Opposing effects of increasing numbers of added resources on producer richness but not biomass in terrestrial and aquatic ecosystems.

While resource enrichment can shape community structure and ecosystem functioning, how species diversity and biomass production respond to the input of multiple resources across habitats-particularly between terrestrial and aquatic ecosystems-remains poorly understood. Using a meta-analysis, we show that multiple resource addition consistently increased primary production regardless of the study system, indicative of widespread resource co-limitation. Multiple resource addition, however, differentially affected plant and algal richness. As the number of added resources increased, terrestrial plant richness tended to decline, which was best explained by diminished niche dimensions and soil acidification. In contrast, aquatic algal richness tended to increase with the number of added resources, which is in agreement with the benthic model for coexistence. Resource identity also mattered as nutrient and water addition had opposing effects on terrestrial plant richness, and nitrogen addition had a weaker positive effect on algal richness than phosphorus and micronutrients. These findings suggest that different ecological mechanisms may regulate terrestrial and aquatic producer diversity, with important implications for understanding biodiversity responses to environmental change.