Decreased nematode energy flow triggers soil microbial necromass loss in degraded grasslands

Human activities and climate change cause large-scale degradation of global grasslands, leading to significant loss of soil organic carbon (SOC). As pivotal multitrophic components within the soil food web, nematode-mediated energy flows constitute a critical link in deciphering the coupling mechanisms between soil biota and carbon (C) cycling. Although nematodes are highly sensitive to environmental changes, how nematode energy flow responds to grassland degradation and its relationship with SOC remain unclear, particularly in microbial necromass dynamics. Herein, we separately established grassland degradation sequences across six distinct sites spanning 2000 km. Using linear mixed-effects models, we examined the overall responses of soil nematode biomass, diversity, and energy flux to degradation across large scales, along with the linkages to microbial necromass. We found that grassland degradation decreased microbial necromass C (46–72 %), while increasing the contribution of bacterial necromass C to SOC (1.2–1.7 times) (p < 0.05). Compared to non-degraded grasslands, nematode abundance, biomass, diversity, and energy flux increased under light degradation but decreased under moderate and heavy degradation (p < 0.05). Energy flow in the bacterial channel decreased by 90–92 % under heavy degradation, significantly higher than the reductions in the fungal channel (49–59 %) and plant channel (12–20 %). This notable reduction ultimately reduced energy flow uniformity by 44 %, indicating uneven energy distribution among channels (bacterial, fungal, and plant). Plant and soil properties were the primary factors influencing microbial necromass, and also affected microbial necromass by regulating nematode energy flow. Overall, grassland degradation preferentially inhibited the bacterial energy channel, disrupting energy allocation balance within the nematode food web, and thereby potentially reducing microbial necromass accumulation. These findings advance our understanding of the biological drivers behind grassland SOC loss and provide theoretical foundations for developing soil C restoration strategies.