Estimates of mean residence times of phosphorus in commonly-considered inorganic soil phosphorus pools

Quantification of turnover of inorganic soil phosphorus (P) pools is essential to improve our understanding of P cycling in soil-plant systems at different spatial scales. Turnover can be quantified using mean residence time (MRT), however, to date there is little information on MRT of P in soil P pools. We introduce an approach to quantify MRT of P in sequentially-extracted inorganic soil P pools using data from isotope exchange kinetic experiments. Our analyses of 53 soil samples from the literature showed that MRT of labile P (resin- and bicarbonate extractable P) was on the order of minutes to hours for most soils, MRT in NaOH-extractable P was in the range of days to months, and MRT in HCl-extractable P was on the order of years to millennia. Multiple regression models were able to capture 54–63 % of the variability in MRT among samples, and showed that land use was the most important predictor of MRT of P in labile and NaOH pools. MRT of P in HCl-P was strongly dependent on pH, as high pH soils tended to have longer MRTs. This was interpreted to be related to the composition of HCl-P. Under high pH, HCl-P contains mostly apatite, with a low solubility, whereas under low pH conditions, HCl-P may contain more exchangeable P forms. The estimates of MRT of P in inorganic pools improve our interpretation of soil P dynamics at the laboratory-, field- and ecosystem scale, and will also be useful to constrain P dynamics in global land surface models.ISSN:1810-6277ISSN:1810-628