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.