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.