Crop photosynthesis and the flux of carbon dioxide below the canopy.
The upward flux of CO2 at the soil surface was calculated from the weight increase of soda lime granules exposed inside a glass tank covering 400 cm2 soil. Over bare soil the flux varied annually with a summer maximum of about 7 g/m2/day of CO2, a winter minimum of 1 g/m2/day of CO2 and a Q10 of 3. The CO2 contribution of root respiration on a cropped soil was usually about 1-3 g/m2/day. From dry-matter determinations the amount of soil C assimilated was about 6 % of the net C uptake for rapidly growing grass in spring and about 20% for other crops during summer. The top 46 cm of soil contained 12 kg/m2 of C and lost 0.4 kg/m2 of this annually by respiration. The corresponding half-life of soil organic matter was 22 years. The variation of CO2 concentration and rate of photosynthesis with soil flux depended on wind speed and atmospheric stability. In most weathers atmospheric mixing was so vigourous that the concentrations in the canopy and in the free atmosphere were very similar (about 300 ppm), and photosynthesis was independent of soil flux. In glass houses, where mixing is less, the concentration may be much less than 300 ppm in the canopy. In chosen conditions gross photosynthesis increased by 30% when the upward flux of CO2 below the canopy increased by 10 g/m2/day.