Determination of the total nitrogen deposition by the 15N isotope dilution method and problems in extrapolating results to field scale

In order to determine the total input of reactive nitrogen from the atmosphere into the soil-plant system, a method was developed on the basis of 15N isotope dilution called Integrated Total Nitrogen Input (ITNI). This is the only method known which measures directly the active uptake of nitrogen by aboveground plant parts. It is applied at the pot scale. As in all pot experiments, extrapolating the findings to a large area is problematic. Moreover, the method's advantage-quantification of the uptake of nitrogen by aboveground plant parts-also entails a disadvantage in that vegetation type has a pronounced effect on the total deposition of nitrogen. This influence exerted by the plant is examined and discussed using both published and new findings. Based on this critical analysis, proposals are derived for improving the process of extrapolating the measurements from the pot to field scale. These proposals are verified using existing measurements.