Beijing Forestery University, Beijing, China (People's Republic)
Abstract: Drought can impact the dynamics of ecosystem carbon sink-source by regulating the processes of gross ecosystem primary productivity (GPP) and ecosystem respiration (Reco). However, how ecosystem carbon fluxes at seasonal and interannual timescales respond to frequent drought, particularly with drought duration, remains to be clarified for arid and semi-arid areas. Using the eddy-covariance technique, we investigated temporal variations of the net exchange of CO2 (NEE), and its components of GPP and Reco, in a semiarid shrubland of northern China during 2012–2022. Our results showed that the semi-arid shrubland switched back and forth from being a carbon sink and source across 2012–2022, and was a carbon sink over a decade (–96 g C m−2). Seasonal peak of net carbon uptake ranged from –1.77 g C m-2 d-1 in 2014 to – 4.17 g C m-2 d-1 in 2012. The seasonal variations in NEE was mainly regulated by canopy structure (as reflected by NDVI), while changes of NEE over finer timescale such as half-hour were responsible by soil moisture and air dryness. The limitation from soil moisture on half-hour daytime NEE strengthened with the extension in drought duration. Annual rainfall was independent with annual GPP and Reco, while closely correlated with annual NEE. Droughts impose a greater reduction in GPP than those in Reco, thus causing decreases in the net carbon uptake. The dynamics of the ecosystem carbon sink-source mainly depended on drought duration, as drought suppresses new leaf formation in spring and causes defoliation in summer or autumn. The shrubland in this study may be more vulnerable to being a carbon source with further perturbations in carbon fluxes from more frequent occurrences of continuous droughts in future.
