What we know about managing soil carbon in prairies – a complete (but disappointing) guide

I’ve delayed writing a post about soil carbon and soil health in prairies for years because I haven’t been able to figure out how to do it.  It’s a difficult subject to write about because we (scientists) know disappointingly little about the subject.  In fact, I thought seriously about making this blog post nothing but a title and a single hyphenated word of text.  Something like this:

<<< >>>

What do we know about prairie management and soil health?

Diddly-squat.

<<< >>>

The idea made me chuckle, but based on my experience telling jokes at home, I probably would have been the only one laughing.  Most of you would have felt disappointed because you were hoping for some helpful information.  Well, join the club.  I’m fortunate to know quite a few brilliant people who are well-educated on the subject of soils, soil carbon, soil health (whatever that is – definitions vary wildly), and related topics.  However, my numerous queries to them about how we should manage prairies to improve or sustain soil health have pretty much yielded me diddly-squat.

 That’s not completely true, of course, but it’s also not much of an exaggeration.  We know much more about how farming practices affect soils in crop land than we do about how fire, grazing, or other management affects soils in prairies.  There are actual useful tips farmers can use to improve their soil productivity and health – e.g., “don’t till your field more than necessary.”  People can use that information to do good.  Wouldn’t it be great to have something similar for prairie management?

Soils are an integral part of prairie ecology but they stay hidden below ground where it is hard to know what they’re up to. We need to better understand how our management strategies affect soils in order to better conserve prairies.

In just a minute, I’m going to give you some useful information about soil carbon in prairies, but trust me, it’s not going to be very satisfying.  You might ask yourself why I would even write a blog post if there isn’t much information on my chosen topic.  Good question.  There are two reasons.  First, a lot of people ask me about it.  Second, there is a whole lot of mythology and just bad information out there about soil health and grasslands, and I’m getting increasingly frustrated by that. 

If you hear someone talk about how some kind of grassland management strategy (fire, grazing, etc.) affects soil health or soil carbon sequestration, be skeptical.  Remember that loud confident voices aren’t necessarily right, and anecdotal results or even data from a single prairie, farm, or ranch operation can be biased, wrong, or at least minimally extrapolatable.  I’m not saying someone who loudly advocates for a particular approach is being dishonest.  I’m just saying that it would be smart to do some searching for peer-reviewed research that backs up any claim before you invest in a new strategy.

How does fire frequency affect soil carbon? The answer is not fully known, and what we do know is very complex.

Ok, I’ve written nearly 500 words without giving you any useful information.  Here are the few statements about soil carbon and soil organic matter in prairies that seem to be generally agreed upon by most soil experts I’ve talked to, including several I reached out to while working on this post: 

Soil organic matter is added to grassland soil primarily through roots, their exudates (substances secreted into the soil by roots), and root turnover.

Experts are quick to point out that this can be less true in other ecosystems, including forests, and that even in grasslands, there are other important sources of soil carbon, including charcoal (aka biochar) from fires.  Regardless, it’s really important to understand the important contributions of plant roots when you think about soil organic matter in prairies. Inputs from grass litter aboveground (vegetative matter from previous growing seasons) can also add to soil organic matter.  However, there is general skepticism among soil scientists that the trampling of grass litter by livestock (for example, in mob grazing or similar intensive rotational grazing systems) has much influence on overall levels of soil organic matter. 

Most experts seem to agree that moderate grazing can be positive for soil carbon, at least when compared to chronic overgrazing or letting prairies sit idle. However, there’s clearly much more to investigate along those lines.

The amount of total soil carbon changes very slowly in prairies. 

Never-cultivated prairies tend to have high levels of organic matter because production has exceeded decomposition for a very long time. In these prairies, increases in soil carbon are hard to detect because of how much carbon is already present.  Picture how little the waterline in a nearly-full bathtub changes when you dump in a cup of water.  Prairies that have been re-planted in former crop land start with lower carbon levels (much less water in the bathtub) and so often show more marked changes in soil carbon over time.  However, those rates of change can be highly variable between sites.

In addition, the amount of soil carbon in soils is not uniformly distributed within the soil profile (the vertical section of soil from the surface down to underlying rock).  For example, there tends to be more carbon nearer the surface where grass roots are most concentrated.  Also, the rate at which soil carbon levels change can vary quite a bit by depth, which can make it hard to get good measurements of the overall trends. 

To make things more complicated, not all organic matter is equally decomposable. Some soil organic matter is labile; it is decomposed by soil microbes and plants can take up the released nutrients.  Other forms of carbon are harder to decompose (e.g., charcoal) or inaccessible to microbes (organic matter bound onto soil minerals or within aggregations of soil particles). These recalcitrant forms can be stored in soils for longer time periods (centuries!) and are much less a part of the active carbon cycle.  As a result, changes in total soil carbon may not directly reflect how soil functions or processes are changing.

