Geomorphic Influence and Hydrologic Controls On Greenhouse Gas Fluxes At the Soil-Atmosphere Interface in Northern Forests.

Greenhouse gas (GHG) fluxes at the soil-atmosphere interface of northern forests vary greatly in time and space rendering it difficult to accurately evaluate the contribution of forest soils to total forest GHG budgets.  In order to address this issue, we investigated the influence of landscape hydrogeomorphic characteristics on GHG dynamics to better constrain estimates of soil GHG fluxes at the watershed scale. Monthly GHG sampling between July 2011 and July 2013 took place in Archer Creek Watershed, in the Adirondack Mountains, NY.   Gas fluxes at the soil-atmosphere interface were measured at 10 locations representing the range of landscape hydrogeomorphic classes most commonly found in northern forested landscapes. These monthly measurements were complemented by high temporal resolution GHG sampling over a 24 hr period on four occasions to account for the effect of photosynthesis on GHG fluxes. Gas fluxes were related to various indicators of overall watershed conditions, namely stream discharge, stream chemistry, meteorological conditions, antecedent moisture conditions, and topographic position.  Topographic position was an important explaining variable for gas fluxes. Wetland areas were a hot spot for methane emissions with an order of magnitude or higher emissions than other spatial classes in the landscape.  Upper hillslope soils were generally methane sinks and major CO­₂ producers when considered in terms of relative surface area.  Comparatively, nitrous oxide fluxes were more variable in space, though headwater wetland soils tended to have higher fluxes than other spatial classes. All GHG fluxes fluctuated throughout the diel timescale.  Winter CO₂ efflux to the atmosphere was positive but was smaller than the annual median CO₂ flux; CH₄ sinks were amplified, indicating that continuous soil moisture and temperatures above freezing allow for significant biogeochemical activity to continue under the snowpack and should not be ignored in the development of a GHG budget for northern forest soils.