Roger Gates: Sequestered carbon and grassland management
We are privileged to live in a country where we are represented by those we elect and we can offer our sentiments to those who represent us. The political season is upon us and the volume of the discussion seems to amplify daily. While I regret with many the limited civility in our current civil discourse, election season does draw attention to fundamental and important issues. Perhaps indirectly, but sometimes persuasively, the issue of climate change does focus many on the importance of natural resources and the manner in which we care for them.
It is not difficult to predict with confidence that climate will change. From historical records it’s abundantly clear that current climate conditions are not identical to those in the past. The contentious issue is whether we can assign causative factors to the modifications we observe and with what level of confidence we assign causation and predict future change.
A highly visible current premise is that atmospheric carbon dioxide is increasing and that this accumulation leads to changes in climate conditions such as average temperature. A further presumption is that increased atmospheric carbon dioxide results from human activity, such as combustion of fossil fuels. I won’t attempt to weigh in on the appropriate level of confidence to assign to this line of reasoning, only to comment on what we may have learned about management of grasslands in relation carbon cycling. If elevated carbon dioxide is the result of man’s activities, perhaps alternative activities hold the promise of reducing those levels.
Remembering that climate change is certain, regardless of whether we can predict the direction or magnitude of the change and whether we can assign cause and effect, it will be beneficial to care for our resources as well as we can.
So, what do we know about grassland management and atmospheric carbon levels? Carbon dioxide is an essential ingredient in the cycle of plant life. Fixation of carbon dioxide through photosynthesis is required to accumulate the energy needed for plant processes and the accumulation of plant tissue. Photosynthesis removes atmospheric carbon dioxide and deposits it in living plants. One pathway for that carbon is to be oxidized, through respiration by the plant or animals that consume plants or by the processes of decomposition. This returns carbon to the atmosphere as carbon dioxide resulting in little net change.
An alternative pathway in grasslands is for plant material to mature and breakdown as part of the litter or thatch layer that accumulates on the soil surface. This litter may be further reduced contributing to the accumulation of soil organic matter, which contains a substantial carbon component. Dying roots also contribute to the soil organic matter fraction.
Soil carbon is less labile than above ground plant material and may be termed “sequestered,” a more-or-less permanent removal of carbon from the atmosphere. One alternative procedure then, that might “remove” carbon from the atmosphere, would be grassland management that would result in increased soil carbon. Benefits to plant production from increased soil organic matter are well documented, so such management should also improve grassland production.
One of our grad students examined some grazing studies in the northern plains for a recent seminar. These included studies in the fescue grasslands of Canada and from the ARS lab at Mandan, ND.
One surprising conclusion was that a vegetation community dominated by shallow-rooted grasses led to higher levels of soil carbon. In the North Dakota study, which was initiated in 1916, three treatments were compared: no grazing; moderate grazing; and heavy grazing. Over time, no grazing shifted vegetation from native cool-season mid-grasses to a Kentucky bluegrass-dominated community. Compared to moderate grazing, heavy grazing shifted vegetation to a community dominated by warm-season shortgrasses. Soil carbon accumulation was lower in the moderately-grazed mid-grass community that either of the alternatives.
Current proposals for management incentives for grassland management (carbon credits) are based on the presumption that management which moves vegetation toward greater diversity and productivity results in greater carbon sequestration. Research support for this conclusion is not unequivocal. Further, the testimonial evidence of practitioners of contemporary intensive management approaches such as mob grazing are that procedures are available to improve soil organic matter dramatically and rapidly.
The societal benefits of improved grassland habitat and/or increased soil carbon are not inconsequential. One way to achieve this end is to provide incentives to those who manage the grasslands. Relationships between grassland management alternatives and soil carbon levels remains to be clearly quantified.
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