Changes in soil nutrients and stoichiometric ratios reveal increasing phosphorus deficiency along a tropical soil chronosequence

Understanding the variations and controls of soil nutrients during different stages of soil evolution can provide guidance for ecosystem nutrient management and regulation. This study investigated the changes in and factors associated with soil nutrients and stoichiometric ratios during different stages of soil development using a well-established soil chronosequence (0.09, 0.146, 0.64, 1.12, 1.81, and 2.30 million years) developed on basalt in southern China. Results showed that available nitrogen (AN) decreased continuously with increasing soil age. Total nitrogen (TN), total phosphorus (TP), available phosphorus (AP), and soil organic carbon (SOC) increased initially during the early stage of soil development, reaching peak values in the 0.146 million-year-old soil profiles. Thereafter, TP and AP fluctuated with increasing soil age, whereas SOC exhibited a decreasing trend with aging soil. The profile-averaged C:N (9.81-12.76), C:P (8.57-23.30), and N:P (0.85-2.00) remained rela-tively stable across the soil chronosequence (all the ranges < 15), while the profile-averaged AN:AP (29.73-97.36) varied greatly with increasing soil age (the range > 60). Moreover, the decreasing rate of AP (by 64%) and AP:TP (by 71%) was much higher than that of AN (by 46%) and AN:TN (by 26%) when the soil age was greater than 0.64 million years, demonstrating more severe P deficiency and possible greater P limitation in the late stage of soil development. Bayesian structural equation modeling showed that NDVI, average annual evaporation, clay content, and pH considerably influenced the variations in C:N, C:P, N:P, and AN:AP, but the degree of influence differed among the various factors. Our study demonstrates that it is necessary to apply P-fertilizers in older tropical soils (>0.64 million years) developed on basalt to alleviate the increasing P deficiency in the late stage of soil evolution.