The carbon‐dioxide flux over a field of sugar beet

The difference of CO 2 concentration at two heights above a sugar beet crop was measured with a sensitive infra‐red gas analyser. The ratio of evaporation to humidity gradient and of total heat flux to wet‐bulb temperature gradient gave a transport number for calculating CO 2 flux, assumed proportional to CO 2 gradient and wind speed. In August and September 1958 there was often a constant upward flux at night (mean 0·05 mg cm −2 hr −1 ) attributed to respiration by plants and soil micro‐organisms; but during daylight, photosynthesis gave a downward flux varying with solar radiation and reaching a maximum of 0·4 mg cm −2 hr −1 . From 31 July to 11 September the net downward flux of atmospheric CO 2 was 158 ± 29 mg cm −2 compared with an uptake of 217 ± 38 mg cm −2 estimated from dry matter increase of sampled plants. During October there was a net upward flux of 4·9 mg cm −2 day −1 attributed to a striking increase in amount of CO 2 produced in the soil.