Responses of CH4, CO2 and N2O Fluxes to Increasing Nitrogen Deposition in Alpine Grassland of the Tianshan Mountains

Anthropogenic activities, mainly in the form of fertilizer application and fossil fuel use, have greatly accelerated the emissions of reactive nitrogen species worldwide and N addition to ecosystems alters physiology of soil microbes and vegetation in a way that leads to altered biogenic fluxes of CO₂, CH₄ and N₂O contributing to global warming. Most research stresses that greenhouse gas (GHG) production and consumption can be controlled by temperature and soil water content. Simulated N deposition experiments in temperate grasslands/steppes resulted in reduced CO₂ emission and CH₄ consumption and increased N₂O emission, but the reverse or no significant effects have also been reported.

Recently the ecological effects of simulated atmospheric N deposition on some natural ecosystems in China have been studied by some researchers but few studies have assessed the effect of simulated N deposition on all the three GHGs and these measurements were short-term and have been carried out only during the growing season.

In order to contribute to understanding the effects of N deposition on GHGs in grassland ecosystems, and helping to assess the effects of N globally, parameterize Earth System models and inform decision makers, CH₄, CO₂ and N₂O fluxes were measured from June 2010 to May 2011 in the Bayinbuluk Grassland Ecosystem Research Station, Chinese Academy of Sciences. The Bayinbuluk alpine grassland is located in the southern Tianshan Mountains of Central Asia, Xinjiang. To better understand the effects of N deposition on GHGs in alpine grassland, five N addition treatment sites were selected.

The results indicated that N deposition tended to significantly increase CH₄ uptake, CO₂ and N₂O emissions at sites receiving N addition compared with those at site without N addition during the growing season, but no significant differences were found for all sites outside the growing season. Air temperature, soil temperature and water content were the important factors that influence CO₂ and N₂O emissions at year-round scale, indicating that increased temperature and precipitation in the future will exert greater impacts on CO₂ and N₂O emissions in the alpine grassland. In addition, plant coverage in July was also positively correlated with CO₂ and N₂O emissions under elevated N deposition rates. The present study will deepen our understanding of N deposition impacts on GHG balance in the alpine grassland ecosystem, and help us assess the global N effects, parameterize Earth System models and inform decision makers.

The main finding has been published on Chemosphere, 2012, doi: 10.1016/j.chemosphere.2012.02.077. The paper is also archived at http://www.sciencedirect.com/science/article/pii/S0045653512003050.