Two years in-situ phosphorus addition drives soil fungal network complexity and multifunctionality in a tropical cloud forest

Phosphorus (P) is a limiting nutrient in tropical forest, where soil microorganisms play a crucial role in nutrient cycling and ecosystem functioning. However, how microbial communities regulate soil multifunctionality under altered P availability remains poorly understood. To address these gaps, we conducted a two year (2022–2024) in-situ P addition experiment in a tropical cloud forest in Bawangling, Hainan Tropical Rainforest National Park. We then examined microbial species diversity, phylogenetic diversity, and network complexity and their roles in driving soil multifunctionality. Results showed that P addition significantly increased soil available phosphorus (AP) and acid phosphatase activity (ACP). Fungal diversity and network complexity decreased with increasing P concentration, whereas bacterial diversity remained nosignificant changes. Variance partitioning revealed that fungal network complexity was the primary driver of soil multifunctionality (31.8 %), with synergistic effects from fungal species and phylogenetic diversity (12.7 % and 11.1 %, respectively). Structural equation modeling indicated that P addition enhanced multifunctionality primarily through an indirect pathway by improving soil conditions. Conversely, P exerted significant direct negative effects on microbial network complexity and species diversity. Despite these negative direct effects, both microbial network complexity and species diversity themselves had strong positive direct effects on multifunctionality. The overall model explained 63.2 % of the variance in soil multifunctionality. These findings suggest that moderate P input can enhance soil nutrient status while preserving fungal network complexity, a key bio-indicator of ecosystem recovery, thus offering a scientific basis for managing P inputs in tropical cloud forests and cautioning against the ecological risks of excessive P enrichment.