Soil biodiversity and ecosystem functions in grasslands: Is more always better?

Given the biodiversity crisis, research on soil biodiversity and ecosystem functioning (BEF) has grown rapidly. While a positive BEF relationship is often reported, whether it holds across different soils with distinct soil and plant communities remains understudied. Here, we conducted a greenhouse experiment containing five experimental grassland systems representing different (semi)natural grasslands. Each grassland system contained four biodiversity levels established by sequential filtering of the field soil community by size, creating a gradient in their presence, richness and thus community completeness. We found that shoot biomass remained unaffected by treatments. However, consistent with expectations of a positive BEF relationship, nitrification potential and microbial nitrogen content generally increased with biodiversity increase, whereas the relative abundance of predatory/parasitic bacteria decreased. On the contrary, high soil biodiversity led to a decrease in plant nitrogen content and soil urea degradation potential, suggesting that soil biodiversity may influence competition for nitrogen between plants and microbes. Moreover, while microbial biomass carbon was promoted by soil biodiversity in relatively fertile grassland soils and root biomass was unaffected, they were both reduced in poorer soils. These findings highlight that soil biodiversity may promote certain grassland functions but suppress others and that the direction of these trade-offs may depend on the soil characteristics or the biotic community it harbours. The conservation and management of soil biodiversity thus need to be evaluated in the context of the functions that are to be maximised and the grassland soil context.