Functional traits explain the enhancement of denitrification by wetland plants

Abstract Denitrification is one of the most important ecosystem functions in wetland ecosystems, affecting nutrient cycling, water purification and the global nitrogen budget. However, the effects of wetland plants on denitrification remain unclear because of the complex plant‒microbe‒substrate interactions involved. In this study, we evaluated the effects of 25 wetland plant species traits on denitrification based on data from meta-analyses on denitrification, global wetland plant trait databases and structural equation modelling (SEM). We use plant functional traits as an indicator to quantitatively model the multifaceted ecological processes controlling denitrification as well as the overall denitrification enhancement by wetland plants. We found that both direct and indirect (i.e., the effect of one variable on another is mediated by a third variable) pathways strongly contribute to the overall enhancement of denitrification by wetland plants. Specifically, wetland plant root traits jointly control substrate oxygen availability, which reduces the enhancement of denitrification by wetland plants through oxygen inhibition effects. Plant nitrogen status facilitates denitrification enhancement probably through organic nitrogen input by plant leaves and root exudates. Root biomass directly decreases denitrification enhancement through competition for nitrogen uptake with denitrifying bacteria. Our results provide a systematic view of the effect of wetland plant traits on denitrification enhancement. The application of plant functional traits in revealing the complex denitrification processes helps us to better understand the impacts of plants on wetland ecosystem functioning and inspire new practices in ecological engineering and ecological management on denitrification through a trait-based perspective.