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]
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)
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.