Abstract
The ecosystem light response parameters, i.e. apparent quantum yield (α), maximum rate of ecosystem gross photosynthesis (Amax), and daytime ecosystem respiration (Rd), 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, Amax, and Rd, and to investigate variation mechanism on an annual scale. The three parameters exhibited marked fluctuations during the observation years. The annual α, Amax, and Rd 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 Rd declined significantly when vapor pressure deficit (VPD) exceeded 1.05 kPa and increased significantly when canopy conductance (gc) exceed 6.33 mm/s at YC, but changed slightly when VPD and gc exceeded 1.16 kPa and 7.77 mm/s at LC, respectively. The fact that the negative effects of VPD and gc on α and Rd 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 gc. Therefore, the VPD, through its negative effect on gc, inhibited α and Rd indirectly. Among-year Amax variation was mainly influenced by the annual mean surface soil temperature (Ts) 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.
Highlights
A large number of studies began to focus on the carbon (CO2) exchange flux at ecosystem level with the eddy covariance (EC) technique[1,2,3,4]
The ecosystem α, Amax, and Rd could be affected by vapor pressure deficit (VPD)[3,17,18,19], soil water content (SWC)[1,20,21], temperature[4,22], and biotic factors, such as leaf area index (LAI) and canopy conductance[15]
The seasonal SWC variation often produced two peaks in one year, which occurred during the two growing seasons, respectively
Summary
A large number of studies began to focus on the carbon (CO2) exchange flux at ecosystem level with the eddy covariance (EC) technique[1,2,3,4]. The net ecosystem carbon exchange (NEE) obtained from the eddy towers often shows high dependence on the photosynthetic proton flux density (PPFD), which can be described well by the Michaelis-Menten rectangular hyperbola model[5] This model is extremely important because it is often utilized in gap filling strategies when there is missing flux data[6,7]. 50% of the photosynthate is back to atmosphere through canopy dark respiration (Rd)[12], so Rd is an essential part of ecosystem carbon cycling Because of their important roles in affecting the shape of NEE light response curves and attempting to examine the balance between plant photosynthesis and respiration[13], the three parameters have been extensively researched as part of the overall assessment of the global carbon budget[14]. Based on CO2 flux and micrometeorological measurements, the objectives of this study were to (1) describe the among-year variations in annual ecosystem α, Amax, and Rd by using continuous eddy covariance data for Yucheng (YC) from 2003 to 2012 and Luancheng (LC) from 2008 to 2012; and (2) analyze how environmental and biotic factors affect the annual ecosystem α, Amax, and Rd
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