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?


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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.

Prairie Word of the Day – Phenology

Hello, and welcome to the fifth edition of the popular series, “Prairie Word of the Day.” This is the series that has previously brought you such inspiring words/phrases as Tiller, Habitat Heterogeneity, Disturbance and Shifting Mosaic of Habitat. Thank you for the many cards and letters expressing your gratitude for the explanations of these words, and suggesting future topics.

Today’s featured word is Phenology. In short, phenology is the study of the timing of various events in the lives of plants and animals and the factors that influence that timing. Phenology should not be confused with Phrenology, which is the long discredited study of how the shape and size of the human skull supposedly correlates with character traits and mental capacity. Phrenology has been used to bilk people of their money, support racist and sexist stereotypes, and bolster Nazi eugenics. Let’s not talk about that today.

Phenology, without the “r”, is a complex and important topic in ecology. You might hear someone talk about the phenology of plants related to when they begin emerging from the ground, when they flower, and when they begin to wilt and senesce at the end of the growing season. Additionally, however, phenology includes the timing of the emergence of insects from dormancy or their final molt into adulthood. It also includes the timing of animal migrations and hibernation, as well as many other events in the lives of myriad organisms.

This bee (either Melissodes agilis or M. trinodis) is a specialist feeder on sunflower pollen and is only active during the period of summer when sunflowers are blooming. If the bee emerged before sunflowers started blooming, it might not find anything to eat.

The factors that influence a species’ phenology often include temperature, light, and moisture – in combination with genetic signals. We still have a lot to learn about the phenology of most prairie species, especially in terms of how they might adapt to changing climate. In fact, rapid climate change has brought much recent attention to phenology because changes in the flowering time of plants, for example, have already helped illustrate the occurrence and impacts of climate change. In addition, there is great concern that species may not be able to adapt the timing of their lives quickly enough to match the changing climate, and/or that timing of interdependent species might not remain synchronized. For example, flowers might start blooming before or after their particular pollinators are active, or birds or insects might migrate to breeding areas before food is available at those sites. A couple years ago, monarch butterflies arrived in Nebraska way ahead of schedule, but fortunately they were still able to something to eat and lay eggs on.

When monarchs arrived in Nebraska much earlier than normal, dandelions were one of the few abundant wildflowers for them to feed on and they laid eggs on whorled milkweed because common milkweed hadn’t emerged yet.

Here in Nebraska, we got some interesting insight into the phenology of plants during 2012. The year ended up giving us the most severe single year drought in recorded history and it started out as a year of extraordinarily warm temperatures. In fact, spring and summer temperatures arrived so early that we recorded many plant species blooming weeks or months ahead of their typical schedule. I wrote a short blog post about this back in May of 2012 and a number of people from around North America responded with their own sightings. The observation that stood out most to me was the blooming of asters in May. I had never seen heath aster (Aster ericoides) or New England aster (Aster novae-engliae) bloom before late August or September.

Phenology is also important to land managers trying to sustain biological diversity in prairies. For example, around here, we are constantly fighting cool-season invasive grasses. The growth period for those species starts earlier and ends later than that of most native prairie plants. That gives us some opportunities to use herbicides to kill or suppress smooth brome, Kentucky bluegrass, or other invasive grasses when the chance of harming other plants is very low. In addition, we can use prescribed fire, grazing or mowing to target those grasses when they are most vulnerable. For example, we might try to burn a prairie right as those species are starting to bloom because it wipes out those plants’ entire season of energy investment in growth and flowering. The fire doesn’t kill those grasses, but it can knock them back enough to allow other plants – especially those just starting their growth periods – to flourish while the vigor of the invasive grasses is low.

We timed this burn to suppress cool-season invasive grasses, which were just starting to bloom. After the fire, many warm-season grasses (and other plants) responded quickly because they were just beginning their period of most active growth.

Timing of burns can also be aimed at suppressing many other kinds of plants. For example, we sometimes try to burn prairies when encroaching trees are just leafing out and highly vulnerable. Alternatively, burns can be timed to limit impacts on animal or plant species. That might include strategically scheduling a fire based on the emergence of rare insect species or before sensitive reptiles become active in the spring. Prescribed grazing can be employed in much the same way – strategically moving livestock in and out of an area to suppress the growth of particular plants or to create desired habitat structure prior to the arrival or emergence of particular animal species. In all these cases, land managers are acutely aware of the phenology of the species they are trying to suppress or assist.

If you’re someone who enjoys keeping track of when things happen each year, you might enjoy joining a citizen science effort to document changes in the phenology of many different phenomenon. You could start at the National Phenology Network and peruse some of the options they provide. Or, if you already have years of field notes that document when you see your first bumblebee, prairie clover flower, or grasshopper sparrow each year, I’d encourage you to contact a local expert on that/those particular species and let them know about your data. You might have information of great value to conservation.