Aim The Tibetan Plateau accounts for about a quarter of the total land area of China and has a variety of ecosystems ranging from alpine tundra to evergreen tropics. Its soils are dominated by permafrost and are rich in organic carbon. Its climate is unique due to the influence of the Asian monsoon and its complex topography. To date, the carbon dynamics of the Tibetan Plateau have not been well quantified under changes of climate and permafrost conditions. Here we use a process-based biogeochemistry model, the Terrestrial Ecosystem Model (TEM), which was incorporated with a soil thermal model, to examine the permafrost dynamics and their effects on carbon dynamics on the plateau during the past century.
Location The Tibetan Plateau.
Methods We parameterize and verify the TEM using the existing data for soil temperature, permafrost distribution and carbon and nitrogen from the region. We then extrapolate the model and parameters to the whole plateau.
Results During the 20th century, the Tibetan Plateau changed from a small carbon source or neutral in the early part of the century to a sink later, with a large inter-annual and spatial variability due to changes of climate and permafrost conditions. Net primary production and soil respiration increased by 0.52 and 0.22 Tg C year−1, respectively, resulting in a regional carbon sink increase of 0.3 Tg C year−1. By the end of the century, the regional carbon sink reached 36 Tg C year−1 and carbon storage in vegetation and soils is 32 and 16 Pg C, respectively. On the plateau, from west to east, the net primary production, soil respiration and net ecosystem production increased, due primarily to the increase of air temperature and precipitation and lowering elevation. In contrast, the decrease of carbon fluxes from south to north was primarily controlled by precipitation gradient. Dynamics of air temperature and associated soil temperature and active layer depth resulted in a higher plant carbon uptake than soil carbon release, strengthening the regional carbon sink during the century.
Main conclusions We found that increasing soil temperature and deepening active layer depth enhanced soil respiration, increasing the net nitrogen mineralization rate. Together with the effects of warming air temperature and rising CO2 concentrations on photosynthesis, the stronger plant nitrogen uptake due to the enhanced available nitrogen stimulates plant carbon uptake, thereby strengthening the regional carbon sink as the rate of increase net primary production was faster than that of soil respiration. Further, the warming and associated soil thermal dynamics shifted the regional carbon sink from the middle of July in the early 20th century to early July by the end of the century. Our study suggests that soil thermal dynamics should be considered for future quantification of carbon dynamics in this climate-sensitive region.