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:

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What do we know about prairie management and soil health?

Diddly-squat.

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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.

Hubbard Fellowship Blog – The Nature of Human Intervention

This post is written by Eric Chien, one of our Hubbard Fellows.  Eric has a solid background in land management and apparently thinks quite a bit while he’s doing stewardship work.  Here are some of his latest thoughts – I think you’ll find them thought-provoking.

What if I told you our most resilient prairies will likely experience burning, mowing, cutting, shredding, chemical spraying, and fencing for decades to come? Among splendidly diverse native wildflowers and grasses, and a rich assemblage of insects, birds, herps, and mammals, there will be the consistent imprints of boot tread in the soil. The sounds of wind-blown grass, and meadowlarks will be occasionally interrupted by the clamor of metal and engines. We will know resilience not only by the existence of a vibrant prairie pulsing with life, but also by the presence of a sturdy 4-strand barb wire fence, and a two-track road worn to mineral soil.

Managers know that maintaining the function and diversity of prairies is highly involved work. I think the image of that monumental work is viewed somewhat quizzically by much of the public that has not had an opportunity or guide to understand prairies. The notion of conservation as the process of removing human presence and intervention is still widely circulated. Once removed from the yoke of human imposition, the natural world is supposed to largely perpetuate itself; growing more abundant, diverse, and resilient in its respite. That is the idea at least. My experience on prairies tells me that conservation landscapes characterized by little human presence is a mold not applicable to prairies. It probably has not been for 150 years. Considering the long history of Native American land management, it may never have been. What’s more, the intensive management in prairie conservation is representative of what many of the world’s ecosystems will require to maintain their functions into the future. Decades of prairie management suggest that we consider ourselves and our presence not as obstacles or crutches to the diversity of life, but as integral drivers of the processes and forces that maintain integrity and functioning of ecosystems.

The author cuts down a cottonwood tree on the edge of one of our Platte River Prairies. Photo by Katharine Hogan.

On the prairie, we light the fires, control the grazers, and suppress the invasive plants. In doing so we drive species composition and distribution, habitat heterogeneity, and the presence or absence of ecosystem functions; the most fundamental ecological attributes. Our involvement is not out of hubris, nor does it make prairies an artifice. Science and experience tells us that without our involvement prairies nearly always slip into measurably degraded states, or entirely disappear. Chris has written thoroughly on the science and implications around the myth of the self-sustaining prairie and the reasons why management is necessary. Seeing our new role with clear eyes has important implications for our approach to conservation.

Rather than thinking of ourselves as prairie doctors, we should see ourselves as prairie organs. Organs are not optional, and cannot be removed from the whole when the budget it tight. When we set up prairie conservation complexes we need to consider humans with the same gravity we consider plant diversity. Whether it is land management professionals, volunteer cohorts, or farming and ranching families, thoughtful and capable human managers are as important as the native grass community.

People have been lighting North American prairies on fire since the last glaciers retreated and grasslands emerged as the dominant ecosystem in the Great Plains.

What does recognition of that human importance look like at The Nature Conservancy? Since 1994, standard operating procedure in TNC has mandated setting aside an endowment for every new land acquisition with the principal set at minimum 20% of the fair market value of the land. It is a small but key step in maintaining essential human capacity in our conservation lands. We also strive to recognize human importance by making our conservation work relevant to ranching families. The ecological and management knowledge we seek out strives to reconcile economic and conservation needs. The gold standard in our work are solutions that allow people and nature to thrive. This is not just because supporting human communities is important, but because prairies with deep human presence are healthy, resilient prairies.

If at this point you’re thinking- “This sounds like an overly involved prairie person issue.” I say this- Prairies are likely a vanguard for where many of our natural systems are headed. Our ability to find success as drivers of ecosystem integrity and resilience through active management have implications for the future integrity of countless ecosystem types. Resilience processes in forests, reefs, tundra, and countless other systems are being broken down by ongoing fragmentation, and novel disturbances. There is already a need for us to step in and play a key role in the ecology that reinforces the biodiversity, functions, and services those systems provide. That need is only growing. North American Prairies are a proving ground for our ability to do that effectively.

I hope you consider sharing your thoughts.