Variations in diurnal and seasonal net ecosystem carbon dioxide exchange in a semiarid sandy grassland ecosystem in China's Horqin Sandy Land

Abstract. Grassland ecosystems are major components of the terrestrial ecosystems in arid and semiarid regions, and play important roles in the regional carbon dioxide (CO2) balance and cycle. Sandy grasslands are sensitive to climate change, yet the magnitudes, patterns, and environmental controls of their CO2 flows are poorly understood. Here, we report the results from continuous year-round CO2 observations in 5 years from a sandy grassland in the Horqin Sandy Land. The result showed that the sandy grassland was a CO2 source at annual scale (with mean annual net ecosystem CO2 exchange (NEE) of 48.88 ± 8.10 g C m−2 yr−1 in completely year 2015, 2016 and 2018), the total precipitation was the most important factor for NEE. At the seasonal scale, the sandy grassland showed a CO2 absorption during the summer, and in the rest of the seasons, it were all expressed as CO2 release. The main environmental factor of NEE were temperature and soil water content (SWC) in spring, radiation in summer, soil heat flux (SHF) and temperature in autumn, and SWC and temperature in winter. At the diel scale, net radiation (Rn) was the most important factor of NEE in all seasons. The sandy grassland may have been a net annual CO2 source at annual scale because the study site is recovering from degradation, thus vegetation productivity is still relatively low. Therefore, the ecosystem has not yet transitioned to a CO2 sink and long-term observations will be necessary to reveal the true source or sink intensity and its response to environmental and biological factors.