ABSTRACT
Nitrate leached from grazed leguminous pastures has contaminated groundwater in several areas of Australia, New Zealand and temperate regions of several other countries. This paper focuses on those pastures which do not receive nitrogen fertiliser or slurry applications, where fixation of atmospheric nitrogen by clovers is the dominant nitrogen input. Pasture residue and livestock waste products mineralise and are available for uptake by the pasture and for leaching. The non-uniform distribution of livestock waste on a pasture in space and time leads to nitrate concentrations which locally exceed the uptake/attenuation capacity.
The irregular pattern of leaching of nitrate complicates experiments at paddock scale (10 to 40 ha). Observations at pasture plots subject to applied nitrogen loads and models of nitrogen transformation and leaching processes have been applied to estimate the nitrate leached to groundwater beneath pastures at paddock-scale. These models have also been used to predict the effects of changes in pasture management on the leaching of nitrate. Furthermore the fate of nitrate in groundwater may be simulated to forecast groundwater nitrate concentrations under alternative land management scenarios. These methods may have applications in other cases where diffuse sources of contamination are in fact a collocation of point sources.
