Tropical tree-mycorrhizal types show divergent phosphorus adaptive strategies after 12-year simulated acid rain

Acid rain is believed to exacerbate phosphorus (P) limitation in tropical forests, but how tropical trees respond and adapt to acid-induced P limitation, particularly after long-term acid rain events, remains poorly understood. We conducted a 12-year simulated acid rain (SAR) experiment by irrigating plots with water of different pH values (i.e., 3.0, 3.5, 4.0, and 4.5 as a control) in a tropical forest in southern China. Five tree species associated with either ectomycorrhizal (ECM) or arbuscular mycorrhizal fungi (AMF) were chosen to examine the changes of P fractions in their rhizosphere soils and green leaves. In ECM tree rhizospheres, SAR treatments significantly increased labile P by 27.3 % (p < 0.05) and decreased occluded P by 11.7 % (p < 0.05), which were positively correlated with increased phosphodiesterase activity and related gene abundance. However, in AMF trees, SAR treatments significantly reduced rhizosphere available P and foliar P by 45.9 % and 28.7 % (p < 0.05 for both), respectively. In response, AMF trees exhibited greater plasticity in foliar P fractions than ECM trees, shifting from structural P (phospholipids and phosphorylated proteins) to metabolic P (P-containing metabolites and nucleic acid P) fractions under SAR treatments. These findings suggest that, to cope with acid-induced P limitation, ECM trees tend to adopt an acquisitive nutrient-use strategy for greater P mobilization, while AMF trees favor a conservative strategy with more efficient foliar P utilization.