilization can result in changes in the nitrogen (N) cycle in maize rhizosphere soil. However, there have been few reports on the impacts of plant–soil–microbe regulatory mechanisms on the N cycle in soil. In this study, soil samples were collected from a long-term experimental site located at Jilin Agricultural University, Changchun City, Jilin Province, Northeast China. We then analyzed the changes in the functional genes related to the N cycle, soil enzyme activity, and maize root exudates under long-term fertilizer application using metagenomics and liquid chromatography analysis. We aimed to investigate the response of the N cycle to long-term fertilizers, the interaction among plant, soil, and microbes, and the effect of the plant–soil–microbe system on the N cycle. Long-term fertilization had a significant effect on soil N contents, N2O emissions, and enzyme activity related to the N cycle in maize rhizosphere soil. The functional genes of the N cycle were mainly enriched in the N degradation pathway in maize rhizosphere soil. N fertilizer application decreased the abundance of functional genes related to N fixation and degradation, denitrification, and assimilatory nitrate reduction (ANRA) and increased the abundance of functional genes participating in dissimilatory nitrate reduction (DNRA) and anaerobic ammonia oxidation/hydroxylamine oxidation. The soil environment was positively related to soil enzyme activity and negatively related to the microbial community composition and amino acids in root exudates. The contribution rate of microorganisms to the N cycle was the highest (r2 = 0.900), followed by amino acids (r2 = 0.836) and the soil environment (r2 = 0.832). Therefore, we concluded that N fertilizer is the main factor limiting the soil N cycle and that microorganisms are the main factor regulating the N cycle in the plant–soil–microorganism system.