Simulating phosphorus leaching from two agricultural soils as affected by different rates of phosphorus application based on the geochemical model PHREEQC

Phosphorus (P) leaching from agricultural soils, in consequence of long-term utilization of P fertilizers, decreases the water quality and leads to eutrophication. The effect of monopotassium phosphate (MKP) at the rates of 0, 50, 200, 400, and 800 mg P kg⁻¹ on P and certain cations leaching from two agricultural soils (loam and sandy loam soils) was investigated in a laboratory study. Soil treatments were packed in columns with 5 cm in diameter, up to 10 cm. Soil columns were leached using distilled water solution for 20 pore volumes, and the leachates were analyzed for pH, electrical conductivity (EC), calcium (Ca), sodium (Na), potassium (K), and P. To simulate the concentrations of K and P in leachates, the PHREEQC model was utilized. In addition, the P vertical distribution in different depths of the soil columns after the leaching experiment was investigated using Olsen-extractable P (Olsen-P). Generally, as the MKP rates increased, the mean (mean of 20 pore volumes) value of pH and Ca concentration in leachates decreased, but the mean value of EC, Na, and K concentrations in leachates increased. In early pore volumes, the P concentration in all treatments begins to rise, then begins to fall. The application of different rates of MKP fertilizer increased the cumulative amount of P leached in both studied soils. Significant relations were obtained for the rates of MKP application and the cumulative amount of P leached. Overall, the model did a good job of simulating K and P concentrations in leachates, as well as the trend of K and P leaching. In both treated soils with increasing of fertilizer rates, the Olsen-P status in all depths increased, and the P content increased with depth. The Olsen-P contents before the leaching experiment for each treatment were predicted, and power equations significantly described its relation with mean P concentration in leachates. Higher application rates of MKP (400 and 800 mg P kg⁻¹) resulted in much higher P concentrations in leachates than the threshold value (0.1 mg l⁻¹), and these rates should not be used in agricultural soils, whereas applying 50 mg P kg⁻¹ to agricultural soil could be a reasonable rate for preventing P losses.