Soil properties alter plant and microbial communities to modulate denitrification rates in subtropical riparian wetlands

The capacity of riparian wetlands to remove nitrogen from upland runoff through the denitrification process is mainly controlled by soil properties and biological communities. However, the pathways or mechanisms by which soil properties and biological communities may regulate riparian denitrification rates are still largely unknown. Here, we measured the potential and unamended denitrification rates in sediments from riverine wetlands along a nitrogen-rich river in subtropical China. We also investigated the plant richness, vegetation cover, and abundance of denitrifying microorganisms in wetland soils. The results show that both potential and unamended denitrification rates presented strong seasonality, and were found to be significantly associated with nitrogen concentration in soils, implying that denitrification process in riparian wetlands is probably nitrogen limited. Moreover, potential and unamended denitrification rates were positively related to denitrifier abundance, plant richness, and vegetation cover. Using structural equation modeling, we found that moisture and nitrogen concentration could influence denitrification rates directly; the indirect effects of these factors on denitrification rates mediated through modulating the structure of plant and microbial communities. These findings suggest that denitrification rates and nitrogen removal capacities in riparian wetlands are jointly modulated by soil properties and biological communities.