Abstract
Few studies have quantified the effects of different levels and forms of nitrogen (N) deposition on soil nitrous oxide (N2O) 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 N2O 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 NH4NO3, NaNO3, and (NH4)2SO4 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 N2O 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 N2O emission. The results showed that the types and levels of N addition significantly increased soil inorganic N contents, and the accumulation of soil NO3–-N was significantly higher than that of soil NH4+–N due to N addition. The three N forms significantly increased the average N2O emissions (P < 0.05) in the order of NH4NO3 > (NH4)2SO4 > NaNO3 by 355.95%, 266.35%, and 187.71%, respectively, compared with control. The promotion of N2O emission via the NH4+–N addition was significantly more than that via the NO3––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 N2O emissions in the initial years, especially the third year when the increased cumulative N2O 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 NH4+–N, NO3–-N, temperature, and water-filled pore space were all positively correlated with soil N2O emissions. These findings suggest that atmospheric N deposition can significantly promote soil N2O emission, and that exogenous NH4+–N and NO3–-N inputs into temperate forests can have synergic effects on soil N2O emission. In future research, both aspects should be better distinguished in the N cycle and balance of terrestrial ecosystems by using 15N tracer methods.
Highlights
Nitrous oxide (N2O) is a potent greenhouse gas whose global warming potential is 298- and 21-fold that of CO2 and CH4, but it contributes to stratospheric ozone depletion [1]
We report the results of continuous measurements of soil N2O emissions over a 5-year period from a temperate forest in northern China
The maximal emissions were obtained in August 2013 for the low and high nitrogen addition serials, which were 163.23 and 276.33 μg m–2 h–1 in the L-NH4NO3 and H-NH4NO3 addition plots respectively, for all the three added nitrogen forms (Fig 2)
Summary
Nitrous oxide (N2O) is a potent greenhouse gas whose global warming potential is 298- and 21-fold that of CO2 and CH4, but it contributes to stratospheric ozone depletion [1]. N2O is produced by denitrifying bacteria during the reduction of NO3– or NO2– to N2O and N2, or released as an intermediate product when nitrifying bacteria oxidize NH4+–N to NO3– and NO2– [3] These two processes may be affected by soil water content, temperature, N availability and pH, as well as other particular biotic or abiotic properties [4,5,6]. Some previous studies indicated soil N2O emissions were increased under conditions of higher soil water content and soil temperature [10] The latter may regulate soil N2O emission by influencing N2O-producing microorganisms, such as nitrifying and denitrifying bacteria [11]. Low soil moisture can reduce the temperature sensitivity of soil microbes, so that the diffusion of extracellular enzymes in the substrate are lowered [12]
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