Within-field variability of mineral nitrogen in grassland

The within-field variability of soil mineral nitrogen (N-min) in a grazed g rassland of 8000 m(2) was examined. NO3--N concentrations were characterize d by a high spatial variability. This can be explained by the uneven deposi tion of animal excreta. All NH4+-N as well as NO3--N values were lognormall y distributed, be fore and after the grazing season. At the end of the graz ing season the largest part of the variability of NO3--N was found for NO3- -N concentrations measured within a distance of a few metres. A high variab ility for NO3--N over very short distances was also indicated by a large nu gget variance. During the grazing season, observed mean N-mn, values increa sed from 22 to 132 kg N ha(-1). Regions with clearly higher NO3--N concentr ations could be identified. These zones matched with the drinking place and the entrance of the pasture, places which were more frequently visited tha n others. High residual N levels in autumn led to relatively high losses of N, mostly by leaching, during the subsequent drainage period. Knowing the variability of N-min, the number of samples needed to estimate the average N-min in a field could be calculated for different probabilities and variou s degrees of precision. From the spatial distribution of the N-min concentr ations and the restrictions imposed by the new European decree, adapted fer tilizer strategies can be proposed at least for places where systematically higher N-min concentrations can be expected.