The phosphate desorption rate in soil limits phosphorus bioavailability to crops

Abstract

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

Dynamics of soil phosphorus measured by ammonium lactate extraction as a function of the soil phosphorus balance and soil properties

“…This logically follows the two P pool concept, i.e. passage of time promotes movement of P out of the extractable pool in a positive balance and promotes weathering or mobilization into the extractable pool under P depletion scenarios (Smolders et al, 2020). There was no significant main effect of time in our model (Equation 4), so the effect of time only emerges in combination with large net P balances.…”

“…Crops with high P demand rate per unit of root area (e.g. maize) can start experiencing yield reductions when P-AL values drop below 180 mg P kg -1 under prolonged negative soil P balances (Smolders et al, 2020).…”

“…There are few generic data and few models to calculate the P supply or soil P balance (inputs minus outputs) needed to reach the target values, or to maintain the target P at field scale. Several mechanistic models exist to predict the changes in soil P (Johnston et al, 2014;Karpinets et al, 2004;Smolders et al, 2020), but these models require measurements or estimations of several input variables. Since these data are often not available or have to be estimated by transfer functions which are not always accurate, models with limited input data requirement are preferred for predictions of general soil P changes (Uusitalo et al, 2016).…”

Diffusive gradients in thin films predicts crop response better than calcium-acetate-lactate extraction

“…The kinetics of P release from the solid phase have been found to follow one fast and one subsequent slow reaction, assigned to two labile P pools with differential desorption kinetics (De Jager and Claassens 2005;Lookman et al 1995;Maguire et al 2001;Menezes-Blackburn 2016;Smolders 2021;Taddesse et al 2008). A recent experimental and modelling study demonstrates that P desorption rates control P availability to fast growing crops with small specific root area.…”

Investigation of the soil properties that affect Olsen P critical values in different soil types and impact on P fertiliser recommendations

“…However, the same critical value is given for most crops (except vegetables) despite it being known that different crops require different amounts of soil available P (Nawara et al, 2017;Sandaña, Orena, Santos Rojas, Kalazich, & Uribe, 2018). One reason for different crops having different critical values is that faster growing species such as maize require P more quickly than slower growing species (Smolders et al, 2020). If the desorption rate of P from soil is too slow to keep up with plant demand, it will manifest as a higher critical value, because P desorption from soil in extraction tests is unlikely to be affected in the same way.…”