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

Abstract 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 CO 2 release remains unclear, limiting our ability to quantify the floodplain carbon balance. We measured CO 2 outgassing fluxes and their δ 13 C values along the Bermejo River, Argentina during wet and dry seasons using nonstationary CO 2 accumulation chambers on the water surface, sediment deposited on recently exposed riverbeds, overbank deposits, and paleochannels. Moist riverbed sediments had low CO 2 fluxes, suggesting rapid equilibration with atmospheric CO 2 ; overbanks and paleochannels had higher CO 2 fluxes (correlated with lower δ 13 C values), indicating that fresh OM respiration. δ 13 C CO2 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 CO 2 fluxes. Wet‐season conditions enhance recalcitrant OM respiration from paleochannels, increasing CO 2 fluxes and δ 13 C CO2 values. Total floodplain CO 2 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 ). CO 2 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.