Leaf Litter Stunner

Photo by David Foster / Harvard Forest Archives

A team of researchers has been studying the effects of adding extra leaf litter and other organic matter to the soil in Harvard Forest for more than 25 years. The original purpose was to test the limits of how much carbon the soil could store, because scientists believed that the soil had an infinite capacity to hold carbon. But the results so far have been completely unexpected.

The data appear to show that, as temperatures rise, the organic matter in forests breaks down more quickly, accelerating the release of carbon dioxide into the atmosphere. While the study intended to explore a promising avenue to mitigate global warming, the results seem to indicate that additional organic material in the soil may exacerbate it.

The leaf litter manipulation study, which is still going on at Harvard Forest and other sites around the world, involves doubling the leaf litter in 3x3-meter plots for 20 years. “With climate change, one of the current projections is that, as temperatures warm, we will have longer growing seasons, larger trees, a more productive forest, and more plant material going into the soil,” said Myrna Simpson of the University of Toronto. Researchers also installed root barriers to prevent belowground organic matter from making its way into the soil of the test plots.

The scientists expected to see an increase in carbon in the mineral soil layer, the place where most long-term carbon is stored. “But after 20 years we didn’t see any carbon gain at all,” said Simpson.

Using laboratory techniques that Simpson developed to examine the molecular composition of the soil, the researchers found evidence that the bacteria and fungi in the soil processed the excess organic matter and converted it to carbon dioxide. “We didn’t get any more carbon in the soil because the bacteria and fungi ate that carbon,” she said. “Rather than sequestering carbon in the soil, we saw an increase in soil respiration, which could contribute to global warming even more.”

Though the scientists admitted that their results were a bit depressing, they aren’t giving up. They will continue the study for at least another five years and process data from other sites to see if the results elsewhere are different. Simpson said that the balance between carbon storage and degradation in the soil is a function of temperature, moisture, nutrients, the type of bacteria and fungi living in the soil, and the physical characteristics of the soil, like pH and drainage.

“These characteristics can alter how the processes happen and the rate that they happen, so we’re looking at other forests, because not every forest is the same,” Simpson said. “At some point, the microbes will probably run out of usable substrates and slow down, but it didn’t happen after 20 years. We’ll keep monitoring to see if the trend continues.”

  1. Steve Steiner → in Virginia
    Aug 12, 2016

    One of the ways extra carbon gets stored is by the roots of plants cycling under the increased carbon so, by blocking out root infiltration, they removed a huge part of the natural process. I don’t understand the point of removing the largest contributor to carbon sequestration and then being surprised when it doesn’t happen.

  2. Michael Connor → in Vermont
    Aug 12, 2016

    The experiment seems odd. Forest soils are crowded with roots; this seems to be nothing more than a pile of leaves on some soil. It seems to me that 20 years of roots growing and dying would be an important variable in determining the % of organic matter that would accumulate in the soil.

  3. Glyn Mitchell → in Jersey Channel Islands
    Aug 12, 2016

    Fungi and other microorganisms have high C:N values, were qualitative microbial assessments taken before and after mulch addition?

    Thanks Glyn

  4. Pieter Leenhouts → in Kars, ON, Canada
    Aug 13, 2016

    Our petroleum and Coal are created by sequested carbon through peat bogs and other anarobic sites and processes one would think. We should be studying those areas for effective carbon capture.

  5. Peggy Lynch → in Cambridge MA
    Aug 14, 2016

    That is very, very interesting…. And important to know. Is work being done comparing the CO2 release from bare soil vs leaf littered soil vs plant covered soil? Or has that work been done?

  6. JM Stettner → in VT
    Aug 17, 2016

    The notion that our petroleum and coal are created by peat bogs is debatable at best. Though it is under-reported and completely untaught in the public school system, abiotic fossil fuel origin has been a theory since the 16th Century.  It has been widely disbelieved, but was pursued by Russian and Soviet scientists. As recently as this year, serious research has been reignited: “Fossils From Animals And Plants Are Not Necessary For Crude Oil And Natural Gas, Swedish Researchers Find.”. ScienceDaily. Vetenskapsrådet (The Swedish Research Council). 12 September 2009. Retrieved 9 March 2016.

  7. Tom Sadlowski → in Pioneer Valley MA
    Aug 23, 2016

    @Steve Steiner nailed it…

    Carbon is sequestered through deep roots, not the dander added to the top which primarily feeds the soil microbes, and thereby services the soil health.  Thus, in agriculture, it is important for roots to be driven deep by removing the practice of surface watering or drip irrigation to build soil carbon and nitrogen stores (thereby rely upon stable moisture in carbon-dense soils with moisture pulled upward), and covering the ground for soil health, which will then lead to more nutrient-dense cells in all life produced from it, as well as a more stable micro and then macro climate.

  8. Bendrix → in Rochester MA
    Oct 03, 2016

    Understanding carbon creation and storage is still a science in infancy, as this study helps to show.  The consequences of radical fuel plans are easy to understand (wind turbines and bird mortality for example), but real understanding of the current apparent warming of the earth is yet elusive.  We should be cautious putting limitations on energy use, as at its hear, carbon generation directly correlates with a higher standard of living.  Something people will need a very good and clear reason to forego.

  9. Daryl Burtnett → in Vermont
    Oct 25, 2016

    The quote in the piece “Rather than sequestering carbon in the soil, we saw an increase in soil respiration, which could contribute to global warming even more.” leads me to ask, “even more” than what?

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