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One of our board members recently posed a question about the carbon cycle and fire. The concern is that fire obviously releases carbon into the atmosphere that might otherwise be sequestered (stored) deep into the soil profile. Wouldn’t this then be counter to our organizational goals? A very legitimate question given the current state of climate concerns across the globe. Intuitively, it seems like a no-brainer that smokes coming off a grassland fire would pump carbon particulate matter into the atmosphere, but not much about fire science is actually intuitive.
The majority of ‘material’ in smoke is water vapor with the remainder being a mix of compounds and particles depending on the type of fuel that is burning. The exact amount of water vapor is highly dependent on the moisture content of the fuel. In grasslands, burning green grass in May produces smoke with a much higher water vapor content than does burning dry grass in the fall. The remaining compounds in the smoke, including carbon from the plant, remain relatively unchanged.
Here’s where things get interesting, and we need to go ‘deeper’. In preparing for this article, I reviewed several resources and spoke directly with Dr. Ryan Limb, Associate Professor of NDSU’s Range Science Program. He and his colleagues are in the midst of a long-term research project assessing the impacts of both fire and grazing on a broad array of plant community dynamics including nutrients and biological cycles. This research is being conducted at the NDSU Central Grasslands Research Extension Center near Jamestown, ND. Dr. Limb and his colleagues are striving to shed some light on the truths and myths of the role of fire in our northern prairie systems.
The primary mechanism of carbon sequestration by grasslands is capture of atmospheric CO2 through photosynthesis. Through this process, carbon is eventually transferred into the roots of the plant into the deeper soil profile (beyond about 12 to 18 inches deep). This process represents true carbon sequestration or storage.
Carbon released by the burning plant, whether alive or dead, is somewhat irrelevant to the total soil carbon sequestration question. The carbon that makes up the above ground ‘body’ of the burning plant is released into the atmosphere through smoke, but it cannot be assumed to be carbon that would have otherwise been sequestered into the deeper soil profile. Rather, this is plant material that would have either possibly been grazed (and the carbon would cycle ‘above ground’) or it would have generally broken down over time and become part of the duff layer and eventually part of the surface organic layer of the grassland. In any case, the majority of the carbon that makes up the physical plant is eventually released as atmospheric carbon, not sequestered into the deeper soil profile.
Certainly grazing can shift plant community dynamics. Whether that shift is positive or negative in relation to carbon sequestration is largely dependent on the application (timing, intensity, duration) of the grazing tool. In this way, grazing and fire are similar. However, the fire tool offers the manager an enhanced ability to target specific impacts because it can be applied in a singular event across a much larger (or smaller) area. For the most part, it is inappropriate to compare fire and grazing as surrogates for each other. Both are very important ecological processes that merit use in grassland management.
While all plants utilize photosynthesis, the ability of a plant to capture and sequester carbon into the deeper soil profile depends on the plant. Our native plants, both grasses and forbs, are generally very deeply rooted and many have tap roots that extend deeply into the soil, well beyond 18 inches. Conversely, some of our most problematic rangeland plants are sod-forming exotic species, such as Kentucky bluegrass, which is very shallow-rooted. If this plant dominates rangeland, true carbon sequestration is compromised because its root system, while still processing carbon, doesn’t necessarily ‘sequester’ it deeply into the ground and it remains more or less in the organic layer of the soil. Thus this carbon is generally not considered captured and can easily cycle back into the atmosphere.
It depends on some variables, but the fair answer is yes, fire can actually increase a grassland’s ability to sequester carbon. One example would be a native plant community that is not as productive as it should be, thus not sequestering carbon. Lack of production could be due to poor management or invasion by exotic species that compete with the native grassland population. At any rate, fire is proven to stimulate native plant community production and diversity, increasing both leaf area and volume, stimulating seed production and root growth, and in some cases plant density. If well-timed, fire can also drastically reduce the competition from invasive plants which may not be contributing to carbon sequestration anyway. Stimulation of the deeper-rooted native plant community via fire can ultimately result in a plant community that has greater ability to sequester carbon over time and space.
All land management actions have positive and negative impacts. Our grasslands have perpetual internal threats that include such things as poor management and invasive species as well as large external threats such as conversion and negative public perceptions of grazing, fire, and other management. For perspective, The Environmental Protection Agency’s website includes a synopsis of sources of Greenhouse Gas emissions. US agriculture accounts for about 10% of that total, and over half of that 10% is directly related to the current management of agriculture soils and modern Ag inputs. Livestock production accounts for nearly one-fourth of the total, with the report citing methane as the primary culprit. The report does not specifically address the fossil fuel carbon footprint involved in feeding livestock, but it does mention manure management as contributing 12% of the agricultural sector’s emissions total. So in answer to the question posed in the title of this article, it is important that the Coalition continue to deliver programming and education on all tools that can help any grassland manager keep healthy grass ‘greenside up’ to sequester carbon; whether it be a western rancher using fire for beef production or an eastern landowner utilizing fire to maintain pollinator habitat for bees or pheasants.
The Coalition will continue to host prescribed fire trainings over the next few years as funding allows. For the upcoming spring, we will be hosting the school in two different location. The first will be a return to the Oak Lake Station while a second opportunity is being planned for the Mitchell area. Both events will be held in mid-late May. More information on both events will be forthcoming, as will more articles in this space that address fire-related questions. Next month look for an article on how fire use relates to the desire to ‘armor’ the soil by keeping it covered.
Pete Bauman is a Natural Resources and Wildlife Field Specialist for SDSU Extension located in Watertown, SD.
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