Sequestering Carbon in Soil and Trees

Martha RobertsThis post is by Martha Roberts, an economist at Environmental Defense. It's the first of a three-part series on carbon sequestration – storing carbon or carbon dioxide (CO2) in soils, trees, geological formations, and oceans.

1. Biological Sequestration
2. Geological Sequestration
3. Ocean Sequestration

Global warming is occurring because – day after day, hour after hour – human activities pump large amounts of greenhouse gases into the atmosphere. One way to decrease emissions is to store carbon or CO2 someplace other than the atmosphere.

There are two vastly different ways of sequestering carbon: biological and geological. The topic of this post is biological sequestration, which is among the biggest of the "low hanging fruits" for making quick, substantial cuts in emissions.

Our lands and forests have huge potential for storing carbon – they are nature's "carbon sinks". Green plants take CO2 out of the atmosphere and convert it into organic carbon as they grow – a process called photosynthesis. Organic carbon is converted back to CO2 when it is eaten or decomposed – a process called respiration.

Farmers and foresters can do many things to increase photosynthesis and/or decrease respiration. For example, they can replant forests or delay timber harvests to increase photosynthesis. Tilling increases the respiration of microbes in the soil by improving the conditions for decomposition, so no-till farming reduces respiration.

Are trees the only plants that can store carbon? No, but trees store the most carbon because they're large and long-lived. As long as the wood doesn't decompose or burn, it stores carbon away from the atmosphere. Still, restoring tilled fields to grasslands can help. Since the grass isn't harvested and the land isn't tilled, more of its organic carbon remains in the soil.

How do we get farmers and foresters to use better land management practices? The answer is a carbon market in which farmers and foresters can participate. In an inclusive carbon market, those who emit carbon pollution pay for credits, whereas farmers and foresters sell them through agricultural offsets.

Agricultural offsets require a robust monitoring and verification process – a reliable way to measure the carbon stored in soils and wood. This is challenging but doable, as Bill Chameides and his colleagues demonstrated in their technical manual on the topic. (Duke University, which published the book, offers excerpts online [PDF].)

Globally, soil carbon sequestration alone could offset as much as 15 percent of fossil fuel emissions. In addition, thoughtful offset projects can have side benefits such as improved soil quality, increased crop yields, and better wildlife habitat. And all that's keeping us from these benefits is a lack of economic incentives – a carbon market where farmers can sell their carbon credits.

In 2004, the USDA published an economic analysis of biological carbon sequestration in the U.S. agricultural sector. The report evaluated how landowners would respond if they could be paid to sequester carbon.

Not surprisingly, the results showed that the higher the price for carbon, the more farmers would do to enhance carbon sinks. At the highest price considered in the analysis, U.S. farmers and landowners would implement practices that could sequester as much as 160 million additional tons of carbon in forests and agricultural soils every year. That's equivalent to nearly 10 percent of America's CO2 emissions in 2005! Even at lower carbon prices, agriculture can play a very important role in our fight against global warming.

When people talk about reducing emissions, they often focus on high-tech solutions. But let's not forget the low-tech strategies that can bring immediate results, like improved agricultural practices and increased energy efficiency.

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  1. Rapidrex
    Posted December 18, 2007 at 6:13 pm | Permalink

    If you are not a vegan…then you are not doing ALL you can do if you truly believe that global warming is an issue. If you do…maybe you need a head doctor too. What a waste of time!

  2. Posted December 18, 2007 at 10:47 pm | Permalink

    Don't trees that lose their leaves in the Fall respire (return to atmosphere) CO2? (that is, the dead leaves return CO2). Then in the spring and summer, they sequester again, right? If so, shouldn't we be planting only evergreen trees instead of, say, oak trees? Or am I missing something? Thanks.

  3. debisbooked
    Posted December 19, 2007 at 1:37 pm | Permalink

    How about getting people in general to practice better land management? I agree with this author that "Our lands and forests have huge potential for storing carbon – they are nature's "carbon sinks."' However, I do not believe it is the small group of forester and farmer who by their land management practices are the most remiss. The farmers I know care deeply for their land and the environment. One practice (besides no-till farming) is planted tree wind breaks to keep the soil from blowing away – these trees act as 'carbon sinks' also. No, it's not the farmer and forester doing the most damage by lack of good practice. What needs to be curtailed is turning our fields and forests into malls, box stores, and endless housing tracks. It's overconstruction of buildings and asphalted roads that put an end to biological carbon sequestration. Instead, refurbish cities and towns into pleasant, livable areas and leave our fields and forests alone to do their job.

  4. Posted December 19, 2007 at 4:44 pm | Permalink

    Hi kenzrw,

    Great question. Yes, dead leaves are decomposed and most of their organic carbon is released as CO2. However, there's more going on. First, there is a lot of long-term carbon storage in the wood, which does not die back every year. Second, during the growing season the trees' roots add carbon to the soil, and some of this will end up in long-term storage in the soil. Even a tiny fraction of the leaf carbon can end up in soil carbon storage if the leaf litter doesn't decompose completely.

  5. Posted December 19, 2007 at 4:50 pm | Permalink

    PS kenzrw,

    Your question also brings up the interesting issue of deciduous vs. evergreen trees for offset projects. I asked Bill Chameides and Zach Willey (the editors of the ag offsets book) if they'd like to jump in with some of the nuances involved. When I hear back from them I'll post another comment here.

