Variations of carbon dioxide exchange in paddy field ecosystem under water-saving irrigation in Southeast China

Based on the results of continuous flux measurements in the Taihu Lake Region of China, the carbon dioxide (CO2) exchange of paddy fields under water-saving irrigation and its influence factors were analyzed. The net ecosystem exchange of CO2 (NEE) were negative during the daytime and positive at night, and the minimum value appeared around noon. The peak CO2 uptake in the flourishing period was approximately 1.5 times the values observed in the tillering and ripening stages. Frequent alternation of wetting and drying in paddy fields with water-saving irrigation resulted in larger NEE for the typical day before soil rewatering than that for the day after soil rewatering in the early growth stage. These alterations had a minimal effect on diurnal variation of NEE in the late rice growth stage. The average NEE for the typical days after soil rewatering in July and August were 2.30 and 1.38 times that before soil rewatering, respectively. CO2 exchange in paddy field ecosystem under water-saving irrigation displayed seasonal variation with the change of rice plant growth. The total ecosystem respiration (Reco), NEE and gross primary productivity (GPP) were 1996.88, −1284.15 and 3281.03gm−2, respectively, throughout the growth stage. These results showed that the paddy field ecosystem under water-saving irrigation was a sink for atmospheric CO2. Frequent alternation of wetting and drying processes increased the ecosystem reparation of paddy fields under water-saving irrigation and produced smaller net CO2 absorption compared to that of flooding irrigation. The net CO2 absorption because of plant photosynthesis during the daytime and the net CO2 release at night measured by static transparent chamber method were larger than those measured by eddy covariance method (P<0.05). The nighttime respiration of paddy field ecosystem under water-saving irrigation increased exponentially with temperature. Additionally, a larger temperature sensitivity coefficient (Q10) was observed for paddy fields under water-saving irrigation compared to that under flooding irrigation. The photosynthetic photon flux density (PPFD) was the most important factor for regulating daytime GPP. And the light saturation point during the heading-flowering and milk stages of paddy field ecosystem with water-saving irrigation (1500μmolm−2s−1) was higher than that of fields with flooding irrigation in previous studies. A significant positive correlation was found between the leaf area index (LAI) and the daily GPP.