The anthropogenic emission of nitrogen (N) compounds is increasing globally. As a developing country, China has a dramatic increase in atmospheric N deposition since 1980s, and the rate of N addition in China is increasing at a speed of 0.41 kg N hm^-2a^-1 from 1980 to 2010. China has become one of the most important N deposition zones in the world. By comparison with the national average, the rates of N deposition are higher in southeast China. Excess N deposition has aroused concerns about its negative impacts on ecosystem health and services such as loss of biodiversity, N saturation and soil acidification, and inhibition of the capability of methane (CH₄) uptake in upland soils. Forest soils are an important CH₄ sink or source through the activity of methanotrophic and methanogenes bacteria. The diffusivity of CH₄ through the soil profile is the primary limiting factor upon CH₄ uptake, which is influenced by soil moisture and bulk density. Furthermore, soil available N contents (NH₄^- and NO₃^-) can limit CH₄ uptake directly by competing with the monooxygenase enzyme of methanotrophs. Many studies including field and laboratory studies have reported that increased N availability due to N deposition may inhibit the uptake capacity of forest soils for atmospheric CH₄. However, some studies in subtropical China get different results. Further studies should be undertaken to better understand the mechanisms responsible for N deposition-induced suppression of CH₄ uptake in forest soils.
In this research, we studied the responses of soil CH₄ uptake rates to simulated N deposition in a natural forest in Jianou, Fujian, China based on the design and methods used in the European N ITREX project. Treatments included three N levels (three replicates for each level), viz. 0, 50, and 100 kg N·hm^-2·a^-1 for control (CK), low-N (LN), and high-N (HN) treatment, respectively. From December 2009 to November 2012, monthly CH₄ uptake rates were measured using a static chamber and gas chromatography technique. Average CH₄ uptake rate in control was (-62.78±14.39) μg·m^-2·h^-1. Comparing with the control, average CH₄ uptake rate of the LN ((-43.82±4.72) μg·m^-2·h^-1) was decreased by 30.21%. However, there was no significant difference between HN ((-58.23±5.58) μg·m^-2·h^-1) and the control (P > 0.05). The average rates of CH₄ uptake for the LN and HN were both significantly lower than the control in the third year (P < 0.05), which demonstrated that, the CH₄ uptake rates for all N treatments might be seriously inhibited with continuing N addition. Seasonal changes of CH₄ uptake rate in all N treatments were similar. Correlation analysis showed that CH₄ uptake rates were significantly correlated with soil temperature and soil moisture both in the control and HN treatments (P < 0.05), but only significant correlation between CH₄ uptake rates and soil moisture was found for LN (P < 0.05). It suggested that the change of soil moisture was the most important factor which regulated the dynamics of CH₄ uptake rates. In addition, soil pH values for the LN and HN were significantly lower than that in the control (P < 0.01). Also, the LN treatment had lower soil pH value than the HN treatment. Soil C/N ratio for the LN treatment increased significantly compared with the control (P < 0.01), while the reverse was for the HN treatment. There were no significant effects of N treatments on soil NH₄⁺-N, NO₃^--N, total dissolved N, dissolved organic carbon(C), litterfall mass, fine root biomass in 0-10 cm soil (P > 0.05). Thus, it indicated that N deposition might firstly induce the changes of soil pH and soil C/N ratio.