Objective Plants and soil fungi are key biotic components in forest ecosystems, playing a crucial role in regulating soil carbon and other nutrient cycling. According to plant diversity hypothesis, plant diversity can pronouncedly modify soil microbial diversity and community composition. However, we know little of how plant richness and fungal diversity interactively influence soil organic carbon and total nitrogen. In this study, we aimed to investigate the coupling mechanism between tree richness and fungal diversity in shaping soil carbon and nitrogen across tropical, subtropical, warm-temperate and mid-temperate forests.

Method We collected the data of altitude, air temperature, soil properties (including soil carbon (C), nitrogen (N), C/N ratio), as well as fungal diversity from plots with both the tree species mixtures and monocultures. In total, 214 sets of data were used to conduct the meta analysis, with a random-effect model calculating effect size. Additionally, the explanatory weights of biotic and abiotic variables influencing the effect of tree richness were analyzed.

Result We found that the responses of soil C and N contents to tree richness varied across forest ecosystems, with the strength of tree richness effects on soil C and N contents gradually diminishing from tropical to mid-temperate forest ecosystems. Moreover, the effects of tree richness on soil C and N contents in tropical and subtropical forest ecosystems were primarily driven by altitude and soil pH, rather than soil fungal diversity. In contrast, cascading effects of tree richness on soil C and N contents were observed via modulating soil fungal diversity in warm-temperate and mid-temperate forest ecosystems.

Conclusion The findings highlight the variability in the responses of soil C and N contents to tree richness across different climatic zones, promoting our understanding of the mechanisms by which tree species richness affects soil C sequestration.