CANCELLED - Mechanisms on the responses of ANPP to warming with grazing on the Tibetan plateau

Background/Question/Methods

Warming and increase of nitrogen (N) deposition have caused serious ecological-environmental issues. However, the uncertainty about how aboveground net primary production (ANPP) will respond to shifts of plant functional group (PFG) composition and change of soil N availability still limits us to predict global carbon (C) sequestration how to vary with future climate change. Although interactions between species composition and nutrient availabilities control long-term change in tundra productivity, the information on which process is a dominant factor controlling for ANPP is still lacking, especially in alpine meadow ecosystems, in the future climate change. We first determined the propose that PFG (i.e., grasses, forbs and legumes) composition, not N availability, determine the response of ANPP to warming using combined a controlled warming with grazing experiment for three years and with N fertilization experiment for two years in the alpine meadow.

Results/Conclusions A free-air temperature enhancement system (FATE) using infrared heaters and grazing significantly increased soil temperatures (average 0.5-1.6^oC) from 0-40-cm depth for the 3-year. Neither warming nor grazing affected soil moisture except early in the growing seasons at 30-cm soil depth in 2006 and 2007, whereas warming in 2008 significantly decreased soil moisture at 10 cm by 15.6%. Heaters caused greater soil warming at nighttime compared with daytime, but grazing resulted in greater soil warming during daytime compared with nighttime. Warming alone significantly increased average ANPP by 40% due to increased grasses cover and decreased forbs cover, but grazing reduced the response of ANPP to warming because it hampered warming-induced shifts of PFG composition. The soil N availability had inconsistent response to warming and grazing with year. Moreover, N fertilization did not affect ANPP of annual oat pasture and native alpine meadow for two years, which suggest that soil N may be replete in the alpine meadow. The relationships between ANPP and soil temperature and soil moisture were positive and negative, respectively. These findings have important implications that, first, future warming rather than more N deposit from atmosphere may drive ANPP variation; second, increased cover of grasses may reduce carbon release to atmosphere through litter decomposition due to lower litter decomposition rate for grasses than for forbs; third, moderate grazing will benefit to the alpine ecosystem for a long-term because it hampers the shifts of species composition in the future warming conditions.