Plant species richness enhances aboveground primary productivity via net biodiversity effects and bacterial community interactions

The multifunctionality of grassland ecosystems is significantly affected by dominant and subdominant species, particularly in terms of aboveground primary productivity. Few studies have explored how various dominant and subdominant plant species control the interactions between several trophic levels, driving aboveground productivity. To investigate the direct and indirect drivers of productivity in dominant and subdominant plant communities, we selected three dominant and seven subdominant species from the Ningxia northern Yanchi Desert steppe for monoculture and mixture experiments to establish the richness of plant species (monoculture and 4-, 6-, 8-, and 10-species mixtures). We examined the aboveground biomass, soil properties, insect diversity, and microbial diversity of different plant species combinations, linking biotic and abiotic factors with aboveground primary productivity. We found: (1) a significant positive correlation between species richness and net biodiversity effects. Higher aboveground relative productivity was achieved for mixed species than for monoculture, particularly for Elymus dahuricus, and the selection effect had a unimodal relationship with increased species richness, with the highest productivity observed in the 8-species mixture. (2) The co-dominant insect groups in various plant species were the phytophagous Miridae, Agromyzidae, and Cicadellidae families, constituting 69.5 % of all insects collected. Plant species richness had an encouraging influence on phytophagous insect diversity, but the effects on predatory, omnivorous, and parasitic insects remained insignificant. Additionally, insect similarity indices for different plant species richness values were lower than 0.5. (3) Stoichiometric analyses of soil enzymes revealed a reduction in microbial metabolism primarily by the relative limitation of nitrogen and phosphorus, with nitrogen limitations being significantly stronger in the monoculture than in the 8-mixture species; the dominant bacterial phyla, Actinobacteria and Ascomycetes, played essential roles in promoting plant growth in desert steppes, and bacterial biodiversity influenced primary productivity more prominently than fungal biodiversity. Our findings underscore the importance of conserving species diversity and its potential benefits for ecosystem restoration and sustainability.