Iron/Manganese Oxide-Bound Nitrogen Suppresses Microbial Mediated Nitrate Reduction in Soil

Nitrogen cycling plays a pivotal role in sustaining ecosystem productivity and stability, while the chemical forms of nitrogen in soils critically influence both its efficiency and pathways. In this study, farmland soils from the Anning River Basin—an important watershed in Southwest China—were investigated to examine the regulatory effects of environmental factors, particularly iron/manganese oxides-bound nitrogen (IMOF-N), on microbial nitrogen cycling. Measurements of different nitrogen forms, nitrogen transformation percentage, metagenomic sequencing, and multivariate statistical analyses were integrated. Among the five nitrogen forms tested, IMOF-N emerged as the most influential factor affecting the abundance of nitrogen-cycling functional genes. Both random forest modeling and partial least squares path analysis revealed significant correlations between IMOF-N and key genes involved in assimilatory nitrate reduction (e.g., nasB, nasC) as well as dissimilatory nitrate reduction (e.g., nirB, nirD, nrfA). High IMOF-N levels significantly suppressed the expression of genes involved in dissimilatory nitrate reduction, with expression reductions ranging from 46.9% to 69.7%, and reduced the complexity, connectivity, and stability of microbial interaction networks, with Actinobacteria showing high sensitivity to IMOF-N perturbation. In contrast, ammonium nitrogen enhanced nitrogen cycling, while soil water content (SWC) negatively correlated with nitrification conversion percentage but positively with ammonification conversion percentage, likely via regulating oxygen availability. These findings reveal a novel mechanism whereby IMOF-N acts as a stable nitrogen form that suppresses key gene expression and simplifies microbial networks, thereby negatively modulating soil nitrogen cycling. The study provides a scientific basis for optimizing nitrogen management in the Anning River Basin and other Fe/Mn-enriched agricultural regions.