Transformation of Phosphorus Pools in an Agricultural Soil: An Application of Oxygen-18 Labeling in Phosphate

Phosphorus is a key ingredient of fertilizers, and there is no other substitute for P in sustaining life and food production. Excess P in soils may be fixed and become agronomically inactive or removed as a result of leaching and soil erosion. In this research, we aimed to explore how a particular P pool transforms into another pool by applying ¹⁸O-labeled phosphate in an agricultural soil. We analyzed the changes in concentrations and phosphate O-isotope ratios (δ¹⁸OP) of four inorganic P (Pᵢ) pools (H₂O-Pᵢ, NaHCO₃–Pᵢ, NaOH-Pᵢ, and HCl-Pᵢ) along with soil chemistry to understand the roles of different biogeochemical processes changing isotope composition. By monitoring δ¹⁸OP values of four P pools, an active transformation from H₂O-Pᵢ and NaHCO₃–Pᵢ to NaOH-Pᵢ and HCl-Pᵢ was identified. Transformation of originally bioavailable P to unavailable P such as HCl-Pᵢ allowed us to conclude that a suite of HCl-Pᵢ with different δ¹⁸OP values could be precipitated from the originally biologically cycled or bioavailable P pools. Thus, the isotope technique allowed tracking of short-term transformation of readily bioavailable P to a less or non-bioavailable P pool and to discriminate biological and chemical reactions during transformation. These findings support the burgeoning applications of δ¹⁸OP as a tracer of P cycling in soil and are expected to be useful for fertilizer application as well as nutrient management in soils.