The characteristics of CO, diffusion within canopies of rice and soybean were investigated by the heat balance method. There were differences between CO, profiles within rice and soybean stands. The minimum CO, concentration within the rice stand appeared in the lower layer but that within the soybean appeared in the upper layer of the canopy. And in spite of the rice stand having larger leaf area index than soybean, CO_2 source activity in lower layer of rice canopy was smaller than that of snybean stand. These differences might be resulted because light extinction coefficient of rice stand was so small that rice leaves could recieve more light energy than those of soybean and then leaf photosynthesis at the lower layer of the rice was higher than soybean stand. Canopy photosynthetic rates at noon were 8.8g CO_2, /m^2h in rice and 6.8g CO_2 /m^2h in soybean stands.
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