This is Part 3 of an ongoing series. Here is Part 1 and Part 2.
In the first part in this series we told you that decades of grassland research show that grazing does not seem to have much influence on increasing carbon in the soil.
On the other hand, many graziers have noted that their good grazing management seems to be increasing soil organic matter (SOM), and since soil organic matter is about 58% carbon, that should mean that more carbon is being sequestered, right?
About ten years ago, two scientists argued about these two things. One said that it was doubtful carbon increase was occurring due to grazing management, and if there were changes, it would be difficult to measure them. The other said that based on soil organic matter increases alone, soil organic carbon had to be increasing, and it was because of his grazing management. To resolve their disagreement, they did what scientists do – they set up experiments to answer the question.
They now have nine years of data on the effects of management intensive grazing (or Adaptive Multi-Paddock grazing as it’s sometimes called in research circles) and it shows grazing has not increased carbon sequestration in the soil. In fact, the grazed only plots have continued to be a carbon source, not a sink. [pers comm – John Wick]
Other researchers might have told them to expect this result, as it turns out these kinds of studies have been going on for years. One scientist studying grazing and carbon sequestration on the shortgrass steppe told me that when he started he had very little gray hair, but now his hair is mostly gray and he’s retired. When I spoke to another scientist in this field he sighed and said, “We’ve got thirty years of data on this and it feels like people are just ignoring us.”
These scientists, and many more, have focused their research on grasslands since they cover about 1/3 (some even say half) of the earth’s land mass and represent a potentially important carbon sink. And they’ve been thorough and persistent. Since 1990, they’ve looked at different grazing management – light continuous, moderate, heavy (50% removal), short duration-high intensity, and management intensive rotational grazing. They’ve looked at the impacts of grazing at different times of the year, removing forage at different percentages, and comparing grazed vs ungrazed pastures. They’ve measured the results on different soil types, and areas with different historic management. And they’ve looked at seeded pastures, tall grass and mixed-grass prairies, and the shortgrass and sagebrush steppes.
In almost 30 years of looking at all these different things, researchers have yet to find a solid connection between grazing and increases in soil carbon.
So, let’s break down what this research says.
An ongoing, decades-long study of grazing and carbon sequestration on the shortgrass steppe is one of many examples of what scientists have found. Researchers at the Agricultural Research Service’s High Plains Grasslands Research Station found that it wasn’t grazing that increased soil carbon. The key factor to increasing carbon was spring precipitation. Dry springs reduced carbon flow into the system, and even wet summers could not make up the deficit. They also found that grasslands can become CO2 sources during drought, especially in winter, or when there is less biomass and more bare ground.
Another example of this is a study on the northern mixed-grass prairie. ARS researchers at Fort Keogh’s Livestock and Range Research Lab studied the effects of high intensity short duration grazing in the spring and mid-summer. CO2 uptake peaked with biomass production in late June and July. When green biomass was reduced, whether by grazing or dormancy, CO2 uptake was reduced.
As we continued to read, study after study arrived at similar conclusions. Grazing did not increase carbon sequestration in soils. You can find links to the studies we reviewed here. (The papers you find there are related to this and previous articles in the series. We’ll be adding to the list as we add to this series.)
What all these studies provide is some insight into a very old, very complex system. As one group of researchers concluded, based on the results of their research and the work of others, the soil organic matter in these grasslands is “an ancient product of vegetation, climate and soil development over 10,000 years since deglaciation, or even older inert lignite coal ground in the glacial till parent material. Current estimates of organic carbon turnover rates for uncultivated, cold and dry grasslands suggest a significant change in the soil carbon pool may require centuries to millennia. In addition, continuous grazing by large ungulates on the northern Great Plains has been the norm for at least 8,000 years, and the ecosystem is likely in equilibrium under continuous grazing. Short-term modification of the grazing regime may have less impact than complete grazing exclusion on an organic carbon pool subjected to grazing for millennia; yet no consistent grazing exclusion response has emerged from this investigation or others.” (Henderson et al, 2004. Journal of Range Management, pg. 408)
So where does this leave us? When I spoke to some of the scientists involved in these studies, I asked what they would tell grazing managers interested in increasing carbon uptake. Their advice coincides with guidelines that graziers have used for years. Graze moderately, time grazing so that plants can put on good growth in the spring, and keep the soil covered. As one paper concluded, “Current range management in the northern Great Plains aims to optimize forage production through litter carryover for soil moisture conservation and this appears to be consistent with maintaining the soil organic carbon pool.”
Though the literature we reviewed did not support the hypothesis that grazing increases carbon sequestration, that does not mean that grazing is not an important part of grassland systems. Litter and changing plant communities play a role in the health of grasslands as habitat and potential carbon sinks, and grazing plays a role in these processes. Land conversion from grassland to cropland has also played a role in soil health changes, and conversion of poor cropland to healthy pasture can mean improved soil health, another place where grazing can make a difference. Finally, managing grasslands so that forage is used most efficiently is an important part of protecting soil health and can potentially improve productivity for livestock managers while providing food for our communities. We’ll discuss these and other concepts in future articles in this series.
