Caribou roaming the tundra near the experimental snow fence sites. (Credit: Miquel Gonzalez-Meler)
Predicting what will happen in the future as the climate continues to change is especially difficult in sensitive areas, such as the high Arctic regions in Alaska. The area is home to permafrost soils, a small part of them subject to repeated freezing and thawing as annual seasons change.
Various factors influence the freezing and thawing of soil, however, and that affects the carbon balance of the ecosystem. The carbon balance is not simply a matter of gradually increasing temperature equals more thawing of carbon. There is also a delicate interplay of precipitation factors, growing season changes, plant growth and composition, microbial growth and soil thaw depth.
Carbon dioxide and methane uptake and production, important for evaluating effects of climate change, are also affected by seasonal deviations, precipitation and plant growth factors. Examining all these factors and predicting what effects climate change will have on tundra in the future will require some dedicated environmental monitoring and research.
Miquel Gonzalez-Meler, professor of biology at the University of Illinois at Chicago, helped perform multi-year research in the Toolik Lake area in Alaska to determine the effects of climate change there, especially those from snowfall, which is projected to increase 25 to 50 percent by the century’s end.
Gonzalez-Meler and other researchers were able to make use of a piece of equipment constructed in the area 18 years before: a snow fence 200 feet long and 9 feet high, set perpendicular to the wind so snow would pile up behind it. Snow piled highest next to the fence, with less snow depth occurring the farther away from the fence that an area of interest was located. In this way, researchers were able to mimic different levels of snow depth that might occur in future tundra ecosystems.
Areas 30 meters and 70 meters away from the fence mimicked about 50 percent snow increase and a 25 percent decrease in snow, respectively. These areas were measured for soil moisture content, oxygen saturation, concentration of carbon dioxide and methane, isotopic ratios of carbon dioxide and methane and thickness of unfrozen ground. Measurements of soil pH were also taken. Plant species growth in these areas was also characterized during peak growing season from late May to August.
“Our methods range from low-tech to high-tech,” Gonzalez-Meler explains. “Snow density and volume measurements involve getting a knife, cutting out a cubic meter of snow, melting it, and getting density and volume from that. Very low-tech. On the other end of the spectrum, we use a Picarro Cavity Ring Down Spectroscopy system (CRDS) which is very high-tech, giving us parts-per-billion sensitivity levels for methane and carbon dioxide measurements, a big improvement on some older technologies. The Picarro is our star piece of equipment for this study.”
The CRDS uses about 100 milliliters (half a cup) of air for each sample. Although it may not seem portable, it is: Gonzalez-Meler and others used it in their lab trailer, located at the Toolik Field Station established by the National Science Foundation in the 1970s. The trailer also contained scales, temperature probes and pH meters. Infrared gas measurements were also made using a LI-COR 6400 instrument.
Gonzalez-Meler and his team used Omega soil temperature probes placed at 4, 10 and 25 centimeters of depth, accurate to within 0.1 degree Celsius. They also used iButton temperature loggers, which are the size of a nickel. The iButtons are cheap and autonomous, delivering data at five- or 30-minute intervals for up to two years. But they’re not as accurate as the Omega probes, having a roughly 0.5 degree-Celsius accuracy.
Researchers collected a lot of data to try to get a handle on a very complex question: What would happen in the tundra and permafrost with continuing climate change, if winter snow precipitation increases significantly? The answer, Gonzalez-Meler says, depends on a lot of factors.
“For instance, generally the more snow you get, the more it will act as an insulator and more of a warming effect will occur in the soil. The air trapped in the snow is what gives the snow insulating properties — a blanket of snow acts like any other blanket, holding warmth in,” says Gonzalez-Meler. “It will take longer for permafrost soil underneath to reach below-freezing temperature, meaning more of a soil thaw period in the active layer.”
But when carbon dioxide and methane factors are considered, more complexity is added to the picture.
“Where is the soil carbon going to go? If there is more active layer of soil, meaning more soil that is thawed, what could happen is plant growth can be enhanced, meaning more carbon will get sucked up by plants,” says Gonzalez-Meler. “Well that can happen if you start getting more woody plant growth, but not if you have mosses only, as you would with colder and frozen soils. So the type of plant growth is an important factor.”
Another important question is whether the soil has oxygen in it or if it is anoxic. Gonzalez-Meler says lots of methane can be made in anoxic soil. With thawing permafrost, melted water will stay on top of the frozen soil, keeping oxygen from getting in, leading to higher methane produced in the soil. Plants with hollow stems, such as sedge grasses, can transport methane from the soil to the atmosphere. In that case, methane in the atmosphere will rise significantly but it won’t linger in the soil.
Archaea microbes produce methane from carbon in the soil, says Gonzalez-Meler, so long as the soil is not frozen. These archaea are important in the tundra-area ecosystem because methane generally comes from one of two processes: carbon dioxide gets used to make methane, or organic matter will be used to make it.
“So we look at the isotopic ratios of C12 to C13 to figure this out. The C13 form takes more energy for microbes to metabolize, so if we see a relatively low abundance of C13 in methane, that tells us microbes are making the methane and from what source,” says Gonzalez-Meler. “That is how we can determine what the origin of the methane we measure is.”
Snowfall also influences how much methane is produced and where it goes. Generally, less snow means methane will be going from the atmosphere into the soil where it will be consumed by microbes. In that case, soil is a methane sink.
But if there is more snowfall, the ground will be warmer, possibly resulting in more sedge and woody plant growth that can transport methane into the atmosphere, meaning a net gain of methane in the atmosphere.
Microbial type is also an important factor to consider. Gonzalez-Meler points out that some archaea produce methane whereas some aerobic bacteria consume it.
Gonzalez-Meler mentions that the research has been enjoyable and also surprising at times.
“We did not expect that the effects of the woody plants and sedges as methane pipes to the atmosphere would be as large as they were,” he recalls. “And there are no doubt other surprises we have not discovered yet.”
In terms of where the research is going next, Gonzalez-Meler expects to do genomic sequencing on the soil microbes his team has collected using the 16S gene and shotgun sequencing.
The project was funded by the U.S. Department of Energy and the National Science Foundation. Collaborators include Elena Blanc-Beates from the University of Illinois at Chicago; Neil Sturchio from University of Delaware; Jeff Welker of University of Alaska; and Jeff Chanton of Florida State University.
Top image: Caribou roaming the tundra near the experimental snow fence sites. (Credit: Miquel Gonzalez-Meler)