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

Richard L. Smith; J.K. Bohlke

doi:10.1016/j.scitotenv.2019.05.374

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

Richard L. Smith·J.K. Bohlke

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Abstract

Concentrations and emissions of greenhouse gases CO 2 , CH 4 , and N 2 O 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 CH 4 , N 2 O, and CO 2 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 CH 4 and N 2 O, respectively. Mean concentrations were higher in the larger stream than in the smaller stream. In both streams, CH 4 emissions were generally higher in summer-fall and negatively correlated with flow and NO 3 − concentration while N 2 O emissions were generally higher in winter/spring and positively correlated with flow and NO 3 − . In the small stream, diel variations in the concentrations, emissions, and isotopic compositions of CH 4 , N 2 O, and NO 2 − resulted from diel variations in sources, sinks, and air-water gas exchange velocities. Seasonal mean total (CH 4 + N 2 O) area-normalized emission rates, expressed as CO 2 warming potential equivalents, were similar for the two streams, but the total reach-scale emission rate for the larger stream, including CO 2 , was about 2.9 times that of the smaller stream (131.6 vs 46.0 kg CO 2 equivalents km −1 day −1 , respectively). The CH 4 contribution to this flux was 9–28%, despite the relatively high NO 3 − and O 2 concentrations in the streams, indicating contributions from upwelling groundwater or reactions in streambed sediment.

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Source-reported bounding extent: 40.46993497635156° to 40.89067715064627° latitude; -87.8961181640625° to -87.11334228515625° longitude. This indicates report coverage, not an exact sampling location. View area on OpenStreetMap.

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Richard L. Smith, J.K. Bohlke. 2019. Methane and nitrous oxide temporal and spatial variability in two midwestern USA streams containing high nitrate concentrations. https://doi.org/10.1016/j.scitotenv.2019.05.374

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