CO2 Fluxes Driven by Floodplain Morphology and Seasonality at the Rio Bermejo, Argentina

Natural processes drive carbon storage and release in landscapes. On river floodplains, sediment aggradation and organic matter (OM) accumulation can sequester carbon over millennial timescales, suggesting floodplains may be carbon sinks. However, how floodplain morphology and seasonality influence CO2 release remains unclear, limiting our ability to quantify the floodplain carbon balance. We measured CO2 outgassing fluxes and their δ13C values along the Bermejo River, Argentina during wet and dry seasons using nonstationary CO2 accumulation chambers on the water surface, sediment deposited on recently exposed riverbeds, overbank deposits, and paleochannels. Moist riverbed sediments had low CO2 fluxes, suggesting rapid equilibration with atmospheric CO2; overbanks and paleochannels had higher CO2 fluxes (correlated with lower δ13C values), indicating that fresh OM respiration. δ13CCO2 values correlate with distance to the active channel and floodplain sediment depositional age. The supply of fresh OM close to the active channel leads to respiration of labile isotopically light OM. When the floodplain loses connectivity with the active channel, seasonality becomes a dominant control on CO2 fluxes. Wet‐season conditions enhance recalcitrant OM respiration from paleochannels, increasing CO2 fluxes and δ13CCO2 values. Total floodplain CO2 fluxes average 447 ± 138 tC km−2 yr−1, dominated by wet‐season respiration in paleochannels and overbanks (398 ± 155 and 465 ± 256 tC km−2 per season). These fluxes greatly exceed heterotrophic respiration rates of OM in the active river channel (47 ± 108 tC km−2 yr−1). CO2 emissions from sediment exceed the annual carbon storage within the river system, suggesting that the floodplain releases more carbon than transported through the channel annually.