Variations of net ecosystem exchange of CO2 in Arkansas rice fields

Abstract:

Arkansas is the top producer of rice in the United States, producing 471,000 tons of rice in 2019; however, CO₂ flux measurements are still lacking in this area, preventing a holistic understanding of rice’s carbon dynamics and climate-management potential. This study reports the CO₂ fluxes from 8 fields and 12 field-seasons by the eddy covariance method in Arkansas to enhance understanding of these flux dynamics in rice cropping systems. Measurements were collected from 6 flux sites in northeast Arkansas and 2 sites in east central Arkansas during the growing seasons (April–September) of 2015 to 2018. Time-series of CO₂ fluxes were measured as net ecosystem exchange (NEE) and were gap-filled using statistical and process-based models to create estimates of growing season CO₂ fluxes. Different types of conservation-oriented irrigation management (i.e., alternate wetting and drying irrigation, AWD; furrow irrigated rice, FIR; and multiple-inlet rice irrigation, MIRI) were applied to the study sites to assess the rice management strategies for their potential influence on the regional carbon balance. In the 2015 growing season in a pair of adjacent, production-sized rice fields, NEE was ‑6578.9 kg C ha^-1 season^-1 under AWD compared to ‑6411.1 kg C ha^-1 season^-1 under conventional irrigation practice. AWD irrigation in two adjacent sites resulted in an average NEE of ‑7319.8 kg C ha^-1 season^-1 (over years 2017 and 2018) which was 12% and 10% greater in magnitude in contrast to the other adjacent sites with MIRI and FIR treatments (also in 2017 and 2018), respectively. The peak net CO₂ uptake and release were ‑72.7 µmol m^-2 s^-1 and 24 µmol m^-2 s^-1, respectively.