Distribution of inorganic nitrogen in agricultural soils at different dates and scales

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To avoid the leaching of nitrate from agricultural soils to groundwater, appropriate N-fertilization strategies aim at the depletion of the pool of soil inorganic nitrogen (Nmin=NO3−N +NH44−N) during crop growth. Such strategies need a good knowledge of the spatial distribution of inorganic nitrogen pools at the field sites and its change over time. Therefore inorganic nitrogen in arable soils of a farm was determined after harvest in 1990, 1991 and 1992 and in the spring of 1991 and 1992 with a high areal resolution. One bulk sample of four corings was taken per 50x50-m grid cell (n=152−178) from 0−90cm soil depth. At all arable sites, winter wheat was grown in the first, and spring barley in the second year of this study. Results show a wide range of nitrogen contents at the farm level, at the field level and at the level of subdivisions of fields. Almost no spatial dependence of Nmincontents could be found from calculated semivariograms. Because of this small-scale variation the subdivision of fields into fixed plots of homogenous N-fertilizer demand is difficult and can be seen as only a first step towards site specific farming.

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