Methane and Nitrous Oxide Temporal and Spatial Variability in Midwestern Streams Containing High Nitrate Concentrations

Abstract:

Interest in greenhouse gases in fluvial environments, e.g. CH₄ and N₂O, is increasing in relation to atmospheric gas budgets and the relative contribution of streams to drivers of global climate change. Typically these gases are examined individually in environments in which each is expected to be dominant; however their co-occurrence and potential interactions may be important. Spatial and temporal variability of CH₄ and N₂O concentrations were measured in 2 nitrate-rich (40-1200 µM) streams draining >90% agricultural land use in the Midwestern USA and that differed ~12-fold in flow. Long-term (biweekly), short-term (hourly), and transport-oriented (Lagrangian) sampling approaches were compared. Dissolved gas concentrations exceeded atmospheric equilibrium values up to 700x and 16x, for CH₄ and N₂O, respectively. Mean concentrations were higher in the larger stream than in the smaller stream. In both streams, CH₄ was negatively correlated with flow and nitrate while N₂O was positively correlated. N₂O was generally constant with transport (21 km) in the small stream, with variation in localized reaches, and increased somewhat in May/June in the larger stream (38 km), but not during September base flow. Base flow transport trends for CH₄ were similar to N₂O in both streams. In the small stream, substantial diel fluctuations were evident in CH₄ concentrations and N₂O δ¹⁸O values, with more subtle fluctuations in CH₄ isotopes (δ²H & δ¹³C), N₂O concentrations, and N₂O δ¹⁵N values. Seasonal mean total (CH₄ + N₂O) areal 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 was about 2x that of the smaller stream (15.4 vs 8.3 kg CO₂ km^-1 day^-1, respectively). The CH₄ contribution to this flux was 12-30%, despite the relatively high nitrate and oxygen concentrations in the streams, indicating contributions from groundwater or subsurface sediment reactions.