Biophysical Controls on Light Response of Net CO2 Exchange in a Winter Wheat Field in the North China Plain

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 (P-max) was 46.6 +/- 4.0 mu mol CO2 m(-2) s(-1) and initial light use efficiency (alpha) 0.059 +/- 0.006 mu mol mu mol(-1) in April-May, 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 (g(c)) and air temperature (T-a) but declined with increasing vapor pressure deficit (VPD) (P<0.001). The factors influencing Pmax were sorted as LAI, g(c), T-a and VPD. alpha was proportional to ln(LAI), g(c), T-a and VPD (P<0.001). The effects of LAI, gc and Ta on a were larger than that of VPD. When Ta>25 degrees C or VPD>1.1-1.3 kPa, NEE residual increased with the increase in Ta and VPD (P<0.001), indicating that temperature and water stress occurred. When g(c) was more than 14 mm s(-1) in March and May and 26 mm s(-1) in April, the NEE residuals decline disappeared, or even turned into an increase in g(c) (P