Within-field variability of mineral nitrogen in grassland

 The within-field variability of soil mineral nitrogen (N_min) in a grazed grassland of 8000 m² was examined. NO₃ ^–-N concentrations were characterized by a high spatial variability. This can be explained by the uneven deposition of animal excreta. All NH₄ ⁺-N as well as NO₃ ^–-N values were lognormally distributed, before and after the grazing season. At the end of the grazing 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 variability for NO₃ ^–-N over very short distances was also indicated by a large nugget variance. During the grazing season, observed mean N_min values increased from 22 to 132 kg N ha^–1. Regions with clearly higher NO₃ ^–-N concentrations could be identified. These zones matched with the drinking place and the entrance of the pasture, places which were more frequently visited than 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 various degrees of precision. From the spatial distribution of the N_min concentrations and the restrictions imposed by the new European decree, adapted fertilizer strategies can be proposed at least for places where systematically higher N_min concentrations can be expected. Received: 14 December 1999