Grazing induces positive direct effect of shrubs on nematode diversity but suppresses indirect effects through microbial pathways

Background and aims: Dominant plants shape belowground communities, which in turn influences ecosystem functioning. Similarly, herbivores affect belowground communities through physical disturbance and redistribution of organic inputs, but also through their interactions with plants themselves. However, we know little about how grazing moderates the effects of dominant plants on belowground organisms. Methods: We established a three-year shrub removal experiment in a grazed and an ungrazed alpine meadow on the Qinghai-Tibet plateau to assess how grazing moderates the effects of a dominant plant, Dasiphora fruticosa, on nematode communities. We used piecewise structural equation modelling to assess how grazing moderates the direct effects of dominant plants and, indirect effects via changes in edaphic variables and microbial pathways. Results: We found that 1) grazing significantly decreased nematode diversity mainly due to a reduction in omnivores; 2) there was a significant grazing by shrub interaction effect on nematode diversity, whereby shrub had a positive direct effect on nematode diversity only in the grazed meadow; 3) in the ungrazed alpine meadow, microbial diversity was a strong predictor of nematode, while in the grazed meadow, grazing suppressed the microbial pathways of shrub presence. Conclusion: The study shows that grazing decreases the diversity of soil communities and suppresses the interactions among soil biota, while shrub presence plays a vital role in alleviating the negative effects of grazing on understory communities. Therefore, the findings further highlight herbivory in inducing facilitation of dominant shrubs on understory community stabilization and ecosystem functioning in alpine meadows of the Qinghai-Tibetan plateau.