Comparing a process-based agro-ecosystem model to the IPCC methodology fordeveloping a national inventory of N2O emissions from arable lands in China

Nations are now obligated to assess their greenhouse gas emissions under th e protocols of Article 4 of the United Nations Framework Convention on Clim ate Change. The IPCC has developed `spreadsheet-format' methodologies for c ountries to estimate national greenhouse gas emissions by economic sector. Each activity has a magnitude and emission rate and their product is summed over all included activities to generate a national total (IPCC, 1997). Fo r N2O emissions from cropland soils, field studies have shown that there ar e important factors that influence N2O emissions at specific field sites th at are not considered in the IPCC methodology. We used DNDC, a process-orie nted agroecosystem model, to develop an unofficial national inventory of di rect N2O emissions from cropland in China. We assembled county-scale data o n soil properties, daily weather, crop areas, N-fertilizer use, livestock p opulations (for manure inputs to cropland), and agricultural management for the 2500 counties in mainland China. Total 1990 cropland area was 0.95 mil lion km(2). Total N-fertilizer use in China in 1990 was 16.6 Tg N. The aver age fertilization rate was 175 kg N ha(-1) cropland. One-year simulations w ith DNDC were run for each crop type in each county to generate estimates o f direct N2O emissions from soils. National totals were the sum of results for all crop simulations across all counties. Baseline simulations estimate d that total N2O emission from arable land in China in 1990 was 0.31 Tg N2O -N yr(-)1(.) We also ran simulations with zero N-fertilizer input; the diff erence between the zero-fertilizer and the baseline run is an estimate of f ertilizer-induced N2O emissions. The fertilizer-induced emission was 0.13 T g N2O-N yr(-1,) about 0.8% of total N-fertilizer use (lower than the mean b ut within the IPCC range of 1.25 +/-1.0%). We compared these results to our estimates of county-scale IPCC methodology emissions. Total emissions were similar but geographical patterns were quite different.