Terrestrial versus aquatic carbon fluxes in a subtropical agricultural floodplain over an annual cycle

Abstract

Aquatic carbon exports are an understudied component of catchment carbon budgets. For drained agroecosystems, the role of this aquatic pathway in offsetting the terrestrial carbon sink is unknown. Here, we present findings on the complete annual carbon budget of a subtropical agricultural floodplain in Australia. We quantified net ecosystem exchange (NEE) using eddy covariance, and aquatic carbon fluxes from drainage canals over an annual cycle, including atmospheric exchange of aquatic CO2 and CH4, as well as lateral exports of dissolved organic, inorganic and particulate carbon. The floodplain was a large atmospheric CO2 sink, with an annual NEE of −900 g C m−2 yr−1 driven by the sugarcane growing season. Aquatic carbon fluxes were estimated at 24, 16, and 0.05 g C m−2 yr−1 for lateral export, CO2 and CH4 evasion, respectively. Between 70% and 91% of aquatic carbon was lost during flood events which occurred only 12% of the time. From these measurements and estimates of other carbon inputs and outputs from farm operations, the net ecosystem carbon budget was close to neutral at −100 (error range −289 to 215) g C m−2 yr−1. Compared to other drained wetlands, the aquatic carbon flux was a minor component of the carbon budget.