Biophysical controls on net ecosystem CO₂ exchange over a semiarid shrubland in northwest China

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 (R_e) 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 R_e 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 R_e (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 R_e 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.