Phosphorus desorption and dynamics in calcareous Mediterranean agricultural soils assessed by DET (Diffusive Equilibrium in Thin-films) and DGT (Diffusive Gradients in Thin-films) passive sampling combined with analytical modelling

Background and Aims : The passive sampling techniques Diffusive Gradients in Thin-films (DGT) and Diffusive Equilibrium in Thin-films (DET) can suitably assess soil nutrient availability, since they rely on the same diffusion and desorption processes governing nutrient supply dynamics to plant roots. Rigorous interpretation of DET and DGT results requires considering diffusion, adsorption equilibria, and kinetic resupply from the solid phase. Existing DGT interpretation approaches mostly rely on numerical models (such as DIFS) with indirect surrogates (e.g. from conventional extraction methods) for the partition coefficient K _D . This works aims at the direct K _D determination from the fitting of the transient DET response and test it in three soils with contrasting P contents. Methods : In this study, a formula for the time-evolution of an analyte concentration accumulated in DET gels is derived. The formula was validated against single- and double-gel DET configurations deployed in three representative calcareous Mediterranean agricultural soils of Northeastern Spain. Results : The DET-fitted K _D values were used as input to the DIFS model to coherently interpret experimental DGT data from the same soils. Comparison with Olsen-P-derived K _D values reveals a systematic overestimation of the resupply capacity that is most pronounced in the low-P soil ( K _D (Olsen) / K _D (DET) ≈ 12) and approaches unity in the high-P soil (≈ 1.5), indicating that conventional extractions might increasingly misrepresent the kinetically accessible P pool as total P decreases. Conclusion : This integrated DET–DGT strategy provides a mechanistically grounded framework for quantifying phosphorus availability in agricultural soils.