Fertilization Diversity Induces Trophic Asymmetry in Ecosystem Responses to Extreme Drought

The role of fertilization diversity (the variety of nutrient types added) in modulating the effects of extreme drought on multi-trophic communities and their functions remains a critical unknown in ecology. We conducted a multi-nutrient factorial experiment (control, N, NP, and NPK additions) under simulated extreme drought in an alpine meadow on the Qinghai-Tibetan Plateau. We assessed the responses of soil abiotic properties, plant communities, soil microbial (bacteria and fungi) and nematode communities, and ecosystem multifunctionality (EMF). Our results reveal that fertilization diversity induces a trophic asymmetry in ecosystem responses to drought: increasing fertilization diversity (and total nutrient load) significantly altered the response of the plant community to drought by promoting the competitive dominance of drought-tolerant grasses, which in turn modulated ecosystem function of aboveground net primary productivity. Conversely, fertilization diversity amplified the negative effects of drought on soil bacterial and fungal biomass, likely due to exacerbating microbial carbon limitation. For soil nematodes, fertilization diversity modified the response of community composition to drought and simplified nematode communities. Consequently, despite these compensatory and contrasting responses across trophic levels, fertilization diversity led to a net decline in EMF under drought. This net loss occurred because the positive contributions from the plant pathway were decisively outweighed by the substantial suppression of microbial processes, revealing a fundamental decoupling between above- and below-ground components. Our findings demonstrate that the balance and diversity of nutrient inputs, rather than nitrogen dose alone, is a critical factor modulating ecosystem responses to drought, creating contrasting responses across trophic levels that necessitate a holistic, multi-trophic perspective for effective ecosystem management under global change.