Seven years phosphorus addition has no effect on soil acidity in two tropical plantations

Strong acidity accompanied by low phosphorus (P) availability occurs in tropical soils especially under high nitrogen (N) deposition. Commonly, improved P availability can mitigate soil acidity in tropical natural forests via direct reaction with acidic cations or impacting plant uptake and soil microbial community to indirectly regulate soil cations. However, whether P input can alleviate soil acidity in tropical plantations remains unknown. This study selected two tropical typical plantations (dominated by Acacia auriculiformis and Eucalyptus urophylla, respectively), to investigate the effects of seven years of P addition on soil acidity and the underlying mechanisms. The results showed that P addition did not change soil pH or cation concentrations, suggesting no variations in soil acidity under P addition. Moreover, P addition did not affect acidic cation concentrations within plant tissues, soil microbial community, litterfall input, and fine-root biomass, indicating that P addition did not impact the processes of plant and soil microbes to regulate the dynamics of soil cations to alleviate acidity. However, we found significant increases in soil available P, soil total P, and plant P concentrations, indicating that added P led to P enrichment in soil and plants. In addition, a consistent response of soil acidity in two tropical plantation ecosystems was observed under combined N with P addition. Our findings improve the understanding of the relationships between P availability and soil acidification in tropical forests and highlight the necessity to accurately evaluate the role of P in acidic tropical forest soils.