A method to isolate soil organic phosphorus from other soil organic matter to determine its carbon isotope ratio

Abstract

Despite the importance of soil organic phosphorus (OP) for plant nutrition, its dynamics in soil remain elusive due to the lack of multiple stable P isotopes. Here, we developed a method that isolates the soil OP pool from other soil organic matter to measure its carbon isotope signature to elucidate soil OP decomposition. We tested three extractants (H₂SO₄, HCl, and NaOH in 0.5 M) for their capacity to preferentially extract OP. For isolating OP from the extract by precipitation, we evaluated different pH adjustments (pH 1.5 – 10) and iron- or aluminum hydroxide additions. Finally, we determined the carbon isotope ratio (δ¹³C) of the isolated OP pool. We found that the H₂SO₄ extracts had the highest OP content and the lowest organic carbon (OC):OP ratio compared to the HCl and NaOH extracts. The pH adjustments of the H₂SO₄ extracts to pH 4 – 7.5 removed ≥ 93% of the extracted OP from the solution. The OC:OP ratio of the precipitates was the lowest (11 – 16) at pH 7.5, showing a strong preferential OP precipitation caused by the pH alteration. Metal hydroxide addition (combined with pH treatment) did not improve the preferential OP removal. The δ¹³C of the OP reflected differences in the vegetation (C₃/C₄) that grows at the soil sampling sites. Overall, this method provides a simple and effective approach to isolate the soil OP pool from other soil organic matter and determine its carbon isotope ratio, which opens new avenues to study soil OP dynamics.