Accelerated temporal turnover of the soil nematode community under alpine grassland degradation

Soil nematodes play pivotal roles in the soil food web. However, the trophic regulation on nematodes in the soil micro-food web and its temporal dynamics are less explored. Here, we investigated the seasonal dynamics of soil nematode community in an alpine grassland on the Qinghai-Tibet Plateau at non-degraded (ND), moderately degraded (MD) and severely degraded (SD) stages, either under open-top chamber warming (W) or not (NW). Nematode richness was higher in SD than in MD and ND that did not differ, accompanied by a clear shift from the bottom-up regulation via belowground productivity in ND to the top-down regulation via omnivore and predator nematodes in SD. The intensified top-down regulation in SD was explained by increased proportions of predator and omnivore nematodes, likely induced by the expanded soil pore size in SD. As a result, accelerated temporal turnover of nematode community was observed under grassland degradation, which were 0.006 (slope, p = 0.78), 0.045 (p = 0.07), and 0.077 (p = 0.001) in ND, MD, and SD, respectively. Moreover, grassland degradation strengthened the association between nematode density and ecosystem respiration, whose slope was higher in SD (0.0025, p < 0.05) than in MD (0.0008, p < 0.01) or ND (insignificant). In contrast, warming did not affect the temporal turnover of the nematode community, richness, nor its role in ecosystem respiration. Overall, we revealed the accelerated temporal turnover of nematode community and its strengthened role in determining ecosystem respiration under alpine grassland degradation, suggesting that trophic cascade changes under ecosystem degradation may alter the ecosystem functioning.