Silver Nanoparticles Alter the Diazotrophic Community Structure and Co-Occurrence Patterns in Maize Rhizosphere

Biological nitrogen (N) fixation is an ecological method used to provide nutrition for crops and reduce fertilizer application in terrestrial ecosystems. Silver nanoparticles (AgNPs) are becoming environmental contaminants, and, thus, could negatively affect the activity and diversity of soil diazotrophs. To test this, a greenhouse pot experiment for growing maize was performed under different concentrations of AgNPs (0, 1, 5, 10, 20 mg kg−1). We measured the N2-fixation activity and abundance of nifH gene encoding the nitrogenase reductase subunit and analyzed the diversity, composition and co-occurrence networks of diazotrophic communities in maize rhizospheric soil. Results showed that a lower dose of AgNPs did not show significant influence on soil diazotrophs, while a higher dose of AgNPs decreased both soil N2-fixation activity and nifH gene abundance, though diazotrophic diversity remained unchanged. AgNPs at 10 mg kg−1 and 20 mg kg−1 strongly shifted the community composition of diazotrophs, increasing the proportions of Bradyrhizobium and Paenibacillus, while decreasing Azospirillum and Rhizobium. Network analysis revealed weakened negative associations among species under AgNPs, with keystone taxa shifting from Bradyrhizobium, Geobacter, Azospirillum and Burkholderia to Bradyrhizobium, Paenibacillus and Skermanella under AgNPs. Soil-soluble Ag, dissolved organic carbon and soil pH were identified as the factors most closely driving the diazotrophic community composition. In conclusion, higher doses of AgNPs could inhibit N2-fixation activity and shape the diazotrophic communities. These findings provide empirical evidence of AgNPs’ ecological impacts on soil microbial functions.