Tree Diversity Increases the Diversity of Soil Nematodes and the Density of Persisters in a Temperate Forest Experiment

While tree diversity declines worldwide, the consequences for soil invertebrate communities remain unclear. Nematodes span all trophic levels and are valuable indicators for soil biodiversity, food web structure, and soil health. Here, we examined the effects of tree diversity on soil nematode diversity, community composition, and nematode-based indices in a tree-mycorrhiza diversity experiment with temperate, deciduous trees. Experimental treatments included tree communities with only arbuscular mycorrhizal (AM) species, only ectomycorrhizal (EcM) species (one, two, or four tree species), or a mixture of both (AM + EcM; two or four species). We found that total nematode density, taxonomic richness, and Shannon–Wiener diversity increased significantly with tree species richness. The positive effects were primarily driven by increases in plant- and fungal-feeding nematodes, particularly, persister taxa. Moreover, AM tree communities hosted significantly more plant-feeding nematodes than EcM tree communities, while fungal- and bacterial-feeding nematode densities did not differ between the two mycorrhizal types. The increasing trend of plant-feeding nematodes with tree species richness was reversed in AM + EcM tree communities. Our results suggest that greater tree species richness can increase soil nematode diversity and, particularly, the contribution of persister taxa, but may also enhance plant-feeding nematodes. Mixing tree species with different mycorrhizal types may help suppress plant-feeding nematodes, which could be especially important in AM-dominated stands. These findings provide empirical support for increasing tree diversity, while accounting for mycorrhizal types, in the restoration and management practices of temperate forests.