Five-year study of the effects of simulated nitrogen deposition levels and forms on soil nitrous oxide emissions from a temperate forest in northern China

Few studies have quantified the effects of different levels and forms of nitrogen (N) deposition on soil nitrous oxide (N 2 O) emissions from temperate forest soils. A 5-year field experiment was conducted to investigate the effects of multiple forms and levels of N additions on soil N 2 O emissions, by using the static closed chamber method at Xi Mountain Experimental Forest Station in northern China. The experiment included a control (no N added), and additions of NH 4 NO 3 , NaNO 3 , and (NH 4 ) 2 SO 4 that each had two levels: 50 kg N ha −1 yr −1 and 150 kg N ha −1 yr −1 . All plots were treated to simulate increased N deposition on a monthly schedule during the annual growing season (March to October) and soil N 2 O emissions were measured monthly from March 2011 to February 2016. Simultaneously, the temperature, moisture, and inorganic N contents of soil were also measured to explore how the main factors may have affected soil N 2 O emission. The results showed that the types and levels of N addition significantly increased soil inorganic N contents, and the accumulation of soil NO 3 – -N was significantly higher than that of soil NH 4 + –N due to N addition. The three N forms significantly increased the average N 2 O emissions ( P < 0.05) in the order of NH 4 NO 3 > (NH 4 ) 2 SO 4 > NaNO 3 by 355.95%, 266.35%, and 187.71%, respectively, compared with control. The promotion of N 2 O emission via the NH 4 + –N addition was significantly more than that via the NO 3 – –N addition, while N addition at a high level exerted a stronger effect than at the low-level. N addition exerted significantly stronger effects on cumulative N 2 O emissions in the initial years, especially the third year when the increased cumulative N 2 O emission reached their maximum. In the later years, the increases persisted but were weakened. Increasing inorganic N concentration could change soil from being N-limited to N-rich, and then N-saturated, and so the promotion on soil available N effect increased and then decreased. Moreover, the soil NH 4 + –N, NO 3 – -N, temperature, and water-filled pore space were all positively correlated with soil N 2 O emissions. These findings suggest that atmospheric N deposition can significantly promote soil N 2 O emission, and that exogenous NH 4 + –N and NO 3 – -N inputs into temperate forests can have synergic effects on soil N 2 O emission. In future research, both aspects should be better distinguished in the N cycle and balance of terrestrial ecosystems by using 15 N tracer methods.