Plant diversity promotes soil nitrogen retention and removal processes in wetlands

Summary Although nitrogen (N) cycling processes are fundamental to the functioning of wetland ecosystems, the effects and underlying mechanisms of plant diversity on soil N cycling remain insufficiently understood. We conducted a field survey (123 soil samples) and a pot experiment (528 soil samples) to examine how multidimensional plant diversity (species diversity, phylogenetic diversity, and functional group diversity) affects key N cycling processes in wetland soils across different phenological stages (seedling, fast‐growing, reproductive, and wilting). The results showed that multidimensional plant diversity was positively correlated with dissimilatory nitrate reduction to ammonium (DNRA), a N retention process, and denitrification, a N removal process, with greater effects on denitrification. Among the diversity metrics, phylogenetic diversity explained more variance in denitrification than species or functional group diversity, a pattern not observed for DNRA. Additionally, both plant diversity and phenological stages influenced soil N cycling processes directly or indirectly through their effects on soil dissolved organic carbon and functional gene abundances. Together, these findings highlight the critical role of plant diversity in regulating soil N cycling and offer a mechanistic understanding that can be incorporated into Earth system models to improve predictions of soil N dynamics in response to biodiversity change.