Study On The Responses Of Soil Ch₄Uptake To Nitrogen Iuput From Daxing’ Anling Boreal Forest

Currently, the process of forest soil methane (CH4) uptake and its driving mechanisms have become the focus of terrestrial ecosystem carbon cycle research. Forest soil CH4uptake is the comprehensive result of CH4production, transportation and oxidation, which is affected by environmental factors such as variations in heat-water conditions and human factors such as land-use change and exogenous nitrogen (N) input. Researches have shown that increase of atmospheric N deposition caused by human activities has had and will continue to have impacts on the forest soil-atmosphere CH4exchange flux. There are three main responses of forest soil CH4uptake to N input, including inhibition, no significant effects and stimlation, which depends on soil N-status, composition of plants and microorganisms, levels and forms of N input, response time and soil properties. Correspondingly, the N regulatory mechanisms of forest soil CH4oxidation remain controversary, and most of which are focused on the NH4+-N regulation. Moreover, related studies are mainly conducted in temperate, subtropical and tropical forests, and the responses of CH4uptake in the N-limited boreal forest soil to enhanced N deposition are few reported. To our knowledge, there is no such information from boreal forest in China.In this study, a boreal cold-temperate coniferous forest soil in the Daxing’anling region was selected for conducting simulated N deposition experiment. Three N levels(Control,0kgNha-1yr-1; Low-N,10kgNha-1yr-1; High-N,40kgN ha-1yr-1) and two N forms (NH4Cl and KNO3) were set, and each N treatment with three repetitions. Net flux of CH4uptake was measured on a ten-day basis using closed clambers on field plots repeated monthly with N fertilizer during the2010growing season (from June to October). Simultaneously, soil environmental variables including soil moisture, soil temperature in the0-10cm horizon, soil NH4+-N and NO3--N contents in the organic and mineral horizons, and soil pH values in mineral soil horizon were measured. In the early, the aims of our researches were (1) the net flux of soil CH4uptake in the cold-temperate coniferous forest and its key control factor and (2) the responses of soil CH4uptake flux and its driving factors such as soil moisture and temperature, soil NH4+-N and NO3--N and soil pH to N input and (3) the environmental driving mechanisms of the responses of soil CH4uptake to N input. The main research results were summarized as follows:(1) Under normal circumstances, the average rates of CH4uptake in the cold-temperate coniferous forest soil were51.5±4.70μgm-2h-1during the measurement period, which was mainly controlled by soil moisture content in the0-10cm horizon.(2) There were no significant differences in the average fluxes of soil CH4uptake among different N level treatments. However, the average fluxes of CH4uptake in the NO3--N-amended soil was obviously high relative to control, especially under low N treatment.(3) In the N-limited cold-temperate coniferous forest, soil inorganic N at different depths was dominated by NH4+-N, and soil pH values in the0～10cm mineral horizon ranged form4.8to5.2under normal conditions. Further analysis revealed that soil NH4+-N contents in the organic and0～10cm mineral horizons increased greatly with increasing N input. Soil NO3--N content in the organic horizon was insensitive to N input, while soil NO3--N content in the0～10cm mineral horizon was sensitive to NO3--N input. Soil pH values in the0～10cm mineral horizon were not obviously affected by N input. Moreover, no significant differences in soil moisture and temperature in the0-1Ocm horizon among N treatments could be found.This study to some degree suggested NO3--N input, rather than NH4+-N input, showed a trend of stimulation on soil CH4uptake from the N-poor boreal forest ecosystem in the short term. The sensitivity of N-limited boreal soil CH4uptake in response to N input may be closely associated with the maximum active CH4oxidizing zone, the vertical distribution and relative contents of soil NH4+-N and NO3--N. Thus, both the levels and forms of N deposition should be taken into account in the analysis of the responses of CH4uptake to N deposition from the N-limited boreal soils and its control factors.