Sub-arctic Wetland Greenhouse Gases (CO2, CH4 & N2O) Emissions are Driven by Interactions of Environmental Controls and Herbivore Grazers

Abstract:

Climate change is increasing temperatures, altering precipitation regimes and causing earlier growing seasons, particularly at northern latitudes. Such changes in local environmental conditions have the potential to affect biogeochemical cycling including the exchange of greenhouses gases between the atmosphere and the terrestrial biosphere. In addition to the effects of these environmental controls, animals such as migratory geese also influence biogeochemical cycles through grazing, trampling and delivering nutrient-rich fecal matter. In this work we aimed to quantify how local environmental conditions and the presence of grazing interact as drivers of emissions of three key greenhouse gases, CO₂, CH₄ and N₂O, in coastal wetlands of the Yukon Kuskokwim Delta. We explored the magnitude of emissions across gradients of soil temperature and water table depth, and across vegetation types related to the presence of grazing, ranging from no vegetation through grazed and ungrazed vegetation. We also investigated emissions from grazed areas using experimental manipulations of the timing of grazing and advancement of the growing season.

We found that local environmental conditions and use by grazers exert interacting controls on emissions of CO₂, CH₄ and N₂O. Emissions of CO₂ and CH₄ were positively related to soil temperature and CH₄ emissions were inversely related to water table depth, but the relationship varied by vegetation type. Net emissions of CO₂ were greatest in ungrazed vegetation types (6.62 umols CO₂ m^-2 sec^-1; p=0.0007) whereas CH₄ emissions were greatest in the grazed vegetation (122.56 nmols CH₄ m^-2 sec^-1; p=0.037). Flux of N₂O was less than 1 nmol N₂O m^-2 sec^-1 across all landscape positions under typical grazing and temperature conditions, but emissions were stimulated to over 10 nmols m^-2 sec^-1 when grazing occurred early relative to a typical season. Our results indicate that environmental conditions and the presence of migratory herbivores are both important controls on gas fluxes. Future climate change may alter regional gas flux and biosphere-atmosphere feedbacks both via direct environmental drivers and through climate-driven changes to populations or habits of grazers that also exert important controls on biogeochemical cycling in this region.