Greenhouse gas (CO ₂, CH ₄, H ₂O) fluxes from drained and flooded agricultural peatlands in the Sacramento-San Joaquin Delta

The Sacramento-San Joaquin Delta in California was drained and converted to agriculture more than a century ago, and since then has experienced extreme rates of soil subsidence from peat oxidation. To reverse subsidence and capture carbon there is increasing interest in converting drained agricultural land-use types to flooded conditions. Rice agriculture is proposed as a flooded land-use type with CO ₂ sequestration potential for this region. We conducted two years of simultaneous eddy covariance measurements at a conventional drained and grazed degraded peatland and a newly converted rice paddy to evaluate the impact of drained to flooded land-use change on CO ₂, CH ₄, and evaporation fluxes.We found that the grazed degraded peatland emitted 175-299g-Cm ^-2yr ^-1 as CO ₂ and 3.3g-Cm ^-2yr ^-1 as CH ₄, while the rice paddy sequestered 84-283g-Cm ^-2yr ^-1 of CO ₂ from the atmosphere and released 2.5-6.6g-Cm ^-2yr ^-1 as CH ₄. The rice paddy evaporated 45-95% more water than the grazed degraded peatland. Annual photosynthesis was similar between sites, but flooding at the rice paddy inhibited ecosystem respiration, making it a net CO ₂ sink. The rice paddy had reduced rates of soil subsidence due to oxidation compared with the drained peatland, but did not completely reverse subsidence.