Biophysical controls on net ecosystem CO<sub>2</sub> exchange over a semiarid shrubland in northwest China

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.

Soil water regulates the control of photosynthesis on diel hysteresis between soil respiration and temperature in a desert shrubland

“…The station is located at the southern edge of the Mu Us desert in the transition between the arid and semi-arid climatic zones. Based on 51 years of data ) from the meteorological station at Yanchi, the mean annual air temperature at the station was 8.1 • C and the mean annual total precipitation (PPT, mm) was 292 mm (ranging between 250 and 350 mm), 63 % of which fell in late summer (i.e., July-September; Wang et al, 2014;Jia et al, 2014). Annual potential evaporation was on average 5.5 kg m −2 d −1 (Gong et al, 2016).…”

Modelling the diurnal and seasonal dynamics of soil CO<sub>2</sub> exchange in a semiarid ecosystem with high plant–interspace heterogeneity

“…1a), Ningxia, China (see Wang et al, 2014Wang et al, , 2015. The long-term (1954The long-term ( -2004 mean temperature is 8.1 • C, and the mean annual precipitation is 287 mm, most of which falls from July to September (Jia et al, 2014). The radiation and evaporation demands are high in this area.…”

“…Submodels (iv)-(v) have been developed and described in detail in our previous work , which focused on (i) introducing the plant-interspace heterogeneity into water-energy modelling and (ii) investigating the influences of such heterogeneity on the ecosystem water-energy budgets for a dryland ecosystem. Gong et al (2016) also validated the model in regard to the diurnal to seasonal dynamics of radiation balance, surface energy balance, soil temperature and moisture content in the footprint area of a eddycovariance (EC) site (for details of measurement, see Jia et al, 2014). In this work, we therefore focused on the development of submodels (i)-(iii) and the model validation using F S data measured by multiple automatic respiration chambers from crust-covered and non-crusted soils.…”

“…Water contents measured at 70 cm depth was used as the lower boundary conditions for hydrological simulations (Jia et al, 2014). The calculation of soil temperature extended to 170 cm depth with the no-flow boundary, in regard to the probably strong heat exchange at the lower boundary of rooting zone .…”

“…Arid and semiarid (dryland) ecosystems cover over 40 % of land surface and contribute notably to interannual variations of terrestrial C sink (Poulter et al, 2014). However, the contribution of soil CO 2 flux (F S ) from those ecosystems to the global C budget is less studied (CastilloMonroy et al, 2011;Gao et al, 2012;Jia et al, 2014). The temperature dependency of biological CO 2 production (i.e.…”