Nutrient Cycling in Agroecosystems 50: 291–297, 1998. 291 c 1998 Kluwer Academic Publishers. Printed in the Netherlands. Distribution of inorganic nitrogen in agricultural soils at different dates and scales R. Stenger 1 , E. Priesack 1 & F. Beese 2 1 GSF-Institute of Soil Ecology, P.O. Box 1129, D-85758 Oberschleissheim, Germany; 2 Univ. of G¨ ottingen, Inst. of Soil Science and Forest Nutrition, B¨ usgenweg 2, D-37077 G¨ ottingen, Germany Key words: inorganic nitrogen, agricultural soils, frequency distribution, spatial distribution, site specific agricul- tural management 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 = NO 3 -N + NH 4 -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 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. Introduction Soil inorganic nitrogen (N min = NO 3 -N + NH 4 -N) rep- resents not only an economically important pool from which crops fulfill their nitrogen demands but is also ecologically critical since nitrate may leach below the rooting zone and thus will pollute groundwater when present in excess [9]. The determination of actual N min contents before fertilization [11] or the process- oriented modelling of the N turnover [2] can help to improve fertilization recommendations and reduce environmental risks. But the goal of a “site-specific management” can only be reached, when the spatial variability of the N turnover is accounted for [3,4]. Therefore, the spatial variation of N min contents in arable soils and its change over time was studied in a landscape section under uniform management. Sites and methods The investigations were carried out at the Klostergut Scheyern Experimental Station of the Munich Research Association for Agricultural Ecosystems (FAM). The FAM-Experimental Station (Table 1) is located 45 km north of Munich (Germany), in a hilly landscape (450 m - 495 m altitude) derived from ter- tiary sediments originally covered with loess deposits. Since erosion has worn this cover away from the hill tops and upper slopes, shallow and partially gravelly soils, derived from sandy, silty or clayey tertiary sed- iments, are predominant at these sites. Deep brown soils derived from loess and colluvia are found at the lower slopes and in the depressions. The average tem- perature is 7.4 C; the yearly mean precipitation rate amounts to 833 mm. At all arable sites, winter wheat was grown in the first, and spring barley in the second year of this study. This uniform management permit- ted the investigation of the effect of site conditions on the spatial variation of N min without the confound- ing effect of different crops. Inorganic nitrogen (N min )