Environmental factors contributing to variations in CO flux over a barley-rice double-cropping paddy field in the Korean Peninsula.

Understanding the CO flux over agricultural crop fields is critical because the temporal cycle is driven by both ecological environment and anthropogenic change. We analyzed the net ecosystem exchange of CO measured over a barley-rice double-cropping field using the eddy covariance method for 5 years. We conducted gap-filling based on u-threshold criteria and partitioned the net ecosystem exchange into gross primary production and respiration. The relative importance analysis of solar radiation, temperature, soil heat flux, soil water content, and vapor deficit revealed that solar radiation and temperature were the dominant contributors to net ecosystem exchange. The annual variation in the net ecosystem exchange followed a bimodal pattern driven by CO uptake by both barley and rice, displaying two negative peaks in late April and mid-August. The elongation stages of the crops exhibited the highest flux. Gross primary production and respiration were closely related to solar radiation and nighttime temperature, respectively. The relative importance of the other environmental variables was affected by the cultivation season and irrigation water. In the period of rice cultivation, respiration was approximately 3 µmol m s higher during rice drainage than during the flooded period. The accumulated net ecosystem production was estimated to be 315 gC m and 349 gC m for the barley and rice growing periods, respectively, and 649 gC m for the annual total. These values are comparable with the results of other studies on barley-rice double-cropping fields.