Nitrate loading and CH4 and N2O Flux from headwater streams

Abstract:

Freshwater ecosystems transport and process significant amounts of terrestrial carbon and can be considerable sources of CO₂, CH₄, and N₂O. A great deal of uncertainty, however, remains in both global estimates and our understanding of drivers of freshwater greenhouse gas emissions. Furthermore, small headwater streams have received insufficient attention to date and may contribute disproportionately to global GHG flux. Our objective was to quantify GHG flux and assess the impact of changes in DOC and NO₃ concentrations in surface and subsurface water on flux rates in three streams in the Lamprey River watershed in New Hampshire, USA, that contrast in surface water DOC:NO₃. We measured DOC, NO₃ and dissolved gas concentrations in surface waters of each stream monthly from May 2011 to April 2012. Empirical measurements of reaeration coefficients were used to convert dissolved gas concentrations to fluxes. We found higher GHG concentrations and fluxes in the two streams with high DOC concentrations, particularly gases produced by anaerobic metabolism (CH₄, N₂O from methanogenesis and denitrification, respectively). The stream with high DOC and high NO₃ showed high N₂O and low CH₄ flux, while the high DOC, low NO₃ stream showed high CH₄ and low N₂O flux. Our results are consistent with a model in which C inputs drive total GHG production, while NO₃ input regulates the relative importance of CH₄ and N₂O by suppressing methanogenesis and stimulating denitrification. The magnitude of GHG fluxes suggests that streams in this region are likely to be small sources of CO₂, but potentially important sources of CH₄ and N₂O. Since CH₄ and N₂O are many times more powerful than CO₂ at trapping heat in the atmosphere, freshwater emissions of these gases have the potential to offset a significant proportion of the climate benefits of the terrestrial carbon sink, a possibility that has not been sufficiently incorporated into climate models.