Evaluation of Phosphorus Enrichment in Groundwater by Legacy Phosphorus in Orchard Soils with High Phosphorus Adsorption Capacity Using Phosphate Oxygen Isotope Analysis

Long-term phosphorus (P) fertilization results in P accumulation in agricultural soil and increases the risk of P leaching into water bodies. However, evaluating P leaching into groundwater is challenging, especially in clay soil with a high P sorption capacity. This study examined whether the combination of PO₄ oxygen isotope (δ¹⁸O_PO4) analysis and the P saturation ratio (PSR) was useful to identify P enrichment mechanisms in groundwater. We investigated the groundwater and possible P sources in Kubi, western Japan, with intensive citrus cultivation. Shallow groundwater had oxic conditions with high PO₄ concentrations, and orchard soil P accumulation was high compared with forest soil. Although the soil had a high P sorption capacity, the PSR was above the threshold, indicating a high risk of P leaching from the surface orchard soil. The shallow groundwater δ¹⁸O_PO4 values were higher than the expected isotopic equilibrium with pyrophosphatase. The high PSR and δ¹⁸O_PO4 orchard soil values indicated that P leaching from orchard soil was the major P enrichment mechanism. The Bayesian mixing model estimated that 76.6% of the P supplied from the orchard soil was recycled by microorganisms. This demonstrates the utility of δ¹⁸O_PO4 and the PSR to evaluate the P source and biological recycling in groundwater.