Researchers are trying to catch us up, but it’s going to be a while before we can link soil impacts to many of our common prairie management techniques.

Soil health is a term that isn’t well defined or, perhaps, even useful in grasslands. The term works better in crop land, where it can be an indicator of soil fertility (though it is still often defined and applied quite variably within that context).

This is also where I reiterate the disappointing news about how little is known about how various prairie management strategies affect specific soil traits or qualities.  There’s a lot of research ongoing, and eventually we’ll learn a lot more than we know now.  Impacts of prescribed fire on soils has been studied a lot, but the impacts vary with geography, soil productivity and depth, frequency of burning, and other factors.  In some cases, fire can increase root production and turnover enough to make up for the carbon that goes up in smoke, but that also depends upon how often fires occur and other factors.  

When grazing is added to that mix, it becomes even more difficult to predict impacts on soils.  Consistent overgrazing is probably bad for soil organic matter and most belowground functions, but we don’t know much beyond that.  There is some evidence that moderate grazing might create more soil carbon than no grazing, but again, that seems to vary a lot by geography and soil type.  I know of at least one study currently looking at how different grazing systems might affect soil carbon, but it’s going to take many years of research at many locations to get us much useful information about how something like patch-burn grazing might vary from a deferred rotation or traditional continuous grazing system in terms of impacts on soil organic matter.

At this point, it appears that high levels of soil carbon are linked to high plant species diversity, along with productivity.  It seems fair to assume, then, that managing for plant species diversity should be good for soil carbon – as long as that management doesn’t reduce overall productivity.  Probably.  Hopefully.  With lots of caveats and assumptions in need of testing.  You get the idea.

Maintaining plant diversity is probably a no-regrets strategy for managing prairie soils, and it is clearly good for many other aspects of prairie ecology. However, there are lots of ways to promote plant diversity, and they are probably not all equal in terms of how they affect soil carbon.

Plowing up prairies is bad for soils.

This is the one statement that seems to garner easy consensus among soil experts!  We might not know as much as we’d like about how various fire and grazing treatments affect prairies soils, but there is no question that soil carbon decreases immediately and precipitously when grasslands are tilled up.  Furthermore, the recovery of that carbon if/when grassland vegetation is reestablished can take many decades or centuries.  Protect prairies, folks.

Thank you to Clare Kazanski, John Blair, Hannah Birge, Sara Baer and Stephen Wood for their patient and generous guidance, review, and instruction on this topic and post.  They gave me excellent (if sometimes conflicting) input, based on their own research and that of others.  Any errors in this post are definitely mine, not theirs.

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About Chris Helzer

Chris Helzer is the Director of Science for The Nature Conservancy in Nebraska. His main role is to evaluate and capture lessons from the Conservancy’s land management and restoration work and then share those lessons with other landowners – both private and public. In addition, Chris works to raise awareness about the importance of prairies and their conservation through his writing, photography, and presentations to various groups. Chris is also the author of "The Ecology and Management of Prairies in the Central United States", published by the University of Iowa Press. He lives in Aurora, Nebraska with his wife Kim and their children.

14 thoughts on “What we know about managing soil carbon in prairies – a complete (but disappointing) guide

  1. I’m an Environmental Specialist at NASA’s Plum Brook Station, a 10sq mi NASA research facility in northern Ohio. Long-term, I’m in the process of restoring up to 3000 acres of the Firelands Prairie, the eastern-most large landscape prairie in presettlement times, here in northern Ohio (yes, Ohio had real prairie; more lush than any to the west, because of ample year-long moisture).

    The issues of carbon release or sequestration on NASA’s prairie site here is a concern. Are my annual burns carbon positive or negative. Is there a net gain or loss of carbon in the soils beneath the prairies I’m restoring?

    No doubt, NASA’s prairies are decidedly carbon negative. More carbon ends up sequestered in the prairie soils than is released to the atmosphere by either prairie burns, or by microbial decomposition in the rhizosphere. How so?

    First, about two thirds of the biomass of both prairie grasses and forbs is roots and rhizomes. When those individual plant parts die, a portion of their constituent carbon is microbially converted to soil organic carbon (SOC), bound in various forms of humus, etc.

    But a significant amount of carbon — yet to be accurately determined — resides in prairie soils following prairie burns. Following a burn, the ground is covered with microscopic prairie biochar; black prairie fire soot. At first, this appears to be minimal; a light covering of very thin soot. But, this needs to be measured. Has anyone done anything of the following (we hope to do this at Plum Brook Station).

