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. δ¹³C_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 CO₂ fluxes. Wet-season conditions enhance recalcitrant OM respiration from paleochannels, increasing CO₂ fluxes and δ¹³C_CO2 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.