Studies of Energy and Gas Exchange within Crop Canopies (2)

CO₂ flux within and above a corn plant canopy

Abstract

In order to determine the photosynthetic fixation of carbon dioxide by corn plants the carbon dioxide fluxes within and above the crop were calculated from CO₂-profile data and integral exchange coefficients obatined by heat balance analysis. An infrared gas analyzer was used to measure the CO₂-concentration at nine heights within and above the crop. All the measurements were made over 10 minute periods throughout the daylight hours (0600-1800). The observations were carried out seven times at the interval of about 2-weeks during summer of 1966.
The calculated flux of carbon dioxide of a ten minute mean basis is presented in Fig. 1. An important fact is that the time course of the downward CO₂-flux (P_H) above the crop is in fairly good agreement in the phase with that of imcoming short-wave radiation on each day except on July 14. The CO₂-flux above the dense crop reached about 30×10^-8g CO₂/cm² sec at the noon on a clear day, agreeing well with results reported by LEMON (1963). The total fixations for the hours of daylight were approximately between 4.98 and 8.90mg CO₂/cm² period.
The CO₂-flux (P_s) from the soil beneath the crop was calculated using the carbon dioxide data and integral exchange coefficients in Eqn. (1) and found to be about 10-20% of the total fixation of CO₂ by the crop over the hours of daylight (see Table 2). This indicates that the soil beneath the crop is behaving as an important source of carbon dioxide. The values of CO₂-flux were plotted against the temperature of the soil surface in Fig. 5. The points show a power relationship (P_s=P_so⋅Q^T/10).
It is evident from Fig. 2 that while the points are relatively scatter, the fixation of CO₂ and the radiation intensity relationship obtaind by the bulk aerodynamic method is in accordance with that obtained from Eqn. (7). Photosynthetic efficiencies were calculated and found to range between 3 and 7% with the mean value of 5.1% (see Table 2). Transpiration coefficients (∑E_T/α∑P_T) during the hours of daylight were between 40 and 90. The values presented here were considerably low comparing with those obtained from the measurements of the drymatter and the transpiration during a longer period. Elucidating the main cause for the discrepancy between them is an important problem in production ecology for which further investigations are needed.
The height distributions of the CO₂-flux and of the photosynthetic fixation of the carbon dioxide at several heights within the crop were presented in Figs. 7 and 8. Fig. 7 demonstrates the importance of the upper leaves in the CO₂-fixation, particularly in the crop with dense canopy. The 0.6-1.0 zone of z/H accounted for most of the CO₂-fixation between 50 and 70% of the total fixation during the hours of daylight. The midday profiles of the CO₂-fixation intensity of the leaves were characterized by the establishment of a layer with the high intensity in the center of the crop. The relatively low intensity in the CO₂-fixation of the upper leaves at the midday seems to be caused by the imbalance of water in the leaves exposed to high intensity radiation.
In order to characterize the efficiency in water use of the leaves, the height distribution of the layer's transpiration coefficient (ΔE_T/αΔP) was calculated and given in Fig. 9. The values of the transpiration coefficient decreased with the depth from the top to the middle level. Below the middle level the coefficients were found to go up again with the depth. The height distribution of the coefficient is thus concave to z-axis. This feature indicates that the water use of the upper and lower leav