Carbon storage and greenhouse gases emission from a fluvial reservoir in an agricultural landscape

The significance of organic carbon (C) burial in fluvial reservoirs on the global C cycle and atmosphere composition remains debatable. While the entrapment of eroded C in these systems could represent an important C sequestration mechanism, this must be weighed against the emission of greenhouse gases (GHG) that may evolve in anoxic reservoir sediment. Over a 4-year period (2005–2008), dissolved concentrations and fluxes of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) were monitored at a Central Indiana fluvial reservoir (built in 1967) fed by streams draining a predominantly agricultural watershed. Analysis of sediment cores revealed an annual sedimentation rate of 1.6 cm and C burial rate of 2.3–2.9 Mg C ha− 1. Reservoir waters were supersaturated with CO2 (mean: 2.55 mg CO2 L− 1) and CH4 (9.44 μg CO2 L− 1). In the sediment, concentrations were 4–250 times higher reaching values up to 14,533 μg CH4 L− 1. The δ 13C profile of CH4 (− 57.7‰ at the bottom, − 48.26‰ near the surface) indicated active CH4 oxidation during diffusive transport through the water column. Instances of N2O under-saturation were occasionally noted in mid-summers when NO3− level was < 1 mg N L− 1. Over the study period, GHG emission averaged 2.01 g CO2 m− 2 d− 1, 10.49 mg CH4 m− 2 d− 1 and 2.01 mg N2O m− 2 d− 1. These means were heavily influenced by the high CH4 and N2O fluxes (6 times the average) recorded during a mixing event that was triggered by a large discharge event that followed an extended dry period (discharge: 67% below normal). Based on their global warming potential, diffusive GHG fluxes averaged 2.82 g CO2 equivalents m− 2 d− 1 (range: 2.4–3.36) and completely offset reservoir C burial (2.61 g CO2 m− 2 d− 1). These results underscore the sensitivity of the C budget of fluvial reservoirs to major hydrologic events that, through enhancement of GHG fluxes, can easily tip the reservoir C balance from sink to source.