Urine patches on dairy farmland may cause extra loss of nitrogen through leaching and thus lead to potential contamination of groundwater in underlying aquifers. In this study, mathematical models and numerical simulations were used to quantitatively investigate the most important factors affecting the total quantity and spatial distribution of N-leaching from dairy farmland soils. A comprehensive two-dimensional model was derived by integrating models on pasture growth, heat transport, water flow and solute transport, and various N transformation processes. Through numerical simulations, the influence of soil texture (hydraulic conductivity, dispersivity, immobile water content, and “diffusion” path length and conductivity of macropores), urine patch density, quantity of irrigation and N fertilizer application on total N-leaching, and spatial variations in N-leaching were analyzed. Results showed that for all cases investigated, spatial variations in N-leaching at the bottom of root zone were very large, which is the major reason urine patches would introduce extra N-leaching losses. Spatial variations in N-leaching were strongly affected by the density of urine patch coverage, soil hydraulic conductivity, dispersivity, preferential flow in macropores, and irrigation, but little by N fertilizer application and the presence of soil immobile water. Total N-leaching was strongly affected by density of urine patch coverage, soil hydraulic conductivity, irrigation, N fertilizer application, and preferential flow, but only slightly by the dispersivity and immobile water.

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