Soil solution phosphorus turnover: derivation, interpretation, and insights from a global compilation of isotope exchange kinetic studies

Abstract

<p><strong>Abstract.</strong> The exchange rate of inorganic phosphorus (P) between the soil solution and solid phase, also known as soil solution P turnover, is essential for describing the kinetics of bioavailable P. While soil solution P turnover (<i>K<sub>m</sub></i>) can be determined by tracing radioisotopes in a soil-solution system, few studies have done so. We believe that this is due to a lack of understanding on how to derive <i>K<sub>m</sub></i> from isotopic exchange kinetic (IEK) experiments, a widespread form of radioisotope dilution study. Here, we provide a derivation of calculating <i>K<sub>m</sub></i> using parameters obtained from IEK experiments. We then calculated <i>K<sub>m</sub></i> for 217 soils from published IEK experiments in terrestrial ecosystems, and also for that of 18 long-term P fertilizer field experiments. Analysis of the global compilation dataset revealed a negative relationship between concentrations of soil solution P and <i>K<sub>m</sub></i>. Furthermore, <i>K<sub>m</sub></i> buffered isotopically exchangeable P in soils with low concentrations of soil solution P. This finding was supported by an analysis of long-term P fertilizer field experiments, which revealed a negative relationship between <i>K<sub>m</sub></i> and phosphate buffering capacity. Our study thus highlights the potential of <i>K<sub>m</sub></i> for future studies – not only for P, but also for other environmentally-relevant, strongly-sorbing elements with radioisotopes such as zinc, cadmium, nickel, arsenic, or uranium.</p>