Functional diversity mediates the nonlinear relationship between tree species richness and soil denitrification in young subtropical forests

Soil denitrification is strongly linked to soil nitrogen availability and influences nitrate leaching and greenhouse gas emissions. In turn, it can be regulated by ecosystem productivity through plant nitrogen uptake and organic matter inputs. However, the specific effects of tree diversity on soil denitrification and the underlying mechanisms remain poorly understood in forest ecosystems.

Here, we investigated the effects of tree species richness (across five levels: 1, 4, 8, 16 and 32 species) and the corresponding functional structure on soil denitrification through a large biodiversity experiment in young subtropical forests (3‐year‐old).

Our results revealed a unimodal relationship between tree species richness and denitrification potential, with the peak at four species. The structural equation model indicated that tree species richness was positively associated with the functional dispersion of leaf dry matter content, which in turn increased ground basal area, thereby promoting denitrification potential by enhancing soil water‐filled pore space, elevating the abundance of the nirK gene and reducing the soil C:N ratio. Additionally, the community‐weighted means of root tissue density and specific leaf area positively and negatively influenced soil denitrification potential, respectively.

Synthesis and applications . Our results suggest that soil denitrification is more pronounced in young forest communities characterized by high productivity and dominated by resource‐conservative species with high root tissue density and low specific leaf area. We recommend that forest managers prioritize high functional trait diversity rather than tree species richness to maximize niche complementarity in planted forests, an approach that would improve both economic and ecological benefits. These findings provide a foundation grounded in evidence for integrating biodiversity, nutrient cycling and selection principles based on traits into afforestation and stand transformation.