Soil phosphorus fraction and availability dynamics along a 2-million-year soil chronosequence in northern Hainan Island, China

The low phosphorus (P) content and availability in highly weathered tropical soils may limit ecosystem service and function. However, information on P fraction dynamics and availability during tropical soil development is scarce. Understanding P transformations and their impacts on P availability at various stages of tropical soil evolution is crucial for effective nutrient management strategies. In this study, a soil chronosequence (ranging from 0.09to 2.30 million years) was combined with the sequential P extraction to assess temporal changes in P fractions with soil age in northern Hainan Island, China. Results showed that total P (TP) content increased initially (<0.146-million-year), declined at the 0.64- million-year site, and then rebounded (>1.12-million-year) as soil age increased. Sequential P extraction revealed that the stable P pools, including concentrated HCl extracted P and residual-P, were the dominant P fractions, mirroring the temporal trend of TP. The labile NaHCO3-Pi concentration, which reflects soil P availability, ranged from 2.29 to 5.63 mg kg−1. Despite the low labile P content, it is sufficient to meet plant P requirements, even in the oldest soil. Structural equation modeling indicated that moderately labile NaOH-Pi and the stable P pools significantly influenced soil P availability, together explained 51.0 % of the variation in labile P. This study elucidates the pattern of P transformation during tropical pedogenesis over a 2.30-million-year timescale, highlighting the substantial contribution of stable P pools to P supply in tropical soils with low labile P content.