The partitioning of P in soil determines the fluxes and deliveries of labile P in soil solution

- Soil P partitioning and flux from a 50 year field trial was investigated using DGT and DIFS-model
- P exhausted soils is quickly depleted and provided slower rates of P supply to the DGT.
- The DGT-results confirm that the replenishment of P as a response to offtake, increased by P loads.
- We showed the impact from how Kd is estimated on the DIFS-model outputs.
- The DIFS-modeling should be validated further at different soil biogeochemical regimes.

Phosphorus (P) is an increasingly limited resource for food production, which necessitates studying soil factors controlling rates of P release to soil solution. Phosphorus use efficiency is among other, affected by the plants, the plant root induced displacement of P equilibrium in soil during uptake and the following rates of replenishment from geochemically active P stores in soil. Here, soil P partitioning and flux data collected from a 50 year field trial, based on application of differing P masses, was investigated using a diffusive gradients in thin films (DGT) technique and “DGT-induced fluxes in sediments” (DIFS) model. Partitioning of P in soil was accomplished by deploying DGT at increasing contact times (6 h to 120 h). DGT induced P fluxes increased with soil P, and they were highest the first hours after installation. A Langmuir adsorption approach to determine Kd concealed accumulated P labile stores, whereas Kd estimations from ammonium lactate (AL) extractions did not. The estimated fluxes in the latter situation stabilized after 24 h deployment, and they were more clearly reflecting the long term P treatments. The two Kd approaches showed the importance of considering inherent P-stores for calculating fluxes and deliveries of labile P available for plants during P-uptake. The use of DGT and DIFS in combination enabled good estimations of fluxes. Such fluxes may be used to estimate, In Situ, critical soil solution P concentrations available for plants during growth, in different soil types.

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