Biophysical controls on net ecosystem CO2 exchange over a semiarid shrubland in northwest China
Dryland (semiarid and arid) ecosystems are important to the global carbon (C) budget as they cover over 40% of the Earth’s land surface. The C cycling in desert ecosystems is particularly sensitive to climate and land-use changes, and may feed back to the climate system. Currently, the C dynamics of desert shrub ecosystems is less-well-understood compared to those of forests and grasslands. In order to predict global C cycling under changing climate, it is needed to examine the relative importance of multiple abiotic stresses (e.g., drought, heat and excessive solar radiation) and biotic factors (e.g., leaf area index) in controlling C exchange over dryland ecosystems. Extensive rehabilitation practices have been carried out in desertified areas around the world, but information on the C sequestration potential of recovering vegetation is still largely lacking. Using the eddy-covariance technique, we measured the net ecosystem CO2 exchange (NEE) over a recovering shrub ecosystem in northwest China throughout 2012. We also made year-round measurements of climatic and biotic factors. The questions asked were (1) how the magnitudes of NEE and its components vary seasonally and diurnally, (2) how biophysical factors determine CO2 fluxes at multiple timescales.
The annual budget showed a gross ecosystem productivity (GEP) of 456 ± 8 g C m-2 yr-1 and an ecosystem respiration (Re) of 379 ± 3 g C m-2 yr-1, resulting in a net C sink of -77 ± 7 g C m-2 yr-1. Both the maximum C assimilation rate (i.e., at optimum light intensity) and the quantum yield varied strongly over the growing season, being higher in summer, and lower in spring and autumn. At the half-hour scale, water stress exerted a major control over NEE, and interacted with heat stress and photoinhibition in constraining C fixation. Low soil moisture also reduced the temperature sensitivity of Re (Q10). At the synoptic scale, rain events triggered immediate pulses of C release from the ecosystem, followed by peaks of CO2 uptake 1-2 days later. Over the entire growing season, leaf area index accounted for 45 and 65% of the seasonal variation in NEE and GEP, respectively. There was a linear dependence of daily Re on GEP, with a slope of 0.34. These results highlight the role of abiotic stresses and their alleviation in regulating C cycling in the face of an increasing frequency and intensity of extreme climatic events.