Nitrogen and phosphorus enrichment differentially affect grassland ecosystem functioning via multi-trophic pathways

The enrichment of nutrients (e.g., N and P) is crucial in shaping the
links between diversity and ecosystem functions. Previous research has
mainly focused on how nutrient enrichment affects community diversity and
composition within a single trophic level, such as plants or soil
microbial communities. However, how nutrient enrichment affects community
composition and diversity across multiple trophic levels and ecosystem
functions has not been extensively explored. By conducting two long-term
N- and P-enrichment experiments in temperate steppes, we explored the
response of diversity and composition at both the community and functional
group level across the soil food webs to nutrient enrichment, as well as
their relationship with ecosystem functions. We found that the responses
of soil biota and plants to nutrient enrichment varied depending on the
community and functional group. N-enrichment resulted in a decrease in
plant and bacterial richness at both the community and functional group
levels, while P-enrichment decreased fungal and nematode richness at both
levels. In addition, N-enrichment changed the community composition of
plants and all soil biota, while P-enrichment only affected the community
composition of fungi and nematodes. Our findings also showed that
community composition across multiple trophic levels played a crucial role
in regulating ecosystem functions (e.g., plant biomass, soil biota
biomass, and soil organic matter (SOM) decomposition) when considering
both community composition and diversity simultaneously. For example,
N-enrichment induced changes in the community composition of plants,
bacteria, and nematodes, which altered plant biomass, while P-enrichment
induced changes in the community composition of nematodes that altered
soil biota biomass and SOM decomposition. Overall, our study suggests that
long-term nutrient enrichment may have a greater impact on community
composition than diversity across multiple trophic levels, and these
changes in community composition can have broad implications for ecosystem
functions. Therefore, more effort should be made not only on community
diversity but also on composition to clarify the relationship between
diversity and ecosystem functions in future studies under climate change.