Tracing of fire-induced soil phosphorus transformations using phosphate oxygen isotope ratio

This study demonstrates that phosphate oxygen isotope (δ¹⁸O_PO4) analysis effectively detects and monitors fire-induced transformation in soil phosphorus (P). Fires increase bioavailable P, potentially limiting primary production in terrestrial ecosystems. However, understanding the effects of fire on soil P dynamics in the field remains challenging due to the interaction between fire spread and soil properties with high spatial heterogeneity. Soil burning experiments were conducted using a surface soil sample collected in central Japan. The soil was burned in an electric furnace from 50 to 550°C for 3 h, and P concentrations and δ¹⁸O_PO4 values were determined. The results revealed that high temperatures (>350°C) depleted the soil of organic P (P_o) and increased labile and stable inorganic P (P_i) concentrations while significantly decreasing δ¹⁸O_PO4 values. By contrast, low temperatures (150°C) increased labile P_i and P_o concentrations without isotopic shift, indicating that low-intensity fires could increase bioavailable P while conserving soil organic matter. These findings indicate that δ¹⁸O_PO4 analysis can provide insight into the relationship between P transformations and fire intensity and track subsequent changes in P dynamics over time. Our research highlights the potential of δ¹⁸O_PO4 in predicting and managing postfire ecological and agricultural impacts.