Degree of short-term drying before rewetting regulates the bicarbonate-extractable and enzymatically hydrolyzable soil phosphorus fractions

Soil drying-rewetting can remarkably affect phosphorus (P) transformation, and thus, alter P distribution among P pools; however, little is known about the effect of the degree of drying before rewetting on labile P fractions and the bioavailability of organic P (Po). In this study, soils with distinct physico-chemical properties were allowed to desiccate to 5%, 10%, 20%, 30%, and 40% water holding capacity (WHC), and soils maintained at 50% WHC were used as controls, and the bicarbonate-extractable P and hydrolyzable Po fractions were analyzed after 5 h of rewetting. Bicarbonate-extractable Po accounted for 33.3-56.4% of extractable total P, and hydrolyzable Po constituted 34.4-79.7% of extractable Po. For extractable Po, 7.7-29.9% was labile monoester P, 6.5-14.9% was diester P, and 17.8-36.5% was phytate-like P. Bicarbonate-extractable inorganic P and Po contents were not affected by 20-40% WHC treatments, but increased by 26.3-48.1% and 5.7-52.9%, respectively, when the soils were dried to 5% WHC. Similarly, labile monoester, diester, and phytate-like P contents increased by 12.5-89.8%, 0-65.2%, and 24.6-65.6%, respectively, in 5% WHC soils. Pearson's correlation analyses showed that the relative increases in bicarbonate-extractable inorganic P and phytate-like P, following extreme drought, were positively correlated with soil organic carbon, oxalate-extractable aluminum (Al), and iron (Fe), suggesting regulatory roles of organic matter and Al/Fe oxides in P transformation during soil drying-rewetting. Taken together, our results suggest that extreme drought events before rainfall or irrigation facilitate an increase in the level of labile P, including considerable proportions of hydrolyzable Po fractions, potentially posing a substantial threat to water bodies in the context of climate change.