title: Water- and plant-mediated responses of ecosystem carbon fluxes to warming and nitrogen addition on the Songnen grassland in northeast China.
abstract: BACKGROUND
  Understanding how grasslands are affected by a long-term increase in temperature is crucial to predict the future impact of global climate change on terrestrial ecosystems. Additionally, it is not clear how the effects of global warming on grassland productivity are going to be altered by increased N deposition and N addition.
  
  
  RESULTS
  In-situ canopy CO(2) exchange rates were measured in a meadow steppe subjected to 4-year warming and nitrogen addition treatments. Warming treatment reduced net ecosystem CO(2) exchange (NEE) and increased ecosystem respiration (ER); but had no significant impacts on gross ecosystem productivity (GEP). N addition increased NEE, ER and GEP. However, there were no significant interactions between N addition and warming. The variation of NEE during the four experimental years was correlated with soil water content, particularly during early spring, suggesting that water availability is a primary driver of carbon fluxes in the studied semi-arid grassland.
  
  
  CONCLUSIONS
  Ecosystem carbon fluxes in grassland ecosystems are sensitive to warming and N addition. In the studied water-limited grassland, both warming and N addition influence ecosystem carbon fluxes by affecting water availability, which is the primary driver in many arid and semiarid ecosystems. It remains unknown to what extent the long-term N addition would affect the turn-over of soil organic matter and the C sink size of this grassland.
authors.affiliation: Key Laboratory for Vegetation Ecology, Ministry of Education, Institute of Grassland Science, Northeast Normal University, Changchun, Jilin Province, PR China.
authors.forename: Li
authors.initials: L
authors.lastname: Jiang
authors.forename: Rui
authors.initials: R
authors.lastname: Guo
authors.forename: Tingcheng
authors.initials: T
authors.lastname: Zhu
authors.forename: Xuedun
authors.initials: X
authors.lastname: Niu
authors.forename: Jixun
authors.initials: J
authors.lastname: Guo
authors.forename: Wei
authors.initials: W
authors.lastname: Sun
mesh_terms: D002244:Carbon / Q000737:chemistry* / Q000378:metabolism; D057486:Carbon Cycle; D002681:China; D002980:Climate; D057231:Climate Change; D017753:Ecosystem; D009584:Nitrogen / Q000737:chemistry* / Q000378:metabolism; D006109:Poaceae / Q000502:physiology*; D011891:Rain; D012987:Soil; D013696:Temperature; D014867:Water / Q000737:chemistry*
chemical_list: D012987:Soil; D014867:Water; D002244:Carbon; D009584:Nitrogen