Grassland degradation status modulates the relative contribution of soil nematode diversity and community structure to ecosystem functioning under nitrogen fertilization and irrigation management

Restoring degraded grasslands is essential for the provision of ecosystem services and sustainability. Fertilization and irrigation are keys for restoring productivity in semiarid grasslands, but their impact on belowground processes, including those regulated by soil nematode diversity and community structure, remains underexplored. Here, we evaluated how irrigation (+ 50 % of long-term mean growing season precipitation) and nitrogen (N) fertilization (10 g N m⁻² y⁻¹) affect nematode diversity and community structure, and the contributions of nematodes to ecosystem functions across three grasslands (lightly, moderately and severely degraded). Community structure was analyzed via nematode co-occurrence network complexity and compactness. Soil functioning was evaluated against a multifunctionality (EMF) index comprising 10 functional proxies, and systematic models were constructed to link nematode diversity, community structure, and EMF. The irrigation enhanced nematode diversity and network topological parameters, whereas the fertilization had limited effects. Fertilization modified the effects of irrigation on nematode diversity, enhancing its positive effect in severely degraded grassland, while offsetting the positive effects in lightly and moderately degraded grasslands. The irrigation and N fertilization treatments enhanced eight individual functions and EMF. Fertilization further altered the response of EMF to irrigation, diminishing its positive effect on EMF in the severely degraded grassland. In the lightly degraded grassland, nematode diversity primarily affected EMF, whereas in the other two grasslands, network complexity was the key predictor. Our results indicate that irrigation, rather than the N fertilization, primarily contribute to the restoration of the studied degraded grasslands. We emphasize the crucial role of nematode community structure in regulating the response of EMF to restoration measures such as irrigation and fertilization in severely degraded grassland.