Effect of free-air CO2 enrichment (FACE) on CO2 exchange at the flood-water surface in a rice paddy field

Diurnal and seasonal changes in CO2 exchange between flooding water and the atmosphere are reported in a rice (Oryza sativa) paddy field under ambient and free-air CO2 enrichment (FACE) conditions. Measurements of CO2 exchange were made in a randomized complete-block design experiment with two CO2 mole fractions (ambient; FACE, ambient +200 mmol mol(-1)) and four blocks. Evolution of CO2 from the flooding water occurred throughout the entire day under ambient CO2, although the rate decreased in the daytime. By contrast, under FACE, CO2 evolution from the flooding water was observed at night, and CO2 absorption from the atmosphere was observed in the daytime. The time course of CO2 exchange was similar to that of solar radiation. The diurnal pattern is similar to that for photosynthesis of terrestrial plant communities. This suggests that photosynthesis of the free-floating weeds and algae in the flooding water is related to the CO2 exchange between the flooding water and the atmosphere. in addition, the amplitude of the diurnal cycle of CO2 exchange was larger under the FACE than under ambient conditions.