Measurement of CO, concentratlon in air at 25 em below and 100 em above the canopy of a good rice crop Indicated that a severe COa deficit occurred around the photosynthetic surface of crop when light intensity was high. Soil C0 1 flux as measured by the soda lime method in a closed system ranged from 3.9 to 5.7 g•m-~. day-t under flooded conditions and from 6.0 to 8,6g•m-2 •day-t under drained condi• tions. Cropped soil released more C02 than bare soil under both flooded and drained conditions. The estimated contribution of soil C02 to gross photosynthesis was 6~, for the flooded soil and 7% for the drained soil or a contribution of 9 and 12% to net dry matter production. These results together with other information indicate that atmospheric co~ is the most important source of co, in crop photosynthesis, soil co~ released into atmosphere Is second most important, and soil C011 absorbed by plant roots is almost negligible.
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