Quantifying the regional source strength of N-trace gases across agricultural and forest ecosystems with process based models

The process-based models DNDC and PnET-N-DNDC were evaluated with regard to their potential to calculate regional inventories of N-trace gas emissions from agricultural and forest soils. To extend the model predictions to regional scale, we linked the models to a detailed GIS-database for Saxony, Germany, which was holding all the spatially and temporally differentiated input information and other model drivers. Total annual N2O-emissions from agricultural soils in Saxony ranged from 0.5-26.0 kg N2O-N ha(-1) yr(-1) and were calculated to amount to approx. 5475 t N2O-N yr(-1) in the year 1995, which compares quite well with previous estimates based on the IPCC approach (4892 t N2O-N yr(-1)). Compared to the agricultural soils, N2O-emissions from forest soils in Saxony (range: 0.04-19.7 kg N2O-N ha(-1) yr(-1)) were much lower and amounted to 1011 t N2O-N yr(-1). In comparison with other sources of N2O in Saxony our estimates show, that - even in such a highly industrialised region like Saxony - soils contribute more than 50% to the total regional N2O source strength. Simulated emissions of NO from the agricultural and forest soils were approx. in the same magnitude than for N2O. The modelled NO-emission rates ranged from 0.4-26.3 kg NO-N ha(-1) yr(-1) for the agricultural soils and 0.04-28.3 kg NO-N ha(-1) yr(-1) for the forest soils with total emissions of 8868 t NO-N yr(-1) (agricultural soils) and 4155 t NO-N yr(-1) (forest soils). Our results indicated that the agricultural and forest soils were a significant source, which contributed 17.9% of the total NOx emissions from various sources in Saxony. Furthermore, a series of sensitivity tests were carried out, which demonstrated that variations in soil organic carbon content (SOC) and soil texture significantly effect the modelled N-trace gas emissions from agricultural soils at the regional scale, whereas, in addition, for forest soils also the soil pH is within the sensitive factors. Finally, multi-year simulations were conducted for the region with observed meteorological data from 1994-1996. The results demonstrated that the modelled interannual variations, which were obviously induced by only the climate conditions, in the N-gas emissions were as high as 36%. The high interannual variations imply that multi-year (e.g., 5-10 years), instead of single baseline year, simulations would produce more reliable estimates of mean soil N2O-emissions at regional scale. With respect to the Kyoto protocol this means that the mean N2O-emissions from soils in the period 1988-1992 should be evaluated instead of focusing on a single year, 1990.