Contrasting effects of litter removal and root exclusion on soil phosphorus availability in a temperate broad-leaved forest

: Plant detrital inputs sustain soil phosphorus (P) fertility, yet they are increasingly disrupted by global environmental change and forest management. Although P availability frequently constrains temperate forest productivity, the distinct effects of above- versus belowground detrital reductions on soil P dynamics remain poorly understood. To address this, we conducted a long-term (nine-year, 2015–2024) in situ manipulation experiment involving independent litter removal and root exclusion (trenching) in a natural temperate broad-leaved forest. We found that litter removal significantly reduced labile P (−9.9%), moderately labile P (−3.4%), and total P (−3.0%) by concurrently suppressing fine root necromass (FRN), microbial biomass P (MBP), and acid phosphatase (ACP) activity. In contrast, root exclusion unexpectedly elevated labile P (+9.1%) and total P (+2.8%). Structural equation modeling revealed two distinct regulatory pathways: Litter removal directly decreased total P and available N, driving subsequent declines in MBP and ACP activity that constrained microbial P transformations, while root exclusion induced a substantial accumulation of fine-root detritus. This necromass pulse stimulated microbial biomass and phosphatase secretion (increasing MBP and ACP by 21.0% and 31.5%, respectively), thereby elevating soil P availability. Additionally, treatment-induced abiotic shifts strongly mediated these dynamics: Severe moisture accumulation and acute acidification under litter removal synergistically impaired phosphatase efficiency and enhanced geochemical P fixation, whereas moderate moisture enrichment under root exclusion optimized microbial P mobilization. We conclude that while a root necromass pulse facilitates P mobilization for a finite period, continuous aboveground litter inputs act as the irreplaceable driver for long-term P bioavailability. Consequently, preserving forest floor litter is essential for maintaining soil P fertility and ecosystem resilience under intensifying P limitation.