Soil nitrogen loss reduces food web energy flow via nematode community simplification during wild fruit forest degradation in the Tianshan Mountains, China

Wild fruit forests in the Tianshan Mountains are key constructive species of mountain ecosystems in Central Asia, and their degradation has seriously threatened regional ecological security. As typical indicators of soil health, the response mechanisms of nematode community structure and energy flow to wild fruit forest degradation remain unclear. This study focused on typical wild fruit forests in Xinjiang ( Malus sieversii and Armeniaca vulgaris ) and systematically analyzed changes in soil nematode community structure, diversity, and energy flow across different levels of degradation in wild fruit forests. The results showed: (1) wild fruit forest degradation significantly weakened soil nutrient status, which led to marked decreases in the abundance, diversity, ecological indices, and network robustness of the nematode community. Among them, omnivore-predators were the most sensitive to degradation. (2) Under severe degradation, the total energy flux of nematode food webs decreased by 55.2% in A. vulgaris forests and by 78.3% in M. sieversii forests. Flow uniformity declined by 22.2% and 7.35%, respectively. (3) However, in A. vulgaris forests, the abundance of Merlinius increased by 196.3% under light degradation, indicating a compensatory response. (4) The PLS-PM model indicated that soil nitrogen loss due to degradation reduced nematode community diversity, leading to decreased energy flux and distributional imbalance within the food web. Total energy flux significantly influenced soil multifunctionality in M. sieversii forests; however, this pathway failed in A. vulgaris forests, where total energy flux significantly impacted flow uniformity. This study elucidates the multi-level coupling processes among soil nutrients, nematode communities, energy flow, and soil multifunctionality. These findings provide a theoretical basis for early warning and ecological restoration of wild fruit forest ecosystem degradation. • Wild fruit forest degradation reduced soil nematode abundance, diversity, and stability. • Wild fruit forest degradation reduced energy flow and uniformity in nematode food webs. • PLS-PM model revealed the coupling among soil, nematodes, energy flow, and multifunctionality under degradation. • Decline in soil nitrogen was the key factor driving nematode simplification and weakened energy flow.