Environmental influences on light response parameters of net carbon exchange in two rotation croplands on the North...

The ecosystem light response parameters, i.e. apparent quantum yield (α), maximum rate of ecosystem gross photosynthesis (A_max), and daytime ecosystem respiration (R_d), are very important when estimating regional carbon budgets. But they are not well understood in double cropping systems. Here, continuous flux data were collected from two rotation croplands in Yucheng (YC) and in Luancheng (LC) to describe the among-year variations in α, A_max, and R_d, and to investigate variation mechanism on an annual scale. The three parameters exhibited marked fluctuations during the observation years. The annual α, A_max, and R_d ranged from 0.0022-0.0059 mg CO₂ μmol photon^-1, from 2.33-4.43 mg CO₂ m^-2 s^-1, and from 0.19-0.47 mg CO₂ m^-2 s^-1 at YC, and from 0.0016-0.0021 mg CO₂ μmol photon^-1, from 3.00-6.30 mg CO₂ m^-2 s^-1, and from 0.06-0.19 mg CO₂ m^-2 s^-1 at LC, respectively. Annual α and R_d declined significantly when vapor pressure deficit (VPD) exceeded 1.05 kPa and increased significantly when canopy conductance (g_c) exceed 6.33 mm/s at YC, but changed slightly when VPD and g_c exceeded 1.16 kPa and 7.77 mm/s at LC, respectively. The fact that the negative effects of VPD and g_c on α and R_d at LC were not as significant as they were at YC may be attributed to different climate conditions and planting species. A negative relationship (R² = 0.90 for YC and 0.89 for LC) existed between VPD and g_c. Therefore, the VPD, through its negative effect on g_c, inhibited α and R_d indirectly. Among-year A_max variation was mainly influenced by the annual mean surface soil temperature (T_s) of non-growing season of wheat significantly (R² = 0.59, P < 0.01). Therefore, in future climate change scenarios, these environmental effects need to be included in carbon cycle models so that the accuracy of the carbon budget estimation can be improved.