The phosphate desorption rate in soil limits phosphorus bioavailability to crops

This study was set up to identify the role of the phosphorus (P) desorption rate in P diffusion and in P bioavailability in soil. The P desorption kinetics were measured with a zero-sink method in soil suspensions (0-77 days) for a set of soils that either had or had not been mined for P in a glasshouse study. The desorption kinetics was fitted by a serial two-pool model, discriminating a fast desorbing P pool (Q₁) with desorption half-lives of 3-8 days, and a slowly desorbing P pool (Q₂), which replenishes the fast P pool with 100-fold larger half-lives than the fast pool. Phosphate desorption was smaller and slower after soil P mining compared to that in the original soil samples and mining reduced the Q₁/Q₂ ratio. This kinetic model was embedded in a 1D planar diffusion model predicting that the diffusive flux of P to a zero sink in 5 days varies by a factor of 1.4 among the observed Q₁ desorption rate constants, keeping other parameters constant, and that the reduced Q₁/Q₂ ratio upon P mining sharply reduces the diffusible P in soil. The P uptake model of Barber-Cushman was extended with P desorption kinetics and was successfully calibrated to the P uptake data of the glasshouse P mining study. The model correctly predicted that reduced nitrogen (N) fertilization enhances the soil P-use efficiency because of lower critical P demand rates at slower growth. Finally, that new model predicted that maize requires >3-fold more available P in soil than wheat because of a higher P demand rate per unit root area of maize than that of wheat. This confirms a similar factor difference in critical soil P concentrations observed in P-response trials in Belgium between 1973 and 2018. This study shows that the P desorption rate limits P bioavailability for fast growing plants with a small effective root area, especially under negative soil P balances that slow down the desorption rate of P in soil. HIGHLIGHTS: The diffusion coefficient of P in soil is reduced by soil P mining Faster growing plants require more available P in soil because they rely on high diffusive P fluxes P desorption rate can limit the P bioavailability