Nitrogen and phosphorus losses in overland flow from a cattlegrazed pasture in Southland

In response to local concerns about the off-site impacts of intensive cattle grazing on surface water quality, a field study was conducted to quantify N and P losses in overland flow from a cattle-grazed pasture in eastern Southland. Overland flow from drained and undrained areas of a silt loam soil was collected over a 3-year period using a surface guttering system connected to 0.5-litre tipping buckets for recording flow volumes. Results indicated that total overland flow was between two to seven times greater from the undrained than from the drained soil. Losses of N and P in overland flow from drained soils were low relative to that measured in subsurface drainage from these soils. Much of the overland flow, and overland N and P loss, occurred during winter and spring, rather than summer and autumn when annual P fertiliser applications were made. Overland flow and losses of N and P were greatest from the undrained soil receiving 400 kg N ha(-1) year(-1) and stocked at the equivalent of 3.1 cows ha(-1). These N and P losses in overland flow were greatest within, the 1- to 2-week period following spring grazing, rather than the period shortly following P fertiliser applications. These increased losses were probably caused by a combination of (i) the return of nutrients via dung and urine, and (ii) the decline in soil structural condition often observed during grazing under wet soil conditions. Managements targeted to minimise soil-treading damage during spring, and thus maintain good soil infiltration, are therefore suggested as helpful strategies for minimising N and P losses via overland flow, particularly from heavy, poorly-drained soils in Southland.