Variations in the community patterns of soil nematodes at different soil depths across successional stages of subalpine forests

Nematodes are among the most important faunal groups in soil food webs and serve as important bioindicators in evaluating subsurface ecological processes and ecosystem functioning. Although previous studies have reported the effects of vegetation types on soil nematode communities, the community patterns of soil nematodes at different soil depths during secondary forest succession remain unclear. Through a field survey, we investigated how soil nematode community characteristics changed at two soil depths (0-10 cm and 10-20 cm) across subalpine forest successional stages consisting of grasslands, shrublands and forests in Southwest China. We measured soil properties and microbial biomass to analyze significant environmental factors influencing variations in soil nematode communities. With forest succession, we observed increases in the total nematode abundance and maturity index and a decreased plant parasitic index, indicating more stable food webs and lower root herbivory hazards in forests than in grasslands and shrublands. Fungivore diversity was significantly increased and herbivore diversity was significantly decreased during forest succession, despite no significant difference existing in the total nematode diversity among the three successional stages. The abundances of the dominant nematode genera Helicotylenchus and Tylencholaimus were significantly correlated with forest succession. In addition, the variations in soil nematode communities differed between the two soil depths, presumably due to various soil variables. NO-3-N and pH were the dominant determinants of the variations in soil nematode communities at 0-10 cm soil depth, while SOC was the dominant determinant at 10-20 cm soil depth. Overall, our results suggested that forest succession exhibited layer-specific effects on soil nematode communities via different soil variables, and that specific nematode genera could serve as useful bioindicators during forest succession.