Soil microbes’ contribution to the carbon cycle in a warming world
AMHERST, Mass. – Microbiologist Kristen DeAngelis and her graduate student Grace Pold at the University of Massachusetts Amherst, with colleagues at Woods Hole Marine Biological Laboratory (MBL) and in New Hampshire, report results in the Oct. 6 issue of Science from their study of warming-related soil carbon cycling changes in a New England hardwood forest.
Over two and a half decades, the team observed periods of substantial soil carbon loss, punctuated by periods of large changes in microbial communities – an episodic rather than steady pattern of warming-related change. DeAngelis and colleagues write, "We found that soil warming results in a four-phase pattern of soil organic matter decay and carbon dioxide flows to the atmosphere, with phases of substantial soil carbon loss alternating with phases of no detectable loss."
The contribution of soils and terrestrial carbon cycle feedback to the climate system are part of a large and poorly understood component of global warming, the authors point out. Soils are the largest repository of organic carbon in the terrestrial biosphere and represent an important source of the greenhouse gas to the atmosphere. This soil carbon makes soil healthy by holding water and helping plants grow, the microbial ecologist says. Microbes control the depletion of soil carbon.
This research takes advantage of a 26-year, ongoing soil warming experiment at Harvard Forest, a research station in north central Massachusetts. This is the longest running soil warming experiment in the world, DeAngelis says, and allows researchers to peer into the future of climate change.
In their recent experiments, DeAngelis notes, "We found that from the microbial perspective, not only do the different species that are present change, but also the enzymes they produce are changing. They are also probably evolving to adapt to new, poorer and less abundant carbon in the soil. This microbial evolution aspect of long-term warming is a future question we are eager to explore."
Over the course of the 26-year experiment still ongoing, the warmed plots lost 17 percent of the carbon that had been stored in organic matter in the top 60 centimeters of soil.
She adds, "We know that microbial soil respiration is a major, and natural, source of greenhouse gases to the atmosphere. Using the long-term warming experiment as a window into future climate change, we see that warming has a profound but discontinuous effect on greenhouse gas emissions."
This work was led by first author Jerry Melillo of the Woods Hole MBL, who established the long-term soil-warming experiment at the forest experiment station in 1991. There, heating coils similar to those used to keep football and soccer fields from freezing are buried about 4 inches (10 cm) deep in treatment plots to keep the soil surface exactly 5 degrees Celsius warmer than the ambient temperature. With nearby unheated controls for comparison, the plots create an outdoor laboratory for studying artificial climate change.
"To put this in context," Melillo says, "each year, mostly from fossil fuel burning, we are releasing about 10 billion metric tons of carbon into the atmosphere. That's what's causing the increase in atmospheric carbon dioxide concentration and global warming. The world's soils contain about 3,500 billion metric tons of carbon. If a significant amount of that soil carbon is added to the atmosphere due to microbial activity in warmer soils, that will accelerate the global warming process. And once this self-reinforcing feedback begins, there is no easy way to turn it off. There is no switch to flip."
DeAngelis says these findings contribute new information to models that estimate how much the planet will warm but which, at present, do not account in an explicit way for major microbial contributions to global carbon cycling.
Collaborators of DeAngelis, Pold and Melillo include Serita Frey, Mel Knorr and Stuart Grandy at the University of New Hampshire, the New Hampshire Department of Natural Resources and the Environment, W.J. Werner and M.J. Bernard of the Marine Biological Laboratory and F.P. Bowles of Research Designs in Lyme, N.H. This work is supported by the U.S. Department of Energy and the National Science Foundation's Long-Term Ecological Research and Long-Term Research in Environmental Biology programs.