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.

Seeds of Promise

It’s hard to describe the varied emotions involved in cleaning and mixing big piles of prairie seed.  There is incredible optimism embedded in those tiny packages.  After all, each seed has the potential to become a plant; maybe even the first of an entire colony of plants.  On the other hand, most of them will fail to produce anything other than a little food for some animal or microbe.

Alex mixes up a big pile of seed with a grain scoop.  Amber is helping too, but hidden behind a cloud of dust.

Yesterday was seed mixing day at our Platte River Prairies.  We dumped bags and buckets of harvested seed into piles to be mixed and taken out to planting sites.  In recent years, we’ve shifted our seed harvest focus; instead of aiming for the highest possible diversity of prairie species (150-200) to convert crop fields into prairie habitat, we’re now focusing on 30-40 wildflower species that are largely missing from some of our more degraded native prairies.  Those degraded prairies had years of of chronic overgrazing and/or broadcast herbicide use before we obtained them, and haven’t really increased much in plant diversity, despite over 20 years of the best management we could give them.  By increasing the number of plant species in those prairies via overseeding, we hope to increase the quality of pollinator and wildlife habitat, as well as the overall ecological resilience of the prairie community.

We process many of our seeds by running them through a heavy steel fan blade that used to be part of a riding lawn mower.  It’s a quick and effective way of breaking seeds out of pods and off of stems.

To be clear, when I say “we”, I’m mostly talking about the Platte River Prairies staff and volunteers who are actually doing the work these days.  For many happy years harvesting seed, but that torch has now been largely passed to others.  I still harvest a modest amount of seed on my own time, mostly during evenings and weekends, for use at our family prairie, but I’m just an advisor for the bulk of the restoration work going on at the Platte River Prairies.  Regardless, it was immensely gratifying to help out yesterday.  It felt great to run those seeds through my fingers and inhale a little seed dust into my lungs (though we wear masks to minimize the dust inhalation).

Alex dumps a bag of seed onto the pile.

While I like thinking about each seed as a potential plant, I also recognize how few of them will actually make it that far.  Even in cropfield restoration work, when we’re broadcasting seeds onto bare soil with no preestablished competition from other plants, only a small percentage of seeds really end up as plants.  Some are eaten by animals before they get a chance to germinate.  Others don’t land in a place where they get the light and moisture they need.  Still others germinate, but are then outcompeted by neighboring plants, eaten by something, or don’t get rain at the right time to sustain them.

When we’re overseeding an existing prairie, the number of planted seeds that turn into plants is far lower still.  We burn ahead of time to create bare soil, and graze to reduce competition, but there are still very few spots where a seed can land and have a good chance to thrive.  That means that the vast majority of those wonderful little seeds of promise just die.  Though, as we discussed yesterday while we worked, even the ones that die are feeding something – birds, mammals, insects, fungi, etc. – so it’s not that they’re really wasted.  It’s just that we didn’t really spend all that time harvesting seeds just to feed fungi.

Instead of focusing on how many of those seeds will become fungus fodder, though, I’d prefer to think about the good that will come from those that survive.  By harvesting and broadcasting those seeds, we’re transforming prairies with very few summer wildflowers into prairies with enough floristic diversity that they will support a more robust pollinator population and provide better habitat structure to a number of wildlife species.  Even if one tenth of one percent of the seeds we plant germinate, we’ll be making a big difference.

This overseeded prairie is not yet where we’d like it to be in terms of plant diversity, but it’s far better off than before it had even the number of flowers shown here.  Hopefully, now that some of these wildflowers are established, they’ll be able to spread on their own as we provide helpful management.

Soon, we’ll be releasing those seeds into the wild to take their chances in the world.  Most of our planting these days is done by machine, which helps us cover a lot of ground quickly, with fairly even distribution of seeds.  That’s all well and good, but I sure get a lot of joy from hand-tossing seeds at our family prairie.  Not only can I aim the seeds for areas I think (though I’m totally guessing) they might survive best, I can also give them a little good luck wish as they leave my hand.  Later in the season, when I return to look for seedlings, I can congratulate both the seed and myself on our success whenever I find a new plant.  With enough of those successes, we’ll slowly rebuild the diversity and resilience that will carry these prairies well into the future.

Alex, Amber, and a big ol’ pile of potential prairie plants.