Estimating soil P pools and desorption rates using flow-through cells

Understanding and predicting dissolved phosphorus release from soils is critical to proper land management for maximizing P uptake by agronomic crops and minimizing losses to surface waters. In this study, we investigate the amount and rate of P release for 22 diverse soils using flow cells operating at low and high velocity. P release data were fit to a one-dimensional advection-dispersion equation assuming two P pools (Q₁ and Q₂) each with their own kinetic desorption rate. The model provided exceptionally good fits to the P release data at both flow velocities. The high-flow velocity desorbed P at a higher rate, while the slow-flow velocity released more P and at higher concentrations; although when normalized for residence time, the rate of P desorption was greater in the slow-flow velocity columns. Fitted values of the readily available P pool (Q₁) for the two flow velocities were well correlated, with Q₁ values for the slow flow velocity being consistently greater than the high flow velocity. Conversely, kinetic desorption parameters between the two flow velocities were not as well correlated. Fitted Q₁ values were strongly correlated with oxalate-extractable P (P_Ox), Mehlich-3 P (P_M3), and water-extractable P (P_W). Q₁ values were much lower than P_Ox and P_M3 but similar to P_W. We only found weak correlations between the kinetic desorption rates and measured soil properties. Our results show that the use of flow-through cells can provide useful information on P release from soils, but results will be dependent on flow velocity.