Biophysical controls on light response of net co2 exchange in a winter wheat field in the North China Plain

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Journal Article
PLoS One, 2014, 9 (2), pp. 1 - 13
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To investigate the impacts of biophysical factors on light response of net ecosystem exchange (NEE), CO2 flux 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 46.664.0 mmol CO2 m22 s21 and initial light use efficiency (a) 0.05960.006 mmol mmol21 in April2May, two or three times as high as those in March. Stepwise multiple linear regressions showed that Pmax increased 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 Pmax were sorted as LAI, gc, Ta and VPD. a was proportional to ln(LAI), gc, Ta and VPD (P,0.001). The effects of LAI, gc and Ta on a were larger than that of VPD. When Ta.25uC or VPD.1.121.3 kPa, NEE residual increased with the increase in Ta and VPD (P,0.001), indicating that temperature and water stress occurred. When gc was more than 14 mm s21 in March and May and 26 mm s21 in 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 CO2 uptake under the same radiation intensity averaged 18% higher in cloudy days than in sunny days during the year 200322006. It is necessary to include these effects in relevant carbon cycle models to improve our estimation of carbon balance at regional and global scales.
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