Abstract
Abstract. Quantification of turnover of inorganic soil phosphorus (P) pools is essential to improve our understanding of P cycling in soil–plant systems and improve representations of the P cycle in land surface models. 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 (NaOH-P) was in the range of days to months, and MRT in HCl-extractable P (HCl-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. These results suggest that current land surface models underestimate the dynamics of inorganic soil P pools and could be improved by reducing model MRTs of the labile and NaOH-P pools, considering soil-type-dependent MRTs rather than universal exchange rates and allowing for two-way exchange between HCl-P and the soil solution.
Highlights
Since only a small fraction of soil phosphorus (P) is present as phosphate in the dissolved state where it can be taken up by plants and microbes, the rate at which this pool is replenished from other soil P pools is critical to assess the bioavailability of soil P (Syers et al, 2008)
P that was exchangeable within 1 min (E1 min) ranged from 0.99 to 218 mg kg−1, and P that was exchangeable in 3 months ranged from 11.7 to 6311 mg kg−1 between the different soils (Table 1)
While earlier works hypothesized that resin and bicarbonate P have a fast turnover, and NaOH and HCl a slow turnover, data on mean residence time (MRT) of P in these pools for a wide range of soils were previously missing (Cross and Schlesinger, 1995; Tiessen et al, 1984)
Summary
Since only a small fraction (usually < 1 %) of soil phosphorus (P) is present as phosphate in the dissolved state where it can be taken up by plants and microbes, the rate at which this pool is replenished from other soil P pools is critical to assess the bioavailability of soil P (Syers et al, 2008). The extent and the timescale on which unavailable soil P forms can become gradually bioavailable are important factors affecting ecosystem productivity under increasing carbon dioxide concentrations (Sun et al, 2017). Estimates of P availability directly influence inferences on carbon sequestration. Currently P availability is poorly constrained in land surface models, which hampers our ability to project future carbon sequestration rates (Goll et al, 2012). Modeling the rate of replenishment from different pools requires knowledge of the mean residence time of P in these pools.
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