Improving the precision of estimating nitrate (NO ₃ ^-) concentration in seepage water of forests by prestratification with soil samples

Nitrate (NO ₃ ^-) concentrations in seepage water (SW _NO3) of N saturated forests regularly show a high spatial variability. As seepage water sampling is expensive, most studies use small numbers of soil solution samplers. This implies high uncertainties of calculated concentration and flux data. At the N saturated Höglwald site we investigated, whether a preselection of seepage water sampling places by a prior intensive soil sampling can improve the precision of the estimation of SW _NO3. We implemented 121 suction cups and analyzed the soil, removed with the soil auger during installation at the sampling depth of 40 cm, for water extractable NO ₃ ^-. The NO ₃ ^- content in soil (Soil _NO3) was calculated and SW _NO3 was measured. With this data set we tested the correlation between Soil _NO3 and SW _NO3 and simulated a random selection of sampling places (RS) as well as a prestratified sampling based on Soil _NO3 (PS). With bootstrap statistics different numbers of replications (n) were tested and the 95% confidence interval (95% CI) was used to compare RS with PS. Highly significant correlations between Soil _NO3 and SW _NO3 were found (r = 0. 99***). It was even possible to calculate SW _NO3 from Soil _NO3. Here best results where achieved when Soil _NO3 was determined from field moist soil samples. With PS the accuracy of the mean value of SW _NO3 improved considerably. To achieve a 95% CI lower than ±10%, 250 suction cups would be needed for RS compared to n = 20 for PS. Even 6 months after implementation, precision of mean SW _NO3 was higher for suction cup places selected with the PS method. A detailed description is given, how these findings may be applied in studies and monitoring programs focusing on NO ₃ ^- leaching in forest ecosystems.