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.

Dating Sites for Prairies?

One of the biggest, but rarely talked about threats to prairie conservation comes during transitions of land ownership. I’m frequently approached by people who have poured their heart and soul into restoring and/or managing a nice parcel of land and are wrestling with how to make sure their investment isn’t squandered after they’re gone. I think about the same issue with my own family prairie, though I hope I have many decades before the issue becomes urgent.

Our family prairie stayed in the family after my grandparents died because their kids (my dad and his sisters) made it a priority and I was willing and able to take it over its stewardship. I don’t know if any of my kids or their kids will be interested or able to take it when it’s time. That’s definitely a big worry of mine, and many other landowners have similar worries.

Conservation easements are a tool that can provide some help, and they are absolutely valuable in landscapes where prairies are rapidly being turned into crop land. However, easements don’t address all threats and come with a number of complications and disadvantages. If you’re not familiar with conservation easements, they are essentially a legal agreement made between a landowner and a land trust organization in which the landowner gives or sells certain land rights to the land trust. A landowner might agree, for example, not to ever construct a building on the site, till the land for crops, or do other things that would destroy the prairie or threaten the conservation value of the property. That agreement becomes legally binding and is attached to the deed so that all future landowners have to abide by the same restrictions (for the length of the easement, which is often perpetual). Typically, those restrictions are difficult, if not impossible, to alter once everything has been signed.

Easements can help eliminate some clear threats to prairies such as housing development or tillage, but easements are not well-designed to ensure that current or future landowners control invasive plants or otherwise manage the site to benefit plant diversity or habitat quality. A prairie destroyed by chronic overgrazing or invasive trees is just as destroyed as it would be by conversion to a soybean field, but most easements can’t protect against those first two threats.

It’s very difficult to use any kind of legal contract to dictate how a prairie should be managed for the long-term. Challenges to prairies change over time, as do our best ideas about how to address them. Easements, however, are static and inflexible. We need a better option.

A conservation easement could prevent a house from being built on this prairie but can’t force a future landowner to suppress invasive plants or prevent them from managing in a way that damages its biodiversity.

The crux of this issue is that every landowner wants to know that the next landowner will do their best to take care of the property. Sometimes, that assurance comes because land is transferred to a family member who has already invested time, energy and passion into the property. Often, however, family members are uninterested or unable to own or manage the land, so the current owner has to look elsewhere.

What if there was a kind of online dating site for prairie owners and conservation-minded people looking to purchase a prairie? There are myriad ways this could be handled, but the basic idea is that someone looking for a successor could post information about their prairie and the kinds of work they’ve invested in it. Meanwhile, potential buyers could post a profile of themselves that outlines their interest and (potentially) expertise in prairie ownership and conservation. If two people see each other as potential partners, they could set up ways to further explore that relationship.

There are lots of ways to help this idea succeed, including training and certification programs for prospective buyers, educational and financial assistance for both current and future owners, and many others. Clearly, there are also many ways this model could fail, but if even if it only works in some cases, it seems a lot better than our current lack of options. If every passionate prairie owner passes their site on to another passionate prairie owner, it creates a self-perpetuating chain of land protection based on relationships and trust. The model could work equally well for both tiny prairies in the eastern tallgrass prairie and large ranches in the west.

This piece of Sandhills prairie is owned by The Nature Conservancy, and that should keep it conserved. However, TNC and other conservation organizations can and should own a limited amount of land. Many private landowners do a terrific job of prairie conservation and just need help finding someone who will take on that mantle when they’re done.

Legally-binding land protection strategies are typically expensive and limited in scope and effectiveness. Conservation organizations can only buy and manage so many parcels of land, and too much conservation and/or government ownership creates significant social friction in some landscapes. Easements can protect against some threats, but not others, and placing long-term or permanent restrictions on land isn’t a desired solution for many landowners.

Simply helping landowners find appropriate successors for their land seems like a potentially valuable addition to the currently available options. Whether that comes in the form of an online dating-style website or something completely different, I love the idea of helping people find someone they can trust to carry on a conservation legacy. I don’t love it enough to set something up myself, because that kind of thing is not my strength. However, I’d sure be happy if someone else wanted to step up and do it! (Or let me know if something like this already exists – I can’t be the only one thinking along these lines.)