Effects of nitrogen load variation in animal urination events on nitrogen leaching from grazed pasture

Nitrogen (N) leaching losses from grazing systems originate primarily from animal urine patches. The N load in urine patches has strong effects on N leaching and varies largely with variations in volume and N concentration of urination events. The effects of these variations in volume and N concentration of urination events on N leaching have not yet been explored. We present a framework for assessing N leaching losses from grazed pastures that incorporates variations of urination events. We found that, for the same amount of urinary N deposition, annual N leaching losses at paddock level increased logarithmically with increased mean urine volume or N concentration, though N leaching at urine patch level increased exponentially with N deposition rate. The estimates of N leaching losses using mean urine patches, derived from mean urine volume and N concentration, would be less than that estimated using variable urine patches as deposited by animals. Results for a case-study pasture on free-draining pumice soils indicate that, for the same number of urination events, depositing the same amount of urinary N within a year, N leaching would be underestimated by 5–8%, when using average urine patches as opposed to varying both volume and N concentration; and the underestimation would be bigger for pastures on soils of greater water holding capacity. Our results indicate the necessity of more accurate estimation of mean and variation patterns of urine volume and N concentration of urination events for improving estimates of N leaching risks. The results support the development of N leaching mitigation strategies by feeding animal diuretics, and are useful to recommend animal species and age class for areas of high N leaching risks.