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Nitrogen (N) inputs to terrestrial ecosystems are increasing faster than phosphorus (P) inputs, causing N:P imbalances and substantial changes in the diversity and structure of plant, soil microbial, and fauna communities. The responses of the soil micro-food web to N:P imbalances remain unclear but are crucial to understanding the impacts on ecosystem functioning. We investigated the effect of N and P additions on plant communities, edaphic properties, and nematode species diversity and composition, and assessed shifts in soil micro-food web complexity using nematode-based indices and structural equation modeling (SEM) in a nine-year nutrient addition experiment in an alpine meadow on the Tibetan Plateau. We found that N addition decreased the structure index (SI) and enrichment index (EI), indicating that N enrichment led to a simplification of the soil micro-food web. The addition of P alleviated the negative effect of N on soil micro-food web complexity but did not restore nematode community composition. The SEM supported the hypothesis that N addition simplified soil micro-food webs through increases in the N:P ratio and associated reductions in plant species richness, while P addition ameliorated the effect of N by decreasing the soil N:P ratio and increasing plant species richness. Our results highlight the positive effects of plant diversity in maintaining the complexity of the soil micro-food web. Our findings provide information that can be used to guide sustainable management and restoration of grasslands to alleviate the projected effects of future N:P imbalances on the soil micro-food web. However, efforts should be made to avoid nutrient deposition given the observed effects on both plant and nematode communities that could not be restored by alleviating the N:P imbalance through P addition. • Phosphorus alleviates soil micro-food web simplification caused by excess nitrogen. • Nematode community composition remains unrestored despite phosphorus addition. • Plant diversity is crucial for maintaining complex soil micro-food webs. • Modifying nitrogen/phosphorus ratio alone can't fully restore communities. [ABSTRACT FROM AUTHOR]

Grazing by herbivores plays a key role in grassland ecosystems, but its effects on soil fauna biodiversity are not clear in alpine meadow ecosystems, especially on the Tibetan plateau. We investigated the effects of long-term grazing disturbance of varying intensities (low disturbed site, medially disturbed site, and seriously disturbed site) on ecosystem biodiversity and functional process using soil nematode communities. Soil physicochemical characteristics and plant communities were also measured to explore the relationships between soil nematode communities structure and key environment variable. Soil samples were collected in May, June, August, and October, 2013. The abundance of total nematode communities, plant feeders and bacterivores increased with the grazing intensity increase, and predators decreased with grazing intensity increase. Low disturbed site has the lowest abundance of fungivores. Grazing disturbance has no significant influence on abundance of omnivores ( P > 0.05). Species richness was highest on seriously disturbed site and lowest on low disturbed site. MI and SI decreased as grazing intensity increase. The highest NCR was found in low disturbed site and the lowest in medially disturbed site. The highest EI was found in medially disturbed site and the lowest in low disturbed site. Principal component analysis (PCA) showed that nematode communities could be separated by three different grazing disturbance intensity sites. Redundancy analysis (RDA) showed that Helicotylenchus , Acrobeloides and Labronema were positively correlated with higher plant communities diversity. Filenchus was positively correlated with forbs biomass. Some omnivores such as Eudorylaimus and Aporcelaimus was positively correlated with soil moisture and rich nutrient resources, meanwhile, Enchodelus and Coomansus was positively correlated with biomass of sedges and grasses. Anaplectus and Prismatolaimus significantly correlated with soil moisture, nutrient resources and biomass of sedges and grasses. This study revealed that grazing herbivores can change composition, structure and diversity of soil nematode communities, and then reflect radical shift in below-ground soil faunal biodiversity and processes in alpine meadow on the Tibetan plateau. [ABSTRACT FROM AUTHOR]