Thermal conditions control on growing season carbon budget of alpine ecosystems in the Qinghai Lake Basin, Northeast Qinghai-Tibet Plateau

The Qinghai-Tibetan Plateau (QTP) is one of the most influential eco-geographical units in the world, and its alpine ecosystems have a profound impact on the global carbon cycle. Therefore, it is essential to accurately estimate the carbon budget in these fragile ecosystems and their response to climate change. The objective of this study was to evaluate the applicability of Biome-BGC model (BBGC) in a typical closed watershed (Qinghai Lake Basin, QLB) on the QTP; analyze the spatiotemporal characteristics of growing season Net Ecosystem Exchange (NEE); and further identify the main meteorological factors affecting the carbon budget. The results showed that: (1) From 2009 to 2018, the alpine ecosystems acted as a continued carbon sink (-64.45 g C·m−2 between May and October), and exhibited clear monthly variation. NEE became more negative from May to July (-0.79 g C·m−2·day−1), then gradually became less negative to September, and finally shifted from carbon absorption to carbon emission in October (0.29 g C·m−2·day−1). (2) The carbon sink capacity during growing season among varied ecosystems was ordered as: shrub (-0.60 g C·m−2·day−1) > temperate steppe (-0.55 g C·m−2·day−1) > alpine steppe (-0.54 g C·m−2·day−1) > alpine meadow (-0.44 g C·m−2·day−1) > alpine desert (-0.15 g C·m−2·day−1). (3) The growing season carbon budget was mainly regulated by thermal conditions (air temperature and solar radiation), with air temperature dominating at lower altitudes and solar radiation becoming predominant at higher altitudes. Overall, the alpine ecosystems in the QLB performance as a key carbon sink, and exhibit obvious altitude-depended variation characteristics.