Time series analysis of changes in the soil solution: Evidence for approach to nitrogen saturation in Dutch forest soils

Abstract

In the Netherlands, high atmospheric inputs of ammonium N followed by microbial oxidation of ammonium N to nitric acid and low N uptake from poor tree growth, has resulted in high levels of dissolved nitrate and strong soil acidification in forest soils. So-called nitrogen saturated soils where the supply of atmospheric N plus mineralised soil organic N exceeds the capacity of the ecosystem (vegetation plus soil biota) to assimilate N have been observed occasionally. At high levels of N availability, N saturation leads to enhanced soil acidification and groundwater pollution. To test if nitrate saturation is being approached elsewhere, temporal trends in concentrations of dissolved nitrate and some other elements in a number of acidic forest soils were analysed statistically. A procedure was developed to account simultaneously for short-term, mainly seasonal variability plus any longer-term trend on a time scale of 4–7 years. Although results should be interpreted with care, because of the limited period of data collection, strong evidence for a long-term increase in nitrate concentrations has been found in all plots considered. These trends could not be accounted for by trends in atmospheric deposition or soil hydrology in general, suggesting the approach to nitrogen saturation. The procedures used in this study to analyse the monitored data focus on the estimation of trends and calculation of its significance. They can be applied to similar studies dealing with temporally varying data, sampled at irregular intervals and exhibiting missing values.