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

Abstract

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 CH4 contribution 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.