Crop photosynthesis and the flux of carbon dioxide below the canopy

The upward flux of carbon dioxide 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 CO2 m-2 day-1, a winter minimum of 1 g m-2 day-1, and a Q10 of 3. The contribution of root respiration, calculated from the difference between the fluxes over fallow and cropped soil, was usually about 1-3 g m-2 day-1. From dry matter determinations, the amount of soil carbon assimilated by crops was about 6% of the net carbon uptake for rapidly growing grass in spring and about 20% for other crops during the summer. The top 46 cm of the soil profile contained 12 kg carbon m-2 and lost 0.4 kg m-2 annually by respiration. The corresponding half-life of soil organic matter is 22 years. The variation of carbon dioxide concentration and rate of photosynthesis with soil flux depends on wind speed and atmospheric stability. In most weather, atmospheric mixing is so vigorous that the concentrations in the canopy and in the free atmosphere are very similar (c. 300 ppm), and photosynthesis is independent of soil flux. In glass-houses, where mixing is less, the concentration may be much less than 300 ppm. In chosen conditions gross photosynthesis increased by 30% when the upward flux of carbon dioxide below the canopy increased by 10 g m-2 day-1.