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

Abstract

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.