Abstract
Abstract. Grasslands are major terrestrial ecosystems in arid and semiarid regions, and they play important roles in the regional carbon dioxide (CO2) balance and cycles. Sandy grasslands are sensitive to climate change, yet the magnitudes, patterns, and environmental controls of their CO2 flows are poorly understood for some regions (e.g., China's Horqin Sandy Land). Here, we report the results from continuous year-round CO2 flux measurements for 5 years from a sandy grassland in China's Horqin Sandy Land. The grassland was a net CO2 source at an annual scale with a mean annual net ecosystem CO2 exchange (NEE) of 49 ± 8 gCm-2yr-1 for the years for which a complete dataset was available (2015, 2016, and 2018). Annual precipitation had the strongest effect on annual NEE; grassland carbon sequestration increased with the increasing precipitation since NEE depended on annual precipitation. In the spring, NEE decreased (i.e., C sequestration increased) with increasing magnitude of effective precipitation pulses, total monthly precipitation, and soil temperature (Tsoil). In the summer, NEE was dominated by the total seasonal precipitation and high precipitation pulses (> 20 mm). In the autumn, NEE increased (i.e., C sequestration decreased) with increasing effective precipitation pulses, Tsoil, and near-surface soil water content (SWC) but decreased with increased SWC deeper in the soil. In the winter, NEE decreased with increasing Tsoil and SWC. The sandy grassland was a net annual CO2 source because drought decreased carbon sequestration by the annual plants. Long-term observations will be necessary to reveal the true source or sink intensity and its response to environmental and biological factors.
Highlights
Arid and semiarid ecosystems cover 30 % to 40 % of the global terrestrial surface (Poulter et al, 2014)
Our results suggest that the sandy grassland was a net CO2 source at an annual scale with an annual mean net ecosystem CO2 exchange (NEE), Gross primary productivity (GPP), and Rec of 49 ± 8, 303 ± 29, and 352 ± 21 g C m−2 yr−1, respectively, in the years for which a complete dataset was available (2015, 2016, and 2018) (Fig. 3f)
We found that GPP and Rec increased significantly with increasing annual precipitation, whereas NEE decreased significantly with increasing annual precipitation, indicating that the ecosystem’s carbon sequestration capacity increased with increasing precipitation
Summary
Arid and semiarid ecosystems cover 30 % to 40 % of the global terrestrial surface (Poulter et al, 2014). The extent and distribution of these areas are increasing in response to factors such as climate change, changes in wildfire frequency and intensity, and changes in land use (Asner et al, 2003; Hastings et al, 2010) These ecosystems are important because they account for 30 % to 35 % of terrestrial net primary productivity (Gao et al, 2012; Liu et al, 2016a) and approximately 15 % of the global soil organic carbon pool (Lal, 2004; Liu et al, 2016a). We designed the present study to reveal how changes in water availability (e.g., total precipitation, pulse size) affect carbon fluxes in the sandy grassland ecosystems of the Horqin Sandy Land
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