Recently I’ve been thinking a lot about sequestration. No…I’m not delving into the world of political analysis; honing such a craft disregards the value of one’s physical and mental health, if not life.
My thoughts are squarely centered on soil carbon sequestration. It’s been many years since I’ve moved carbon to a place of hierarchy within my brain’s gray matter. The element was shoved to the back row seconds following my final college biochemistry exam. Visions of carbon molecules dancing around something called the Krebs cycle still haunt me to this day.
Much has happened since that last round of biochemistry class. Like a dropped-off barn cat, carbon has found its way back into my life by being at the forefront of some major agricultural, environmental, and energy policy discussions.
Element number 6 on the periodic chart has woven together a complicated linkage among important issues related to crop production, biofuels, and climate change.
As with life itself, this story begins with the soil. We don’t talk in terms of fertilizing with carbon, yet its role in a healthy, productive soil is unquestioned. Carbon binds soil particles together to form aggregates, provides food for microbes, enhances air and water movement, acts as a docking station for essential plant nutrients, reduces erosion, and combats soil compaction.
Most discussions about soil quality speak of organic matter percentage, not soil carbon content. In fact, the two entities are synonymous. Organic matter is 57 percent carbon by weight.
Carbon enters the soil by way of plant photosynthesis and exits into the air as carbon dioxide, primarily the result of plant and soil microbial respiration. How much carbon remains in the soil is largely a function of environmental conditions, crop management, and tillage practices.
It’s a slow and difficult process to raise the organic matter content of a soil. Poor crop management reduces the amount of crop residue that is returned to the soil, while aggressive tillage can send carbon dioxide spewing into the air and organic matter into lakes and streams through erosion. These are the reasons why organic matter generally decreases as landscapes convert from native vegetation to cultivation.
Practices such as no-tillage, strip tillage, cover crops, and growing perennial forages slow the decay of organic matter, which aids in the sequester soil carbon.
Planting seed into piles of soil surface residue rarely meets with success. For this reason innovations and improvements in planter, grain drill, and tillage implement technology have largely been centered on preserving crop residues while at the same time creating favorable seedbed conditions. The result: most planters now weigh and cost as much as an Oshkosh truck.
As this quest for all things good continues, enter the biofuels movement–specifically cellulosic ethanol. Many researchers feel that the most feasible and efficient means of ethanol production is from plant fiber rather than grain.
There is currently a large U.S. research effort looking toward switchgrass, corn stalks, and other plant material as a source of biofuel. At the same time, it’s also understood that routinely removing large amounts of crop residue from the field, especially for annual crops like corn, may have severe negative consequences on soil organic matter content. In essence, the carbon once returned to the soil would now be burned for fuel. It’s just never easy.
If the biofuels dilemma wasn’t enough, the climate change debate also puts carbon front and center and has agriculture sitting in the front half of the room. Carbon dioxide, along with methane and nitrous oxide, are the greenhouse gases most often associated with agriculture. Atmospheric carbon dioxide levels have increased significantly since the Industrial Revolution in the late 1800’s.
Agriculture accounts for only about 7 percent of total greenhouse gas emissions. Though cultivated soils contribute to carbon dioxide emissions, they also provide a potential mitigation solution as a large sink to capture and hold atmospheric carbon. In other words, crop production practices that help maintain or build organic matter not only improve soil quality and increase crop productivity, but they also reduce greenhouse gases.
Both carbon and money sequestration issues have evoked a lot of finger-pointing, index or otherwise. The solutions to these problems are complicated and will involve some level of compromise. Our government leaders and scientists must heed the famous words of basketball coach Jim Valvano, “Don’t give up. Don’t ever give up.” The long-term health of our economy and environment are at stake. So could be our ability to feed the world.
Mike Rankin, Crops and Soils Agent, UW Extension-Fond du Lac County