Short-term consequences of spatial heterogeneity in soil nitrogen concentrations caused by urine patches of different sizes

The scale of spatial heterogeneity in soil nitrogen (N) concentrations varies considerably in grazed systems, because grazers vary in the volume of urine they excrete. This could affect how urine-N is processed, and subsequently how much N is lost from the system, as diffusion and plant effects on soil nutrient concentrations can be scale-dependent. Two field experiments were performed; one measured the impact of urine patch size (small, medium or large) on soil inorganic N pools and fluxes over time, and the other assessed whether urine patch size affected plant responses and system N retention even if the same total amount of urine was applied. Soil from inside small urine patches retained inorganic N for shorter amounts of time, resulting in lower plant biomass and N uptake than that inside larger patches. Although system nitrogen retention was not affected by patch size, it appeared that larger patches had a greater potential to lose N due to the longer period over which soil inorganic N concentrations remained high. This suggests that systems grazed by larger organisms are more prone to lose N through patch size effects than those grazed by smaller ones.