The politics and science of soil carbon

When it comes to discussing climate change, knowing the role of carbon is important. It pops up everywhere: carbon dioxide, carbon sequestration, carbon emissions, and carbon markets. A big driver of the shifting climate is the change in carbon’s location. More and more of it is being stored in the atmosphere, instead of in Earth’s crust and vegetation.

Many solutions for righting this imbalance suggest reducing emissions and recapturing the carbon we pumped into the air. I recently read a blog by the Institute for Agriculture and Trade Policy (IATP) about one such solution: a soil carbon market. The author described the debate about such markets among international climate change policy makers at their October meeting in Panama. That meeting was a primer for the UN’s climate change conference in Durban, South Africa, which began on Monday and finishes December 9th.

Me, doing a little soil science

It seems that there was contentious discussion around soil carbon markets in Panama. Some people argued that targeting soil carbon is a “mirage” or diversion from action that will really make a difference. Whether a market mediating soil carbon storage and release is fast or effective is beyond my scope of expertise. But I did take issue with a couple of the author’s statements about the state of soil carbon itself. I work in a biogeochemistry lab where soil organic carbon is a major research subject and the science of carbon is our specialty.

There were two statements in IATP’s blog with which I disagreed. I will list each and explain why, as a soil scientist, I find them empirically false.

Statement 1:Soil also just can’t hold a lot of carbon. Most scientific studies say that it can range from zero tons in dry and warm regions to one ton in humid and cool zones, making it an even less enticing option for smallholder farmers.

In fact, soil can hold a huge amount of carbon, and does. Soil contains double the amount of carbon stored in the atmosphere, and stores more than vegetation and the atmosphere combined (Houghton 2007, Jobbagy and Jackson 2000, Trumbore 2009). The amount of carbon in soil varies widely across different types of soils and different places but on the whole, there is a lot of carbon in soil.

To put some numbers on it, I’ll use the estimates from Balancing the Global Carbon Budget, an article by R. A. Houghton published in the Annual Review of Earth and Planetary Sciences in 2007. Houghton uses petagrams of carbon (PgC), which may not be a familiar unit but the numbers are still easy to compare (For the mathy types: 1 PgC = 10^15 g = 10^9 metric tons). Below is a table with Houghton’s estimates of the total carbon stored in each category:

Vegetation    550 PgC
Atmosphere    805 PgC
Soil   1750 PgC
Fossil fuel   7500 PgC
Ocean 38,000 PgC

While soil holds less than the oceans, it is still a major player in the carbon game. Additionally, it is difficult to evaluate the blog’s statement that soil carbon ranges from zero to one tons without a unit of scale attached. But it is safe to say that, with one petagram equaling one billion metric tons, the unit the author was referring to must have been quite small.

Statement 2: There are legitimate concerns about how to measure the carbon sequestered in the soil and how to be sure the carbon would stay in the soil, both of which make it unlikely that the carbon markets would embrace it anytime soon.

It may be challenging to efficiently measure soil carbon with a standardized and cost effective method for a market with international scope. However, I am not sure if this statement is referring to that challenge or actually doubting the validity of current measurement techniques. To this point, I will refer to a wonderfully relevant article titled “Measuring and monitoring soil organic carbon stocks in agricultural lands for climate mitigation.” The lead author is Richard Conant of Colorado State University and it was published in Frontiers in Ecology and the Environment this year. Conant et al. agree that quick, cost-effective soil carbon measurements might be a challenge for such things as soil carbon markets. But they point out that “existing methods for quantifying soil organic carbon (SOC) concentration in samples are well established and have a high analytical precision.”

There are a lot of interesting intersections between soil science and policy decisions, climate change and otherwise. Policy developers and scientists have different goals and foci in some cases, but the more communication between the two groups, the better!

Referenced in this post:

Contant, Richard, Stephen Ogle, Eldor Paul and Keith Paustian. 2011. Measuring and monitoring soil organic carbon stocks in agricultural lands for climate mitigation. Frontiers of Ecology and the Environment. 9(3):169-173.

Houghton, R. A. 2007. Balancing the global carbon budget. Annual Review of Earth and Planetary Sciences. 35:313-347.

Jobbagy, Esteban and Robert Jackson. 2000. The vertical distribution of soil organic carbon and its relation to climate and vegetation. Ecological Applications. 10(2):423-436.

Trumbore, Susan. 2009. Radiocarbon and soil carbon dynamics. Annual Review of Earth and Planetary Sciences. 37:47-66.

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3 thoughts on “The politics and science of soil carbon

  1. Hi Emily

    Right on. Some of Conant’s work, as well as that by Ellert, Janzen, and McConkey, convinced us to develop a practical and affordable and accurate method of measuring soil carbon change on small permanent plots. We are using this as the basis of the Soil Carbon Challenge, a competition to recognize land managers for turning atmospheric carbon into soil organic matter. NOT an “offset” scheme or market scheme, but an effort at turning our focus from managing AGAINST what we don’t want, to managing FOR what we want and need, such as soil organic matter in part because of its huge effect on water cycling, on moderating flood and drought.

    Occupy the carbon cycle. Join the Soil Carbon Challenge.

    soilcarboncoalition.org/challenge

  2. Emily, thanks for your concise examination of that IATP blog post. IATP offering a critique of global carbon markets is one thing, but the lack of understanding of the basics of soil carbon, carbon cycling and our total reliance on it was sort of alarming. My personal hunch – and hope – is that basic self-interest will continue to lead communities, cities and nations to figure out policy that incents land managers to manage for soil carbon accrual. I also suspect regional, even watershed approaches, may be most effective, based on lower-input, higher yield ag production, water quality and quantity benefits, mitigation of natural disasters like flooding, drought, wildfire and erosion damage to infrastructure. If people want these things – and we do – improving soil cover and increasing soil carbon is probably the best way to get them. As you say, reliable means of monitoring soil carbon change are well-established, and there are some existing new tools for quantifying all soil properties accurately and precisely that can further accelerate our learning and management. Good on you for your post, and for being a proud soil geek. Nice work. FYI, a grassroots project called the Soil Carbon Challenge is underway right now, establishing baseline soil carbon plots on progressive farms and ranches in the US, Canada, Mexico, with an eye to follow-up monitoring in the near future, looking for those who are achieving topsoil formation, how they’re doing it, how fast they’re doing it. http://www.soilcarboncoalition.org
    Cheers, Abe

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