Describing N leaching under urine patches in pastoral soils

摘要

Urine patches are the major source of nitrogen (N) leaching from grazing systems as the nitrogen load under these spots far exceeds the plants needs. The high concentration of nitrogen under urine patches is also challenging for modelling as some processes, such as ion adsorption, cannot be assumed to be linear. We have employed the APSIM model framework with a newly developed pasture module to describe the results of an experiment where lysimeters were treated with a load of 1000 kg/ha of nitrogen as urine. To model this data we used a mechanistic soil model with NH_4 adsorption modelled by a Freundlich isotherm The model showed good agreement with measured drainage and both NO_3 and NH_4 leaching. The effect of the assumption that NH_4 is completely immobile was compared with the Freundlich isotherm and found to underestimate leaching by 20% in this loamy soil. In soils, such as sandy soils, with lower adsorption capacity the underestimation is expected to be greater.