Arbuscular Mycorrhizae Affect Soil Nitrogen Fertilizer Utilization, Denitrification Functional Genes, and N2O Emissions During Biochar Amendment

Arbuscular mycorrhizal fungi (AMF) can form symbionts with plant roots, acquire soil nitrogen, and affect nitrous oxide (N2O) production. Biochar, as a soil additive for the management of agricultural soil, affects soil nitrogen (N) utilization and plant growth. However, how AMF regulates soil N unitization, the denitrification process, and N2O emissions in plant–soil systems remains largely unknown, particularly under the biochar amendment. In this study, a microcosm experiment was conducted to investigate the impacts of different mycorrhizal treatments (CK: neither AMF hyphae nor plant roots; AMF: only AMF hyphae; AMF + R: AMF hyphae and plant roots) on plant growth, soil N fertilizer utilization, N2O production and consumption, functional gene abundance, and N2O emission at two biochar addition levels (B0: no biochar; B1: biochar addition rate of 10 g·kg−1 soil) in a maize planting soil system. The results revealed that AMF alone and AMF with plant root treatments enhanced the fresh weight of maize plants by 10.15% and 19.23% and decreased soil inorganic N contents by 33.28% and 75.56%, respectively. The combination of biochar, AMF, and plant roots showed the largest increase in maize plant biomass. The AMF and AMF with root treatments all significantly decreased the nirS + nirK/nosZ ratio and N2O emissions at two biochar levels. The presence of AMF and plant roots during biochar amendment showed the smallest nirS + nirK/nosZ ratio and N2O emissions. The AMF combined with biochar and AMF and plant roots combined with biochar treatments increased nirS + nirK/nosZ by 24.32% and 26.90% and decreased N2O accumulation emission by 21.12% and 38.13%, respectively. The results imply that biochar, AMF, and plant roots reduced N2O emissions directly by reducing soil N and increasing soil N unitization efficiency and indirectly by shifting the N2O production and consumption gene abundance in agroecosystems. These findings suggest that the addition of biochar and AMF and/or the presence of plant roots can interact to alleviate soil N2O emissions by manipulating plant inorganic N acquisition and the soil denitrification process.