In Germany nitrate leaching from agricultural soils to ground-water usually occurs during late fall and winter. Therefore, seepage losses of soil nitrate are correlated to the amounts of soil residual nitrate in late fall. In Lower Saxony success ingroundwater protection is controlled by extensive measuring of soil nitrate concentrations in late fall. In our study, late fall soil nitrate data from three water supply protection zones in Lower Saxony are analysed. The results of the statistical analysis suggest, that when interpreting such data one should take into account that the amount of soil nitrate in late fall is a function of climate, crop, site, and sampling effects. Therefore, short-term detection of crop-specific success in groundwater protection by declining amounts of soil nitrate in late fall only is possible when climate effects (e.g. temperature, precipitation) are eliminated by regression, only agricultural soils with similar physical and chemical properties are compared, and trend analysis is based on mean nitrate amounts of several field soils with respect to the generally low precision of measured values of soil nitrate which usually is not better than +- 20 percent.
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