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 (P o ). 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 P o fractions were analyzed after 5   h of rewetting. Bicarbonate-extractable P o accounted for 33.3–56.4% of extractable total P, and hydrolyzable P o constituted 34.4–79.7% of extractable P o . For extractable P o , 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 P o 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 P o fractions, potentially posing a substantial threat to water bodies in the context of climate change .