Methane and nitrous oxide temporal and spatial variability in two midwestern USA streams containing high nitrate concentrations

Concentrations and emissions of greenhouse gases CO₂, CH₄, and N₂O commonly are examined individually in aquatic environments in which each is expected to be relatively important; however, their co-occurrence and dynamic interactions in fluvial settings could provide important information about their controlling biogeochemical processes and potential contributions to global climate change. Spatial and temporal variability of CH₄, N₂O, and CO₂ concentrations were measured from June 1999 to September 2003 in two nitrate-rich (40–1200 μM) streams draining agricultural land in the midwestern USA that differed ~13-fold in flow. Seasonal (biweekly), diel (hourly), and transport-oriented (reach-scale) sampling approaches were compared. Dissolved gas concentrations exceeded atmospheric equilibrium values up to 700- and 16-fold, for CH₄ and N₂O, respectively. Mean concentrations were higher in the larger stream than in the smaller stream. In both streams, CH₄ emissions were generally higher in summer-fall and negatively correlated with flow and NO₃⁻ concentration while N₂O emissions were generally higher in winter/spring and positively correlated with flow and NO₃⁻. In the small stream, diel variations in the concentrations, emissions, and isotopic compositions of CH₄, N₂O, and NO₂⁻ resulted from diel variations in sources, sinks, and air-water gas exchange velocities. Seasonal mean total (CH₄ + N₂O) area-normalized emission rates, expressed as CO₂ warming potential equivalents, were similar for the two streams, but the total reach-scale emission rate for the larger stream, including CO₂, was about 2.9 times that of the smaller stream (131.6 vs 46.0 kg CO₂ equivalents km⁻¹ day⁻¹, respectively). The CH₄contribution to this flux was 9–28%, despite the relatively high NO₃⁻and O₂ concentrations in the streams, indicating contributions from upwelling groundwater or reactions in streambed sediment.