Geoderma (May 2024)
Chemical processes and prediction of dissolved phosphorus leaching in mineral and organic soils
Abstract
In agricultural areas dominated by subsurface drainage, leaching of phosphorus (P) from soils is a concern for downstream water quality. Still, the role of chemical processes in subsoils and organic soils in influencing dissolved P leaching needs to be clarified for better predicting the P leaching. In ten mineral and organic soils, we examined a wide range of chemical characteristics including various P pools and sorption–desorption properties at different soil depths and related those characteristics to leaching of dissolved P at the drain depth in an indoor lysimeter experiment. Results showed significant correlations between different P pools (R2-adj = 0.61 to 0.98, p < 0.001) and between sorption capacity measurements (R2-adj = 0.60 to 0.95, p < 0.001). Some organic soils followed the same patterns in P sorption capacity and P lability as sandy soils but some did not, suggesting organic soils differ among themselves possibly due to differences in origin and/or management. Flow-weighted mean concentrations of dissolved reactive P and dissolved organic P depended on both the labile P pools (labile inorganic and organic P pools, respectively) in the topsoil and P sorption and desorption characteristics in the subsoils. Mass-weighted whole-profile degree of P saturation based on the ammonium lactate extraction method (DPS-AL) was an excellent indicator of flow-weighted mean concentration of total dissolved P (FWMC-TDP) (R2-adj = 0.93, p < 0.001). Two profiles, one with organic soils overlaying on sand and the other with sandy soils in all layers, had the greatest FWMC-TDP among all profiles (316 and 230 µg/L versus 33–84 µg/L) due to the same reason, i.e., large labile P pools in the topsoils, low P sorption capacity in the subsoils, and high whole-profile DPS-AL. All results point to the need to include subsoil characteristics for assessing the risks of dissolved P leaching from both mineral and organic soils. Also, the study suggests the need to investigate further the roles of the origin and management of organic matter and organic P in influencing P lability and dissolved organic P (DOP) leaching, as well as the bioavailability of DOP in recipient waters.
