Biophysical controls on light response of net CO<inf>2</inf>exchange in a winter wheat field in the North China Plain

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Journal Article
PLoS ONE, 2014, 9 (2)
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To investigate the impacts of biophysical factors on light response of net ecosystem exchange (NEE), CO2flux was measured using the eddy covariance technique in a winter wheat field in the North China Plain from 2003 to 2006. A rectangular hyperbolic function was used to describe NEE light response. Maximum photosynthetic capacity (Pmax) was 46.6±4.0 μmol CO2m-2s-1and initial light use efficiency (α) 0.059±0.006 μmol μmol-1in April-May, two or three times as high as those in March. Stepwise multiple linear regressions showed that Pmaxincreased with the increase in leaf area index (LAI), canopy conductance (gc) and air temperature (Ta) but declined with increasing vapor pressure deficit (VPD) (P<0.001). The factors influencing Pmaxwere sorted as LAI, gc, Taand VPD. α was proportional to ln(LAI), gc, Taand VPD (P<0.001). The effects of LAI, gcand Taon α were larger than that of VPD. When Ta>25°C or VPD>1.1-1.3 kPa, NEE residual increased with the increase in Taand VPD (P<0.001), indicating that temperature and water stress occurred. When gcwas more than 14 mm s-1in March and May and 26 mm s-1in April, the NEE residuals decline disappeared, or even turned into an increase in gc(P<0.01), implying shifts from stomatal limitation to non-stomatal limitation on NEE. Although the differences between sunny and cloudy sky conditions were unremarkable for light response parameters, simulated net CO2uptake under the same radiation intensity averaged 18% higher in cloudy days than in sunny days during the year 2003-2006. It is necessary to include these effects in relevant carbon cycle models to improve our estimation of carbon balance at regional and global scales. © 2014 Tong et al.
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