Dynamics of CO2 Flux Over Agricultural Field

Fig. 4 shows the typical seasonal changes of ESFCO2 and plant growth during the soybean cropping and postcropping periods in 3 years. The ESFCO2 consists of photosynthesis and respiration of plant and microbial respiration. The SSFco2 represents the respiration of microorganisms alone during the bare soil periods, while it includes root respiration under vegetated conditions. During the early period with low LAI values, the ESFCO2 was positive and the patterns of diurnal change were similar to those during the postcropping period (bare soil conditions). The diurnal pattern of ESFCO2 during some selected periods is shown in Fig. 5. The diurnal trend was apparently synchronized with those of solar radiation and air temperature. On the Contrary, for the growth period with high LAI, the ESFCO2 showed negative values due to CO2 uptake by plant photosynthesis during the daytime, while it was positive during the nighttime. These measurements were comparable with those in previous reports (e.g., Curtin et al., 2000), and the trend of dynamic change seemed to reasonably represent the underlying mechanisms involved in the emission and uptake of CO2.

Figure 4: Seasonal change of net ecosystem exchange of CO2 flux (ESFco2) and crop growth variables; leaf area index (LAI) and dry biomass (DW). Examples are from soybean cropping periods in year 1995, 1996, 1997, and 1999. Vertical arrows indicate the time of seeding.

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