Effects of different nitrogen addition forms and levels on N₂O emission in the temperate forest soil

Nitrous oxide (N₂O) emission can be affected by the amounts and forms of nitrogen (N) available in soils. To improve our understanding of the response of N₂O emission to different N addition levels and forms, a manipulative field experiment was conducted to investigate the effects of different forms (ammonium sulfate, As:(NH₄)₂SO₄; sodium nitrate, Na:NaNO₃; ammonium nitrate, AN:NH₄NO₃) and levels (control: 0 kg/(ha·year); low N, LN: 50 kg/(ha·year); MN: 100 kg/(ha·year) and high N, HN: 150 kg/(ha·year)) of N addition on N₂O emission of temperate Pinus tabuliformis forest soil using the static closed chamber method at Experimental Forest Station of Beijing Forestry University in Beijing, northern China. Our results showed that soil N₂O emission showed a trend of seasonal variation: higher in summer (May to August) but relatively low in the remaining seasons. The minimum value appeared in January. Different levels of N addition increased annual N₂O emission in the order of HN>MN>LN>control. The three N forms increased annual N₂O emission in the order of AN>As>Na, but the difference was insignificant (P>0.05) between As and AN or As and Na. These results suggested that the N₂O emission from temperate forest soil sensitively responded to N addition forms and levels. In addition, soil temperature, air temperature and water-filled pore space also influenced soil N₂O fluxes. Annual soil N₂O emission factors ranged from 0.34% to 0.94% for different N addition treatments, with an overall emission factor value of 0.364%. The emission factor values were far less than the mean default emission factor proposed by the Intergovernmental Panel on Climate Change (IPCC).