THE SPATIAL VARIABILITY OF SOIL NITRATES IN ARABLE AND PASTURE LANDSCAPES - IMPLICATIONS FOR THE DEVELOPMENT OF GEOGRAPHICAL INFORMATION-SYSTEM MODELS OF NITRATE LEACHING

In response to the European Community Nitrate Directive (91/676) a cat chment scale Geographical Information System (GIS) model of nitrate le aching has been developed to map nitrate vulnerability and predict ave rage weekly fluxes of nitrate from agricultural land units to surface water. This paper presents a pilot study which investigated the spatia l variability of soil nitrates in order to: (1) define an appropriate pixel size for modelling N leaching; (2) quantify the within-unit vari ability of soil nitrate concentrations for pasture and arable fields; and (3) assist in the design of an efficient sampling strategy for est imating mean nitrate concentrations. Soil samples, taken from two 800 m transects in early September 1994, were analysed for water soluble n itrate. The arable soils had a mean nitrate-nitrogen concentration of 0.693 mu g/g (S.E. 0.054 mu g/g) and the pasture soils had a higher me an nitrate-nitrogen concentration of 0.86 mu g/g (S.E. 0.085 mu g/g). Spatial variability was investigated using variograms. The pasture dat a had a weak spatial relationship, whereas the arable data exhibited a strong spatial relationship which fitted a spherical variogram model (r(2) 0.87), with a range of 40 m. A pixel size of 40 m is suggested f or nitrate modelling within the GIS based on the arable variogram and an improved sampling strategy for model validation is suggested, invol ving bulking sub-samples over a 40 m grid for estimating mean nitrate concentrations in combined land use and soil units.