Response of soil CH4 fluxes to stimulated nitrogen deposition in a temperate deciduous forest in northern China: A 5-year nitrogen addition experiment

Abstract

Forests are one of the most important carbon pools on earth and methane (CH4) is the most important greenhouse gas after carbon dioxide. In order to investigate how increasing levels of nitrogen (N) deposition affect CH4 uptake in a forest, we conducted a five-year experimental study on three forms of N (NaNO3, (NH4)2SO4, and NH4NO3) at two levels (50 (L) and 150 (H) kg N ha−1 yr−1) in a temperate deciduous forest in northern China. From 2011 to 2015, additional N was added equally once per month during the growing season (from March to October). Soil CH4 flux was measured three times per month from March 2011 to February 2016. Except for L-NaNO3 addition, which had no significant influence on annual CH4 uptake, all other N additions significantly decreased annual CH4 uptake by 49% (H-(NH4)2SO4), 43% (H-NH4NO3), 40% (L-(NH4)2SO4), 31% (H-NaNO3), and 29% (L-NH4NO3), respectively. As N addition significantly increased soil ammonium (NH4+) and nitrate (NO3−) concentrations, we inferred that the inhibition of CH4 uptake by N addition could be attributed to restricted methanotroph activity caused by higher osmotic stress. High level N addition exerted stronger inhibitory effects on CH4 uptake than low level, and the effects of the different N forms on inhibiting the annual CH4 flux exhibited a general order of (NH4)2SO4 > NH4NO3 > NaNO3. NH4+, rather than NO3−, was the major factor contributing to the inhibitory effect of N input on CH4 uptake which might be attributed to their different mobilities. Considering the decreasing ratio of NH4+ to NO3− in the local N deposition and the stronger suppression of CH4 uptake by NH4+, the inhibition of soil CH4 uptake by the atmospheric N deposition could be alleviated at our study plots in the near future. Due to the time-lag in the suppression of CH4 uptake by N addition, if the experimental period had been less than three years there would have been a different conclusion compared with that observed in the following years. Overall, our findings suggest that N deposition-induced increases in soil organic N concentrations may affect processes linked to CH4 dynamics and change soil-atmospheric CH4 fluxes in a temperate forest in northern China depending on the levels and forms of N addition.