Tree growth is linked to the diversity of belowground fungal functional guilds across nine Chinese fir plantations in subtropics

Chinese fir (Cunninghamia lanceolata) is the principal tree species for planted forests in subtropical China. Yet, whether the growth of this tree is related to belowground fungal diversity and functional guilds remains unknown. In this study, both root- and soil-associated fungal functional guilds and their relationships with tree growth (basal area and increment) were investigated across nine Chinese fir stands. The results showed that significantly higher diversity of fungal guilds was observed in soil compared to root habitats (P < 0.001). In root habitats, soil ammonium nitrogen content (NH₄⁺-N) and acid phosphatase (ACP) were identified as the primary drivers of saprotrophic and pathotrophic fungal diversity, respectively (P < 0.01). Conversely, soil available phosphorus (AP) emerged as the strongest predictor of soil-associated saprotrophic and pathotrophic fungal diversity (P < 0.01). Meanwhile, soil pH was the most significant determinant of symbiotrophic fungal diversity in both root and soil habitats, as well as fungal biomass in soil habitat (P < 0.01). Moreover, the interplay between tree growth and fungal guilds or taxa was intricately mediated by soil properties and enzymatic activities. Taken together, our results provide novel evidences that the augmentation of soil AP content and pH could potentially alter the structure of soil fungal guilds and increase fungal biomass, ultimately correlating with tree growth, thereby offering valuable insights into the sustainable management strategies for Chinese fir plantations.