What is the 'effective area' of a urine patch?

Abstract

Cattle urine patches are widely regarded as a key contributor to nitrate leaching in pastoral systems because the N loading (up to 1000 kg N per hectare) in a single urine patch is far in excess of what the affected pasture can utilise, leaving the remaining N vulnerable to leaching. A urine patch consists of a 'wetted area' (where urine is directly voided) and an 'effective area' which also includes an area outside the wetted area (as well as the wetted area itself) that can access urinary N through plant root extension and N diffusion through the soil. Quantitative data on the effective area is scarce and many studies investigating N cycling dynamics under urine patches have not accounted for pasture uptake and soil N dynamics in the effective area. We present results from a field trial where 15N enriched urine and natural abundance urea fertiliser treatments were applied concurrently to soil under ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) pasture. The objectives of the study were to determine the effective area of a 15N enriched urine patch and also to quantify pasture response and soil N dynamics in the effective area. Circular plots consisting of a urine patch 'wetted area' and the potential 'effective area' were established and the pasture and soil N pools were monitored inside three distinct zones. A total of 22% of the urinary 15N was recovered in pasture outside the wetted area, mainly due to surrounding plant root proliferation. Recovery of urinary 15N in the soil was much smaller than in the pasture and was shortlived. Urinary N was recovered in the pasture up to 0.5 m from the edge of the wetted area, resulting in a total potential effective area of up to 2 m2 (up to 6 times the wetted area).