Click here for a list of the papers reviewed in this series.
Thanks Kathy. I’ve been ruminating on the idea of carbon sequestration in grasslands. I’ve seen some interesting results of farms adopting some sort of managed grazing with relatively rapid (5 yrs or so) increase in SOM. My impression from those places is that their baseline is either arable land that was cropped (poorly?) for many years, or heavily set stocked pastures. Either of those management choices will depress the “natural” SOM level a peice of ground reaches at equilibrium.
The papers I read in your biblio (I didn’t peruse all of them) here looked at more “mature” grasslands at or near equilibrium, and thus showed little or no CO2 sinking.
This is the result I’d expect. There may be some management practices we could tinker with that could bump the SOM up a little higher in a perennial pasture of long standing, but chances are low it would make a big change.
Maybe the answer is not increased carbon sequestration or SOM due to grazing alone per se, which is one of the theories by successful graziers. However, these people manage in a way that promotes ground cover, keeps plants in a vegetative state, and overall land health. Those are the reasons for the carbon sequestration and SOM increase because it allows for water infiltration and limits the effects of drought due to this proper management and their grazing management is the driver behind this.
My thoughts are yes the grazing does do what successful graziers are saying, but the science doesn’t back it up. The studies, according to this article, show that spring precipitation is the reason for the CO2 uptake due to increased biomass and less bare ground which is exactly what good graziers manage for. Compare those two characteristics, biomass and ground cover between properly managed land and poorly managed land during a drought and you will see that grazing management does indeed have a large influence on CO2 sequestration.
Chad, I’m not sure I understand what you mean to say. So to clarify, the research we reviewed shows that good grazing does not have an impact on CO2 sequestration. Bad grazing management, like grazing too early in the season before plants put on enough growth, or grazing that leaves bare ground, can increase respiration of CO2 during the dormant season. Hopefully that helps. Thanks!
Thanks Kathy, what I was trying to say is that the science doesn’t support that grazing has an impact but what mob grazing and HHDPG does is what helps contribute to carbon sequestration, increased biomass and ground cover through the proper management of such systems.
It may be spring precipitation or timely moisture that the research is stating that contributes to the sequestration, but if those areas are poorly managed and bare, then the moisture doesn’t do anything significant. In a round about way, grazing properly more or less causes and is responsible for sequestering carbon because of the improvement of the land and vegetative condition that then sequesters the carbon once timely moisture occurs. Hope that is easier to understand. Thanks again…
Thanks for the clarification. Actually, the areas studied were all well managed, and were on areas that had historically been managed well. So again, grazing was not responsible for sequestering carbon. It was just the precipitation that caused increased biomass. In arid regions scientists have found that in drought times, well managed areas will maintain better, but in wet times, everyone looks good. We’ve seen this over and over. But just because grazing may not increase soil carbon sequestration does not mean it has no role in the system. Managing litter and grazing for wildlife habitat are two important ways grazing contributes to the system. There are others as well that we’ll explore in future articles in this series.
If moisture availability might be important, there will be some farms in hilly country that could direct runoff to ponds or dams, which is very common in Australia. Some of this could be used for irrigation in a dry spring. Selecting some early-maturing and some late-maturing forage varieties might help keep up green growth earlier in spring and later in fall.
The research was probably done in a protocol to eliminate variables. I would suggest testing soils in a real world environment with a known first rate grazier that makes changes throughout the growing season according to natural fluctuations of rainfall.
Scientific study, by its very nature, strives to reduce variables to provide conclusive answers. When you read the articles, you’ll see that because these scientists were working in real world situations, they modified their grazing plans to adjust for natural fluctuations in forage growth and precipitation. In each case they noted the changes they made, how those changes coincide with best grazing practices, and how the changes affected results.
I suspect you will get some rotten tomatoes for this work. Just duck. I am reminded of my favorite Winston Churchill quotation:
“However beautiful the strategy, you should occasionally look at the results.”
The thoughtful graziers I know are trying to do their best to improve things on the ground, and the concept of carbon sequestration is a pretty seductive story. I look forward to more analysis in you coming articles.
Finally, I was heartened by your final paragraph reminding us of all the other good reasons for managing our plants, animals and soils.
Did these studies also review changes in soil organic matter?
While perhaps not a permanent part of the soil carbon, higher SOM helps the system function better. . . healthier soil, more water storage.
another tool worth consideration is the addition of biochar to lower quality soils as a soil amendment, which is more stable than SOM.
We provided a link in the article to all of the studies so you could see everything that was measured.
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