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

Abstract

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 (N_min = NO₃-N + NH₄-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 50×50-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 N_min contents 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.