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 O-18-labeled phosphate in an agricultural soil. We analyzed the changes in concentrations and phosphate O-isotope ratios (delta(OP)-O-18) of four inorganic P (Pi) pools (H2O-Pi, NaHCO3-P-i, NaOH-P-i, and HCl-P-i) along with soil chemistry to understand the roles of different biogeochemical processes changing isotope composition. By monitoring delta(OP)-O-18 values of four P pools, an active transformation from H2O-P-i and NaHCO3-P-i to NaOH-P-i and HCl-P-i was identified. Transformation of originally bioavailable P to unavailable P such as HCl-P-i allowed us to conclude that a suite of HCl-P-i with different delta(OP)-O-18 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 delta(OP)-O-18 as a tracer of P cycling in soil and are expected to be useful for fertilizer application as well as nutrient management in soils.