Mixed Plantations Mitigate Negative Effects of Natural Forest Conversion on Soil Meso- and Micro-Faunal Communities in Zhangguangcailing Mountains, Northeast China

Soil meso- and micro-fauna are key components of forest soil ecosystems, but the effects of converting natural forests to monoculture or mixed plantations on their communities remain unclear. This study compared soil meso- and micro-fauna communities in monoculture (broad-leaved and coniferous) and mixed plantations converted from secondary forests in Northeast China. Results showed that monocultures significantly altered faunal community composition and reduced diversity, whereas mixed plantations maintained similar abundance and diversity to secondary forests. For instance, broadleaf monocultures exhibited a 15–23% lower Shannon–Wiener diversity index, while coniferous monocultures displayed a 28–35% reduction in soil faunal abundance. Notably, different taxonomic groups responded inconsistently to stand type conversion: Oribatida (58.8–72.5% of total individuals) varied significantly among stands (p < 0.001), whereas Collembola (18.8–31.4%) was more affected by season. The community composition was significantly correlated with understory plant diversity/coverage and soil C/N ratio, while α-diversity was mainly driven by total nitrogen. Our findings reveal that mixed plantations, instead of monocultures, are more effective for soil biodiversity conservation, which offers sustainable strategies for forest management.