Applying Biochar to Calcareous Soil Promotes Maize Growth and Reduces Soil N2O Emissions by Enhancing Mycorrhizal Symbiosis

The effects of biochar on soil N2O emissions remain contentious, and the microbiological processes involved are not yet fully understood. Arbuscular mycorrhizal (AM) fungi, key players in soil nitrogen (N) cycling, may mediate the impact of biochar on plant N uptake and N2O emissions, but this interaction remains unclear. This study involved a two-year field experiment to examine how varying biochar application rates affect soil microbial communities, particularly AM fungi at rainfed maize (Zea mays L.) farmland, and to assess how AM fungi influence soil N2O emissions and maize growth under biochar addition with two AM fungi treatments (with and without suppression of native AM fungi). The results revealed that biochar significantly enhanced soil microbial diversity, especially the variety and richness of AM fungi. Biochar addition improved soil physicochemical parameters, including soil water content, bulk density, and inorganic N availability. Biochar also decreased AOA and AOB gene abundances, increased AM fungal gene abundances, lowered (nirK + nirS)/nosZ ratio, and reduced soil N2O emissions. Suppression of native AM fungi increased N2O emissions throughout the rainfed maize growing period, accompanied by a higher (nirK + nirS)/nosZ ratio. Biochar addition combined with non-suppressed AM fungi promoted maize growth, with the highest yield observed at 20 t ha−1 biochar. Overall, biochar decreased N2O emissions and strengthened the performance of AM fungi in rainfed maize farmland, highlighting the vital role of AM fungi s in soil N cycling under biochar addition. This study offers a scientific basis for using biochar in reducing N2O emissions and increasing crop yield in dry farmland.