Nitrate concentrations in soil solutions below Danish forests

Nitrate in the soil water below the root zone is a pre-condition for nitrate leaching, and it indicates loss of nutrients from the forest ecosystem. Nitrate leaching may potentially cause eutrophication of surface water and contamination of groundwater. In order to evaluate the extent of nitrate leaching in relation to land use, a national monitoring programme in Denmark has established sampling routines in a 7x7 km grid including 111 points in forests. During the winters of 1986-93, soil samples were obtained from a depth of 0-25, 25-50, 50-75 and 75-100 cm. Nitrate concentrations in soil solutions were determined by means of a 1 M KCl extraction. The influence of forest size, forest type, soil type, tree species and sampling time on the nitrate concentrations was analysed using a statistical model. The analysis focused on data from depth 75-100 cm, as nitrate is considered potentially lost from the ecosystem at this depth. The range of nitrate concentrations was 0-141 mg NO3--N dm-3 and the estimated mean value was 1.51 mg NO3--N dm-3. The concentration was influenced by (1) forest size (concentrations in forests 50 ha), (2) forest type (afforested arable land had higher concentrations than 'other woodland'), (3) soil type (humus soils showed above average concentrations, and fine textured soils had higher concentrations than coarse textured soils), and (4) sampling time. Unlike results obtained in other investigations, there was no significant effect of tree species. A few sites deviated radically from the general pattern of low concentrations. The increased concentrations recorded there were probably caused by high levels of N deposition due to emission from local sources or temporal disruptions of the N cycle. The nitrate concentration in the soil solution below the root zone was mostly rather low, indicating that, generally, N saturation has not yet occurred in Danish forest ecosystems. However, median concentrations exceeding drinking water standards (11.3 mg NO3--N dm-3) were found at 7% of the sites, and 30% of the sites had median concentrations above 2 mg NO3--N dm-3, which equals annual N losses of more than 2-6 kg ha-1..