Sources and Fate of Acid Extractable Phosphorus in an Agricultural Soil.

Phosphorus (P) is one of the most essential nutrients for all living beings. Application of P in excess of crops requirement builds P in soils which may be fixed in soils or removed as a result of leaching and soil erosion. Among different soil P pools, acid extractable P (1M HCl-P and 10M HNO₃-P) corresponds to P in crystalline minerals that could not be dissolved by 1M NaOH. In this study, we aimed to identify the sources and fate of acid-P pools using phosphate oxygen isotope ratios (d¹⁸O_P). We collected soil core samples from the Agricultural Experiment Station in Newark, DE and analyzed for d¹⁸O_P values and P, Al, Fe, and Ca concentrations in 1M HCl-P and 10M HNO₃-P pools. Our results show that the concentrations of acid-P pools are high at the shallow soil depth (≤ 40 cm) and sharply decrease below it before becoming almost constant for ˃50 cm. d¹⁸O_P values of HCl-P and HNO₃-P pools vary normally between 20-21.5‰ and are within the ranges of equilibrium isotopic compositions under ambient conditions in the soil. Interestingly, d¹⁸O_P values of acid HCl-P and HNO₃-P pools are similar to that of NaOH-P suggesting that strongly bound P pool in soil is eventually transformed into acid-P pools. Furthermore, d¹⁸O_P values of these two pools in deeper soils (>50 cm) where the effect of soil tilling or physical mixing is absent are similar to the shallow soil. One plausible mechanism for this observation is that leached P from top soil later precipitates into or is occluded within more crystalline minerals formed in deeper soils. These findings provide important information on the sources and fate of acid-P in agricultural soils and better insights into the build-up of legacy P in soils.