  6. Posted December 20, 2007 at 1:26 pm | Permalink

    OK, I've heard back from several people about evergreen vs. deciduous species.

    Ultimately, the choice of tree species will be affected by many factors, including local climate, soils, and co-benefits landowners want to get from an offset project, but here's a summary of the general differences between using evergreen and deciduous trees for forestry offsets.

    Many evergreen trees store more carbon, and do so more quickly, than deciduous species. (There are exceptions; check out Figure 6.2 in the book excerpts Martha mentioned.)

    However, even if they store less carbon or do so less quickly, deciduous species have a lot of co-benefits. For example, deciduous forests generally have a rich "understory" community below the main forest canopy, and allow multiple uses, such as grazing animals in the spring. Deciduous trees tend to use less water, too.

    Hope that helps. Happy holidays!

  7. Posted December 20, 2007 at 11:23 pm | Permalink

    Hmmm…sounds like it really doesn't matter that much which type of tree is planted in the long run. Evergreen pines also lose pine needles all the time, so I suppose those dried pine needles release some Co2 as well. I hadn't thought about the roots sequestering carbon in the ground either.

    Of course, all trees eventually die (100-1000 years?), so most of the CO2 they sequester will eventually be released back to the atmosphere except for that stored in the ground by the roots. But I guess we should just keep replanting. I visit the cypress swamps in eastern Arkansas frequently and can just 'see' the carbon being reclaimed.

  8. IMWright
    Posted December 21, 2007 at 2:25 pm | Permalink

    Your first premise is innacurate. Global warming occurs thanks to the Solar cycles (see MAUNDERS MINIMUMS)
    Second, terrestrial plants don't "sequester" nearly the amount photosynthetic planktons do.
    Thirdly, our oceans buffer CO2 level by CO2's high solubility levels in water. All that CO2 is then converted to CALCIUM CARBONATES called CORALS. And by golly those same corals have been converted in the past to our modern day limestone and MARBLE maybe you have heard of these.
    AL Gore's inconvenient truth is actually CO2 levels INCREASE after warming has occurred. Warmer water releases its dissolved CO2 to the atmosphere.
    Last, VEGANS produce 4 X the METHANE real humans do- now don't forget how healthy one of the most famous vegans was- Linda McCartney??

  9. kricnit1
    Posted December 23, 2007 at 12:08 am | Permalink

    Seems that CO2 sequestration in plant materials is at best a very short term solution unless that material is buried in a very deep vault. The effect of sequestering CO2 in plant material would only reduce CO2 levels so long as the material is not burned or allowed to decompose. The idea that we can revert a substantial amount of land that is not already supporting wooded forest and is capable of hard or softwood growth seems quite impractical. Grasses and annual growths do absolutely nothing to reduce atmospheric CO2 concentrations due to the very short term respiration cycles of the materials, unless they are buried to the extent that they do not decompose. Given increased global populations and associated demands for these populations for land and resourced such as lumber how can a hundred years of tree growth replace the millions of years of long term carbon storage the earth provided in the form of oil and coal fields when the world is burning 1 cubic mile of oil and coal a year?
    There is also a current premise that CO2 concentrations increase following an increase in atmospheric temperatures. On the surface this seems a credible assertion do to increased rated of decomposition of stored organic material at elevated temperatures. Are the data points for this type of assertion "cherry picked" from a vast amount of data, or is this an actual long term trend of global increases of CO2 concentrations after the increase in temperature has occured?

  10. Posted January 11, 2008 at 1:43 pm | Permalink

    Hi kricnit1,

    Thanks for your great questions (and apologies for the delay in answering them).

    Studies show that improved land management can substantially increase the amount of carbon in soils. This is true even in ecosystems like grasslands, because not all of the plant biomass is decomposed. A small fraction of each year's growth makes its way into soil storage pools that have turnover times of thousands of years.

    The risk of losing biologically stored carbon (for example, through fire) is certainly something that has to be addressed in any legitimate offset project. This is typically done through "insurance" in which the offset provider holds some credits back or has a pool of unsold offsets.

    You also asked whether warming could counteract biological sequestration. It's true that respiration can increase with temperature. The exact balance between land management and temperature's effects on soil carbon will vary from system to system, and this is an important area of ongoing research. However, at least one study has shown that some soils can acclimate to warming, meaning that soil carbon losses may not always increase with warming.

    – Martha & Lisa

  11. Posted January 22, 2008 at 3:07 pm | Permalink

    Bill Chameides chimes in with the following:

    "Since the agricultural revolution human activities have significantly degraded soils globally so that the amount of carbon stored in soils has been depleted. Carbon sequestration practices just need to partially restore soils to their "natural" carbon content to sequester huge amounts of carbon.

    Land management should always be viewed as a short-term bridge to the deployment of low carbon energy sources (which will take some time). In other words not a permanent solution."

    And, with regard to CO2 and temperature: "How atmospheric CO2 will respond to increases in temperature is indeed uncertain. Future climate simulations take this uncertainty into account and this is, in part, responsible for the range of temperature changes quoted in assessments like that of IPCC."

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