Climate change during the last several decades is having major impacts on northern high latitude systems including Arctic and boreal zones. Permafrost soils (frozen continuously for at least 2 years) are warming and thawing in many locations, especially near the southern limit of permafrost distribution. These soils store huge amounts of carbon (C) which become vulnerable to decomposition upon thaw, and additional net emissions of carbon dioxide (CO2) and methane (CH4) from these soils have the potential to further exacerbate climate change. In addition, changes in dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), and nutrient exports from soils to rivers can occur as a result of newly thawed sources and, importantly, changes in hydrologic flowpaths.
CWEST Participants: Josh Koch, Diane McKnight, Rob Runkel, Paul Schuster, Rob Striegl, Brett Poulin, Kim Wickland
Collaborators: Stephanie Ewing (Montana State University), Colin Kikuchi (University of Arizona), Jason Neff (University of Colorado)
Collaborative Research: Catchment hydrology on boreal hillslopes underlain by continuous permafrostBoreal soils in permafrost regions contain vast quantities of frozen organic material that is released to terrestrial and aquatic environments via subsurface flowpaths as permafrost thaws. Longer flowpaths may enhance chemical reduction of solutes, nutrients, and contaminants, with implications for greenhouse gas emissions and aqueous export. Predicting boreal catchment runoff is complicated by soil heterogeneities related to variability in active layer thickness, soil type, fire history, and preferential flow potential. This work aimed to answer two fundamental questions:
1) How does water move from soils to streams in catchments with continuous permafrost?
We answered this question using soil infiltration simulations and stream tracer tests to quantify the location, magnitude, and chemistry of shallow subsurface inflows reaching the Richardson Tributary in the Yukon River Basin. Our results indicate that water drains the landscape evenly in the early summer when soils are mostly frozen. Later in the summer, soils thaw and water can infiltrate deeper into mineral soils. We found that this deeper flow occurred through preferential pathways that include soil pipes and are focused near steep slopes and surface streams. The magnitude of these inflows can be several times larger than the observed surface flows. Uranium isotopes indicated that this late-summer runoff moves along the frozen boundary, eroding permafrost and transporting ancient material to the stream. This material may be an important source of highly labile organic matter that can be utilized by the stream ecosystem. These initial results were the impetus for ongoing studies by the USGS in interior Alaska.
2) How quickly are the major biogeochemical elements carbon and nitrogen utilized in the soils and streams of boreal catchments?
We answered this question using a combination of soil porewater and streamwater sampling. Seven stream tracer experiments were conducted over two years in the Richardson Tributary to capture seasonal and hydrologic variability that might influence transport and biogeochemical cycling rates. Because these boreal catchments contain thick organic soils, runoff from the landscape acted as a natural tracer, fertilizing the stream and ensuring that measured rates were representative of natural conditions (rather than rates elevated by adding a nutrient tracer). We found that dissolved organic carbon and nitrate were reactive in the soils and that reactions in the stream were often limited by high stream velocities. However, C and N processing occurred in the stream throughout the summer, with the highest rates in the spring when fresh, labile material runs off of frozen soils. Large storm events later in the summer also moved labile material and resulted in a pulse of stream biogeochemical activity.
Koch, J. C., Ewing, S. A., Striegl, R., & McKnight, D. M. (2013). Rapid runoff via shallow throughflow and deeper preferential flow in a boreal catchment underlain by frozen silt (Alaska, USA). Hydrogeology Journal, 21(1), 93-106. DOI: 10.1007/s10040-012-0934-3
Koch, J. C., Runkel, R. L., Striegl, R., & McKnight, D. M. (2013). Hydrologic controls on the transport and cycling of carbon and nitrogen in a boreal catchment underlain by continuous permafrost. Journal of Geophysical Research: Biogeosciences, 118(2), 698-712. DOI: 10.1002/jgrg.20058
Collaborative research: Spatial controls on CO2 emissions from permafrost soils
University of Colorado graduate student and former USGS scientist Kathy Kelsey uses a PVC chamber to measure efflux of CO2 from the soil surface of a black spruce forest in interior Alaska. CO2 is naturally produced in soils by the respiration of soil organisms, and this process is an important component of the global carbon cycle.
This research from the Nome Creek Watershed in the Bureau of Land Management White Mountains Recreation Area highlights how variable soil conditions, including soil temperature, moisture, and small- and large-scale topography can drive large variability in soil CO2 efflux from permafrost-impacted catchments.
Kelsey, K. C., Wickland, K. P., Striegl, R. G., & Neff, J. C. (2012). Variation in soil carbon dioxide efflux at two spatial scales in a topographically complex boreal forest. Arctic, Antarctic, and Alpine Research, 44(4), 457-468. DOI: 10.1657/1938-4246-44.4.457
Collaborative research: Biodegradability of dissolved organic carbon from ancient permafrost soils
USGS researcher and former Environmental Studies and INSTAAR student Travis Drake collects permafrost soils from the Fox Permafrost tunnel outside of Fairbanks, Alaska. For his master’s thesis, Travis examined the biodegradability and mineralization of carbon leached from Pleistocene age permafrost soils in a simulated aquatic setting. He found that the ancient DOC leached from the permafrost soils is comprised largely of biolabile low-molecular weight (LMW) organic acids. These juicy organic acids fuel microbial metabolism and are readily converted to carbon dioxide, which likely escapes to the atmosphere along the aquatic pathway.