Elevated CO₂ concentrations increase carbon sinks in paddy fields: an in situ four-year field experiment

Rice paddies play a crucial role in the global carbon cycle by sequestering or releasing carbon dioxide (CO2) into the atmosphere. However, the effects of net CO2 exchange in paddy fields under elevated CO2 concentrations (e[CO2]) have rarely been assessed in previous literature. In this study, a four-year field experiment (20182021) was conducted to understand the effects of e[CO2] on the CO2 flux in paddy fields. The experimental treatments included environmental [CO2] (CK) and 200 mu mol mol(-1) above the environmental [CO2] (T). A static chamber and a laser greenhouse gas analyzer were used to measure the net CO2 uptake and ecosystem respiration. Moreover, the relationships between CO2 flux and environmental variables, plants, and soil factors were analyzed during the rice-growing season. The results showed that the T treatment did not change the diurnal and seasonal dynamics of CO2 flux in paddy fields compared with CK. However, T treatment significantly promoted net CO2 uptake, where the seasonal cumulative amount of net CO2 exchange (CAC) of the T treatment was 1.131.25 times that of CK treatment over the four-year rice growth period. Net CO2 exchange was significantly correlated with photosynthetically active radiation (PAR), relative humidity, air temperature (Ta), soil temperature, and soil water content, and path analysis indicated that net CO2 exchange variations were mainly controlled by PAR and Ta. Furthermore, e[CO2] increased CH4-C/CO2-C over the four rice-growing seasons. Overall, our findings revealed that e[CO2] had a positive impact on net CO2 exchange, enhanced the capability of paddy fields as a net carbon sink, and also promoted carbon emission to the atmosphere in the form of CH4 from paddy fields.