Shifting community composition determines the biodiversity–productivity relationship under increasing precipitation and N deposition

Questions The relationships between biodiversity and ecosystem functioning (BEF) vary largely across natural ecosystems, with a unimodal, monotonous linear or no relationship. However, it remains unclear how BEF relationships vary under global change. Given future predicted changes in precipitation and nitrogen (N) deposition, it is crucial to determine how precipitation change and N deposition affect grassland biodiversity and productivity, and regulate their relationships. Location A Dry Mixed‐Grass Prairie of western Canada. Methods We established a manipulative field experiment of increased precipitation (water addition with approximately 15% and 30% more monthly precipitation) and N addition (10 g/m2) using a randomized complete block design, including six treatments, each replicated five times (30 plots). We conducted vegetation sampling with a 1 m × 1 m quadrat in each plot from 2016 to 2017, to examine the individual and interactive effects of water and N addition on plant diversity, functional group composition, above‐ and below‐ground productivity, and the diversity–productivity relationships. Results We found a positive linear diversity–above‐ground biomass (AGB) relationship under increased precipitation, which was attributed to the water‐induced increase in the abundance of forbs in the plant community, further promoting the positive effect of plant diversity on AGB. However, N addition caused a negative linear diversity–AGB relationship by increasing AGB and reducing plant diversity. The effects of N on diversity and productivity can be further strengthened under increased precipitation, which was due to the increase of C3 rhizomatous grasses with high above‐ and below‐ground biomass and the decrease of forbs with high richness. Conclusions Our results suggest that changes in functional group composition determine the plant species’ diversity, productivity, and their relationships under increasing precipitation and N deposition, which has significant implications for understanding and modelling ecosystem productivity in the context of global change. Our study examined the effects of increased precipitation and N addition on plant diversity, productivity and their relationships in a dry grassland. We highlighted that the increase of forbs under increased precipitation enhanced a positive diversity–productivity relationship, while the addition of nitrogen under ambient or increased precipitation led to a negative diversity–productivity relation