Abstract
Abstract. The carbon (C) cycling in semiarid and arid areas remains largely unexplored, despite the wide distribution of drylands globally. Rehabilitation practices have been carried out in many desertified areas, but information on the C sequestration capacity 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 in order to (1) quantify NEE and its components and to (2) examine the dependence of C fluxes on biophysical factors at multiple timescales. The annual budget showed a gross ecosystem productivity (GEP) of 456 g C m−2 yr−1 (with a 90% prediction interval of 449–463 g C m−2 yr−1) and an ecosystem respiration (Re) of 379 g C m−2 yr−1 (with a 90% prediction interval of 370–389 g C m−2 yr−1), resulting in a net C sink of 77 g C m−2 yr−1 (with a 90% prediction interval of 68–87 g C m−2 yr−1). The maximum daily NEE, GEP and Re were −4.7, 6.8 and 3.3 g C m−2 day−1, respectively. Both the maximum C assimilation rate (i.e., at the optimum light intensity) and the quantum yield varied over the growing season, being higher in summer and lower in spring and autumn. At the half-hourly scale, water deficit exerted a major control over daytime NEE, and interacted with other stresses (e.g., heat and photoinhibition) in constraining C fixation by the vegetation. 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.
Highlights
Drylands cover over 40 % of the earth’s land surface and have been rapidly expanding as a result of climate change and human activities (Asner et al, 2003)
The total amount of C sequestered by the studied shrubland in 2012 (NEP = 77 g C m−2 yr−1), with an annual rainfall of at least 305 mm and a peak LAI of 1.2, was generally lower than that sequestered by forests and grasslands in humid and subhumid areas (e.g., Suyker and Verma, 2001; Zha et al, 2004; Zhou et al, 2013)
A revegetated shrub ecosystem ∼200 km west of our site dominated by Caragana korshinskii and A. ordosica had an NEP of 14–23 g C m−2 yr−1, with an annual precipitation of < 150 mm (Gao et al, 2012)
Summary
Drylands (semiarid and arid areas) cover over 40 % of the earth’s land surface and have been rapidly expanding as a result of climate change and human activities (Asner et al, 2003). Dryland ecosystems are characterized by low precipitation, soil fertility and productivity, they are important to the global carbon (C) budget as they account for approximately 20 % of total terrestrial net primary productivity (Whittaker, 1975) and 15 % of total soil organic carbon (Lal, 2004). A recent study showed a dramatic contribution of semiarid ecosystems to the interannual variability of the global carbon cycle (Poulter et al, 2014). The C cycling in desert ecosystems is sensitive to climate and land-use changes, and may feed back to the climate system (Li et al, 2005). In order to accurately predict global C cycling under changing climate, it is necessary to understand how CO2 exchange in dry areas responds to variations
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