    Before a prairie burn, set out, on the ground open glass petri dishes. Burn the prairie. Then, immediately cover the petri dishes, enclosing the fallen soot, the prairie biochar. Except for some carbonate minerals, the soot will be primarily elemental carbon.

    How much? Take the prairie-soot containing petri dishes into an analytical chemistry lab and weight the dishes. Determine how many milligrams of soot in each. If competent, also analyze the chemistry of the soot. How much of it is carbon?

    Then, extrapolate those findings to acre or hectare areas of the burned prairie. How many pounds or kilograms of prairie biochar are added each year from prairie burns? It won’t be insignificant, inasmuch as it is elemental carbon; not easily converted back to carbon dioxide.

    Lastly, calculate the lbs C/acre over decadal or centennial time frames. Might the dark colors of Midwestern prairie soils be primarily a result of darkening by hundreds of years of prairie burns, from prairie fire microscopic biochar?

    • The biochar you mention is not always minimal. When burns occur during unfavorable conditions there is a lot of biochar. In contrast, during favorable burn conditions a much smaller amount of biochar remains. I have watched what happens to the biochar left after burning and it seems to mostly disappear. I no longer think much of it is being incorporated into the ground. Biochar is very buoyant. I think most of it floats away during rainy weather. When the biochar is put into a container of water a thin film will be left on the bottom. However, the film is so thin that thousands of years would not be enough for it to accumulate to the point that it would add significantly to the carbon stored in soil.

      • Just the opposite here. I, too, have pondered the fate of prairie biochar (ground-level soot) following a burn. Here (N Ohio) I’ve watched the biochar become incorporated, infiltrated and percolated down into the soil during rainfall events following burns. Very little (if any) is taken off the site in surface runoff — which remains uncolored by the black biochar. With the rainwater, the biochar becomes a component of the prairie soil.

        The question remains; what are typical mg/sq m production numbers for prairie biochar following typical prairie fires? Someone, collect biochar in glass petri dishes and determine those quantities. Then, extrapolate back for 8000 years, the beginning of the Xerothermic Interval, when the tallgrass prairies of the Prairie Peninsula (Iowa to Ohio) had their origins.

        If, say, a prairie burn produces 100 mg/sq m of C each year over 8,000 years, that works out to 1.7 lbs. in every sq meter. That’s not insignificant. A tenth of a gram adds up. (What if the soot is 1000 mg, a gram per sq meter per burn?)

        • I should mention that my observations occurred in locations with soils derived from clay. Cultivation for many decades has given what remains of these soils a consistency that is not much better than concrete. Even when restorations are decades old, the soils have not improved much when compared with remnants.

          If a film of carbon atoms is continuous and one atom thick then by using the Van der Waals radius it can be calculated that 23 mg of carbon would be present per meter. Your calculation assumes that each fire adds more carbon. It is possible that once adsorption sites have been occupied then no further carbon will be deposited.

          I think most of the charcoal that ends of in the soil is brought there by ants.

  2. This article makes some great points about the unknown correlation between carbon sequestration potential and management, but there is good data demonstrating the potential of prairies to store carbon. The World Resources Institute states that grasslands store approximately 34% of the global stock of carbon and that some grassland soils have the potential to store 2 tons per acre, the equivalent of removing up to 48 cars per acre per year. As you point out, those are mature systems and we know it can take up to 40 years to restore soil biota seen in mature grassland systems, but knowing the potential of prairies to sequester carbon is an important tool for us to acknowledge and incorporate as part of a broader strategy to address climate change, and I agree that if we can at least focus upon restoring landscapes so that they are species diverse and rich, we can be fairly sure that we are helping address excessive carbon production. Then we can wait on the science to provide definitive data on how best to manage for the optimization of carbon sequestration.

  3. With the rapid overtaking of prairies by eastern red cedar in some areas, I wonder if there have been any studies examining how long cedar roots (which can be very deep) can persist in the soil and how much carbon they transition into deeper soil layers. I’m certainly not advocating cedar invasion to promote carbon sequestration, but cutting down big cedars probably leaves significant deep root structures behind. Just curious.

  4. Your post (and the resultant comments) is very helpful, even without definitive conclusions or recommendations. I will be establishing sampling points this spring to compare soil carbon, and hopefully some other measurements in the 100 acre restoration I seeded last summer as well as the adjacent cropland. This has already given me a couple ideas, as well as some other names of people knowledgeable about the subject. Soil health has become such a squishy term because of its trendy cachet that most soil scientists roll their eyes at its mention, but we all know there is meat behind the concept both biologically and physically. In other words, we need to keep stumbling forward. Thanks!

