Sediment and phosphorus transfer in overland flow from a maize field receiving manure

The transfer of suspended sediment (SS) and phosphorus (P) in overland flow from 30 m(2) field plots receiving either nil, surface-applied or incorporated manure (slurry) were monitored to determine the vulnerability of land cropped to continuous forage maize to diffuse pollutant transfer in winter runoff. In the absence of slurry, P export was dominated by particulate forms, with up to 1 t SS ha(-1) and 0.75 kg total P ha(-1) collected from an individual storm event. Background concentrations of P in soluble (<0.45 mum) form were large (c. 0.5 mg L-1) by eutrophication standards due to the previous build-up of soil P, and largely independent of SS concentrations. Largest P exports (representing up to 23% of the slurry P applied) were measured when dairy slurry (3-13% dry solids) was surface-applied. The P mobilized from the slurry accounted for up to 60% of total plot P export, with the majority occurring in a soluble bioavailable form during the first storm event. Initial P concentrations in runoff were in proportion to the amount of slurry P applied and significantly lower where rainfall was delayed after application. In one year, splitting the slurry application (3 x 10 kg ha(-1)) reduced total P export by 25% compared to a single surface application (30 kg P ha(-1)). In two years, incorporation of slurry, either by ploughing, or by tine cultivation, reduced the amount of overland flow by 50%, and the amount of P export by up to 60%, compared to the surface-applied slurry treatments. Timeliness of slurry spreading to avoid periods of wet weather and simple cultivation of maize fields after harvest are practical and effective options to minimize SS and P transfer in land runoff from maize fields. The results also draw attention to the need to grow maize, and apply slurry to fields with a low P loss risk.