Mycorrhizal fungi alleviate acidification-induced phosphorus limitation: Evidence from a decade-long field experiment of simulated acid deposition in a tropical forest in south China

Tropical forests in southern China have been suffering high level of acid rain in recent decades, which may alter soil phosphorus (P) supply capacity and thus affect ecosystem productivity. We conducted a 10-yr field experiment of simulated acid rain (SAR) to examine how acidification impacts seasonal changes of soil P fractions in a tropical forest with highly-acidic soils in south China. The results showed that SAR significantly reduced soil P bioavailability, with increased occluded P pool but reduced the other more labile P pools in the dry season. The decreased soil P bioavailability was primarily related to the repressed P desorption capacity and enhanced P sorption during soil acidification, which regulated by acid-activated soil iron/aluminum minerals and soil organic matter. However, in the wet season, SAR did not change microbial P, soluble P and labile organic P pools. Different from the decline of microbial abundance in the dry season, SAR increased ectomycorrhizal fungi and its ratio to arbuscular mycorrhiza fungi in the wet season, which significantly stimulated phosphomonoesterase activities and likely promoted the dissolution of occluded P. Our results suggest that, even in already highly-acidic soils, the acidification-induced P limitation could be alleviated by stimulating ectomycorrhizal fungi and phosphomonoesterase activities. The differential responses and microbial controls of seasonal soil P transformation revealed here should be implemented into ecosystem biogeochemical model for predicting plant productivity under future acid deposition scenarios.