The Tibetan Plateau,the third pole of the earth,is one of the least human-disturbed regions in the world.Its outstanding topographic feature and ecological characteristics have significant effects on modern atmospheric circulation and climate,so the greenhouse gases emission and absorption are linked to climate changes at local,regional and even global scales.Also,to examine or estimate the actual magnitude of carbon uptake or release from this terrestrial ecosystem has become the central issue in global carbon cycle researches.However,relatively few studies have been reported in the literatures about the carbon contributions of the alpine grassland area on the Tibetan Plateau.In this paper,we use a mechanistically based ecological simulation model to describe and analyze the spatial pattern of terrestrial net primary productivity(NPP),carbon fluxes and net ecosystem productivity(NEP) in this alpine steppe area.The NASA-CASA(Carnegie-Ames-Stanford Biosphere) model is designed to simulate seasonal patterns in net carbon fixation and allocation,litterfall,and soil nutrient mineralization,and soil CO2 emissions.Our fundamental approach is to use CASA to estimate net primary production(NPP) in the alpine steppe area on the Tibetan Plateau.We combined MODIS data and CASA to estimate NPP,and soil carbon emissions were calculated by the regression formula obtained in the pre-study.Upon these,we estimated the carbon budget for each of the 0.05° latitude×0.05° longitude grid cells in the alpine steppe area.The potential annual net primary productivity(NPP) of this alpine steppe area is estimated to be 20.57×1012g·a-1 of carbon.The annual NPP has the distinct spatial distribution,whereas vegetation NPP decreased gradually from southeast to northwest due to the temperature and precipitation gradients.Soil carbon emission(heterotrophic respiration) is estimated to be 8.07×1012g·a-1.Net ecosystem productivity(NEP),an indicator for carbon budget,is the difference between annual net primary productivity(NPP) and annual soil respiration.Determination of net ecosystem productivity showed that the alpine steppe ecosystem acts as a distinct carbon sink(12.50×1012 g·a-1 of carbon),although this carbon reservoir is quite small.

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