Combined attributes of soil nematode communities as indicators of grassland degradation

As a prominent component of the terrestrial biosphere, soil nematodes constitute a potential indicator for biomonitoring systems. Although nematodes respond to environmental variables or gradients, the mechanism and reason of changes in nematode assemblages with the retrogression of grasslands induced by overgrazing remain poorly understood. This study aimed to comprehensively assess the response of nematodes to degraded grasslands. We characterized shifts in multiple attributes of nematode communities across three grasslands with different levels of degradation. Importantly, we found a crucial potential threshold for decreases in the relative abundance and diversity of omnivore-predators when grassland degradation exceeds the moderate level (soil salinity ranging from 200 to 400 mu S cm(-1)). Exceeding the threshold (soil salinity above 400 mu S cm(-1)), rare genera with specific requirements disappeared, and the common taxa of Dolichodoridae, Eumonhystera, and Prismatolaimus accounted for 51.6% of the total nematode population. The top-down control was eliminated, which created a simple and less stable soil food web in severely degraded grasslands. Meanwhile, the abundance and functional metabolic footprint of nematode communities showed a hump-shaped pattern, reflecting compensatory mechanisms below the threshold. Our study highlights the dominant role of soil salinity in nematode communities, overriding the importance of food resources. Omnivore-predators are highly sensitive to soil salinity and their relative abundance and diversity can be used in combination as integrated indicators of ecological degradation. This study provides reliable and complementary information for evaluating grassland degradation or attempting restoration.