Fungicide, not nitrogen, reduces soil nematode diversity and multifunctionality in alpine grasslands

Abstract

Fertilizers and pesticides can be used to enhance pasture productivity, but their long-term and combined impacts on soil biodiversity and ecosystem functions remain unclear, particularly in alpine ecosystems. Here, we conducted a decade-long experiment involving fungicide application (with or without) nested within a nitrogen (N) addition gradient in a species-rich alpine pasture on the Tibetan Plateau. Soil nematode communities were assessed by Illumina sequencing, and soil multifunctionality was quantified from eight key ecosystem functions. Fungicide application markedly reduced nematode taxonomic and phylogenetic diversity, simplified nematode food webs and co-occurrence networks, and significantly decreased soil multifunctionality. In contrast, N enrichment increased the relative abundance of r-strategy nematodes but had no effect on nematode diversity or soil multifunctionality. No significant N and fungicide interactive effects were detected, suggesting that these two disturbances may act through independent pathways under alpine conditions. Fungicide had strong direct and indirect effects on nematode communities, and the indirect effects were primarily driven by changes in plant diversity and productivity, rather than by soil fungal community characteristics or soil physicochemical properties. Changes in soil multifunctionality were mostly attributed to the fungicide-induced shifts in nematode community composition and network complexity. This is the first long-term field study to jointly assess N and fungicide effects on soil nematodes and multifunctionality, highlighting fungicide use as a greater threat than N enrichment to belowground biodiversity and ecosystem sustainability in alpine grasslands.