Response of Soil Nitrogen-Cycling Genes to the Coupling Effects of Arbuscular Mycorrhizal Fungi Inoculation and Biochar Application in Maize Rhizosphere

Nitrogen (N) is the primary element that limits crop growth, and improving the nitrogen uptake in crops is a key challenge in sustainable agricultural production. Arbuscular mycorrhizal fungi (AMF), as important symbiotic microbes associated with most plants, can facilitate nitrogen uptake by plants and reduce greenhouse gas emissions, meaning they can play an important role in the development of sustainable agriculture. However, the effects of biochar application on mediating AMF N absorption are not clear, especially regarding the functional genes related to the N cycle in soil. In this study, we conducted a pot experiment with two P application rates (−P and +P) to study the effects of biochar and AMF on the community of soil microorganisms and N-cycle genes using metagenomic methods. The N uptake of both the shoots and roots of maize was measured. It was observed that the N uptake in the maize shoots and roots was significantly increased when they were exposed to a combination of AMF and biochar. Under both the −P and +P application rates, the root weights of the AMF and biochar combined (AMBC) treatments increased significantly by 58.3% and 43.2%, respectively, compared with the control (CN) treatments. Furthermore, there were significant increases in the root lengths, of 78.43% and 53.09%, respectively, as well as increases in the superficial areas of 60.0% and 41.9%, respectively. The combination treatment significantly changed the soil microbe community structure and increased the abundances of Geobacter and Pseudomonas. In addition, the abundances of the N-cycle genes of each process were enhanced. Under the −P condition, the total abundances of the N-cycle genes increased significantly by 1.97–2.19 times in the AMBC treatment compared with the CN treatment. Overall, the results suggest that biochar and AMF can promote plant root growth and lead to changes in the soil microorganism structure, resulting in an increase in the abundances of N-cycle genes which, in turn, increase the N uptake in the shoots and roots of maize. This study provides a biological pathway to improve the efficiency of N utilization in soil and prevent environmental pollution in sustainable agricultural production.