Tree species richness enhances network complexity rather than total diversity of soil nematode in a plantation forest

Increasing tree species diversity is an effective practice for forest restoration. It enhances multitrophic diversity and multifunctionality. Soil nematodes play a vital role in enhancing soil health, yet it has not been fully addressed about how tree species diversity affects the multitrophic diversity and interspecific interactions of soil nematodes. We investigated soil nematode communities in a planted forest ecosystem converted from agricultural lands. Soil nematodes were sampled across four tree species richness levels, and classified into four trophic groups (i.e., herbivores, bacterivores, fungivores and predators-omnivores) based on feeding guilds. We analyzed the effects of tree species richness, tree productivity, soil properties and trophic interactions on soil nematode communities. Results showed that total nematode diversity was not affected by tree species richness. The Shannon index of predators-omnivores decreased with tree species richness, while abundance and genus richness of herbivores declined with tree productivity. Structural equation models revealed that soil pH reduced the abundance and genus richness of herbivores and bacterivores through abiotic stress. Conversely, predators-omnivores increased the abundance and genus richness of herbivores and bacterivores via top-down trophic regulation. Notably, tree species richness intensified the complexity of nematode co-occurrence networks. These findings demonstrate that tree species richness, productivity, soil pH and trophic interactions collectively shape soil nematode communities, and that network complexity rather than taxonomic diversity is strongly affected by the plant-soil biota interactions. Our study provides an empirical basis for designing forest restoration schemes that prioritize belowground ecosystem functions.