Abstract
Abstract. Relationships between methane (CH4) fluxes and environmental conditions have been extensively explored in saturated soils, while research has been less prevalent in aerated soils because of the relatively small magnitudes of CH4 fluxes that occur in dry soils. Our study builds on previous carbon cycle research at Tenderfoot Creek Experimental Forest, Montana, to identify how environmental conditions reflected by topographic metrics can be leveraged to estimate watershed scale CH4 fluxes from point scale measurements. Here, we measured soil CH4 concentrations and fluxes across a range of landscape positions (7 riparian, 25 upland), utilizing topographic and seasonal (29 May–12 September) gradients to examine the relationships between environmental variables, hydrologic dynamics, and CH4 emission and uptake. Riparian areas emitted small fluxes of CH4 throughout the study (median: 0.186 µg CH4–C m−2 h−1) and uplands increased in sink strength with dry-down of the watershed (median: −22.9 µg CH4–C m−2 h−1). Locations with volumetric water content (VWC) below 38 % were methane sinks, and uptake increased with decreasing VWC. Above 43 % VWC, net CH4 efflux occurred, and at intermediate VWC net fluxes were near zero. Riparian sites had near-neutral cumulative seasonal flux, and cumulative uptake of CH4 in the uplands was significantly related to topographic indices. These relationships were used to model the net seasonal CH4 flux of the upper Stringer Creek watershed (−1.75 kg CH4–C ha−1). This spatially distributed estimate was 111 % larger than that obtained by simply extrapolating the mean CH4 flux to the entire watershed area. Our results highlight the importance of quantifying the space–time variability of net CH4 fluxes as predicted by the frequency distribution of landscape positions when assessing watershed scale greenhouse gas balances.
Highlights
Considerable effort has been directed to the study of carbon dioxide (CO2) fluxes in a variety of diverse terrestrial ecosystems using both spatially distributed chamber measurements and eddy covariance methods (e.g., Lavigne et al, 1997; Sotta et al, 2004; Webster et al, 2008; RiverosIregui and McGlynn, 2009; Allaire et al, 2012)
We examined the spatial and temporal variability of CH4 fluxes using data collected across a range of landscape positions in the Stringer Creek watershed
Terrain analysis was performed using both 3 and 10 m digital elevation models (DEMs), and higher-resolution mapping can be beneficial in some scenarios, the 10 m flow accumulation results have been shown to be more reflective of the lateral transport of water in Tenderfoot Creek Experimental Forest (TCEF) and were used in this analysis (Jencso et al, 2009)
Summary
Considerable effort has been directed to the study of carbon dioxide (CO2) fluxes in a variety of diverse terrestrial ecosystems using both spatially distributed chamber measurements and eddy covariance methods (e.g., Lavigne et al, 1997; Sotta et al, 2004; Webster et al, 2008; RiverosIregui and McGlynn, 2009; Allaire et al, 2012). Challenges associated with measuring upland methane (CH4) fluxes (Denmead, 2008; Wolf et al, 2011) have made similar studies less prevalent, despite CH4 being a more potent greenhouse gas (GHG) than CO2. Topography can create predictable physical redistribution of resources across a landscape, suggesting that these patterns (e.g., soil moisture: Western et al, 1999; temperature: Urban et al, 2000; Emanuel et al, 2010; soil organic matter and nutrients: Creed et al, 2002; Mengistu et al, 2014) could produce observable landscape patterns in soil C fluxes (Webster et al, 2008; Riveros-Iregui and McGlynn, 2009; Pacific et al, 2011).
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