Effects Of Simulated Nitrogen Deposition On Greenhouse Gas Emissions Of Alpine Meadows In Northern Tibet

Recently, the increase of atmospheric nitrogen deposition had a profound impact on carbon and nitrogen cycling in the terrestrial ecosystems, and affected the structure and function of entire ecosystem. Grassland is one of the most widespread ecosystems in terrestrial ecosystems and extremely sensitive to nitrogen deposition, which is seriously influenced by human activity at present. Moreover, the greenhouse gas emissions of grasslands which will have an important feedback to climate change were regulated through a series of physiological and ecological activity. In northern Tibet, the grassland is the most important and the largest ecosystem in this region with the characteristics of high altitude, low temperature and harshly natural conditions. The increase of atmospheric nitrogen deposition will have a significant impact on alpine meadow ecosystems in northern Tibet. This study was investigated the effects of the nitrogen deposition on greenhouse gas emissions of alpine meadow in northern Tibet, and explored the relationships among the productivity of alpine meadow community, soil nutrient availability, and greenhouse gas emissions, and understood the nitrogen utilization way and adaptability under the condition of nitrogen deposition in the future. This study will provide a scientific basis for scientific management of alpine meadow ecosystems to adapt to the future climate change in Qinghai-Tibet Plateau, and also provide a theoretical basis on preventing grassland degradation and a support of data for the establishment of greenhouse gases emissions reduction policies of alpine meadows.In this study, we conducted an in situ measurements using nitrogen addition in alpine meadow of northern Tibet, and made four fertilizing levels of 0, 7, 20, and 40 kg?hm-2?a-1 in Nagqu County in 2014 and 2015. We used the static chamber method for measuring annual measurement of carbon dioxide(CO2), methane(CH4) and nitrous oxide(N2O) emissions. We also measured the changes of soil nutrients under simulating nitrogen deposition. Moreover, we discussed the relationships among plant productivity, soil nutrient availability, and greenhouse gases emission fluxes under the condition of nitrogen deposition. The main conclusions as follows:(1) The CO2 emissions were significantly increased by nitrogen addition in alpine meadows of northern Tibet. The CO2 emissions in growing season were significantly higher than that in the non-growing season and showed obvious seasonal variations. The aboveground biomass, soil NO3--N, MBN content, the amount of litter decomposition, litter C content, and soil temperature at 5 cm depth had no significant correlation with CH4 emissions in alpine meadow.(3) The N2 O emissions were not significant impacted by nitrogen addition. There were no significant seasonal variations of N2 O emissions, and also no significant N2 O emission difference between growing season and non-growing season. Soil NO3--N, soil TOC, soil TN content were positively correlated to N2 O emissions, while the plant productivity, litter C content, soil moisture at 5 cm depth were negatively correlated to N2 O emissions in alpine meadow. In addition, soil NH4+-N, soil MBC, soil MBN, litter decomposition rate, litter N content, and soil temperature at 5 cm depth had no significant impacts on N2 O emissions in alpine meadow of the northern Tibet.(4) In the growing season, CO2 emission was significantly increased in ten days after nitrogen addition. CH4 emission had a relatively big fluctuation and was not significantly increased or decreased. In June, N2 O emission was significantly increased in the first six days, but after that, it was decreased significantly after nitrogen addition. In July, N2 O emission was a decreasing tendency in contrast to August after nitrogen addition.(5) After two-year of nitrogen addition, the aboveground biomass was significantly increased of alpine meadows in northern Tibet. Only in June and Novevember, TOC and TN content were increased significantly under N40 treatment. In control plots, soil NH4+-N content was significantly higher than NO3--N content, so soil inorganic nitrogen was mainly derived from NH4+-N content. Different levels of nitrogen addition had different impacts on soil MBC and MBN. Under N20 treatment, soil MBC was significantly lower than CK plots. The soil MBN content in treatment plots were marginally(P > 0.05) higher than that in CK plots. Before the starting of growing season, there was no significantly difference between each treatment of the litter residuals. In the peak of growing season, under N7 and N20 treatments, litter residuals were significantly lower than CK plots. In the end of growing season, litter residuals in treatment plots were decreased significantly compared to CK plots. After one year, the litter carbon and nitrogen content were increased in different degrees in all plots.Finally, nitrogen addition significantly increased CO2 emission, but had no significant impacts on emissions of CH4 and N2 O in alpine meadow ecosystem. CO2 emissions of the whole year were the largest contribution to the totally annual emissions of greenhouse gases, which were significantly increased with increasing in nitrogen deposition conditions.