  5. Thanks Chris for tackling this subject. This is a very contentious subject in some environments.

    USDA’s soil heath definition is: “the capacity of soil to function as a vital living ecosystem that sustains plants, animals, and humans.” Since USDA is driving most of the soil health work, I’ll go with that for now, but it is a wide open definition.

    My experience has found very little correlation to the independently taken soil heath test scores to the productivity of crop ground. I’m not sure what is being found in rangeland environments. I know newer, probably more accurate, soil tests are being evaluated.

    This site is very good at explaining why changing OM is very difficult in the short run.
    https://extension.psu.edu/can-i-increase-soil-organic-matter-by-1-this-year

    The quickest way to change OM readings is to take a shallower soil sample than the previous samples taken in earlier years. I believe this is happening inadvertently and thus getting inaccurate OM results. I find very few, if any, peer reviewed studies that show rapid (0.1-0.2% increases per year in OM) organic matter increases without quite a bit of outside material being brought into the fields. Many popular press articles indicate otherwise.

  6. Thanks Chris. In NZ it’s very difficult to get to the bottom re soil carbon – a lot of inconsistency in an otherwise academically robust environment. My understanding is that whatever the soil C was in it’s natural\pre-technological human state was it’s maximum soil C, be it prairie, tundra, forest, wetland etc. Once changed to agriculture (or otherwise damaged, i.e. losing its mycorhizza and native plants) it is nigh on impossible to return to it’s original C level.

    I meet a lot of landowners who think they can do this or that to dramatically raise soil C who’s enthusiasm I have to temper. There is research in NZ showing higher C sequestration on low-phosphate soils, but this hardly helps farmers! e.g. – Influence of soil P status and N addition on C mineralization from 14C-labelled glucose in pasture soils – I have a postersummarising this but it’s now und=findable on the web.

  7. Since carbon is always in flux (excuse me I just exhaled), your summary is spot on as usual. Plant roots in symbiotic relation with soil biology, especially mycorrhizae fungi, is touted as a “more efficient” way to sequester carbon in soils (liquid carbon pathway). Residue breakdown (dry carbon pathway) appears to be a “less efficient” process due to oxidation loss.

    You point out the “type” of carbon currently found in a prairie makes a difference in overall carbon cycling. We measured soil carbon in 30-year old native grass CRP prior to grazing in 2012 and repeated sampling (top 4 inches) in 2018. After 6 years of grazing, TOTAL carbon dropped slightly but a distinct change in “active” carbon was found. The active portion changed from ~70% in 2012 to over 90% active in 2018. Comparing un-grazed CRP land to grazed, we found over 2X the so called “beneficial fungi” to bacteria ratio in the grazed. With these monitoring changes we feel good about the path we are on and will attempt to do microbial inoculation in 2019 to see if we can influence additional change.

    With ecosystems having so many moving parts we realize these few measurements are just monitoring indicators. As you point out all the time, our prairies are faced with CHANGES on many fronts. The carbon to nitrogen (C:N) ratio is key when discussing overall soil biology and influence on prairies as Dave Wedin UN-L, pointed out recently (https://grassland.unl.edu/images/seminar/Wedin-Slides.pdf). As soil microbes appear to have a huge influence in soil carbon, they function under well understood C:N ratios, if the micro nutrients are available, etc…

    Just for the record, after working with a grassfed beef herd for the past 8+ years, I can only think of once or twice hearing a cow fart or belch. Plenty of carbon cycling and spreading microbes out the back end of cattle all over the prairie but very little farting. Nature has provided a time-honored template to raise nutrient dense food. We like taking extremely high carbon, low protein (C:N) stuff (not edible to humans) and turn it into a low carbon, high protein, vitamin and flavor packed food product while preserving & restoring prairies from the plow.

    Keep up the good work.

  8. Interesting post as always. Thanks Chris. A couple of suggestions/questions. 1) Perhaps you start every post from now on with the phrase “Plowing up Prairies is Bad for Soils”. and 2) I wonder if a better measure of “soil health” in grasslands is biological activity? That’s generically how we measure above ground prairies, why not below too?

    • Yes, plowing is bad for soil. I simply wonder how this has been translated into the extreme where my counties ecologists (Cook County, IL) say any soil disturbance is bad. They tell me I can’t use a tool to loosen the soil so I can pull up weeds and small woody species. They say this causes too much soil disturbance. Only herbicide application is allowed, not digging. I’ve compared digging up/pulling the flush of small stuff that appears after initial clearing work with broadcast herbicide application. My observations are that digging up/pulling small stuff gives a better result, even if it does takes much more effort than broadcast spraying. A plus is volunteers don’t need an herbicide license to dig up invasive species. Allowing small invasive species to be dug up/pulled would allow more people to help. However, my county’s ecologists won’t allow digging up/pulling. This just doesn’t make any sense to me.

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