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Understanding the role of carbon in agriculture – part 2


EAST LANSING, MI. – In part 1 of this series on carbon in agriculture, we discussed how growing crops such as corn assimilate tremendous amounts of carbon (C) through the process of photosynthesis. We also discussed how C flows between the plant/soil continuum and the atmosphere as part of a natural cycle. This article will discuss how a farmer can latch on to some of the C flowing through the plant/soil continuum and hopefully retain a fraction of it in the soil. It is not easily accomplished, but is it really as difficult as catching lightening in a bottle?

The naturally occurring annual microbial decomposition of crop residue such as corn stover remaining in the field results in the respiration of about 80% of the C addition of the stover and root residue plus an incrementally decreasing portion of the previous years’ plant material not decomposed in its first year. By implementing C-smart best management practices such as no-till and cover crops, most estimates for long term stable annual CO2 retention hover in the 300 to 1,000 pounds per acre per year range, which varies significantly based on local soil and climate conditions.

Furthermore, the C accumulation factor for a given best management practice is generally considered valid for a period of about 50 years, after which the soil will reach a new equilibrium level of C and further increases in soil C attributable to that particular management practice would not be likely. Thus, it takes a long time to effectively sequester appreciable amounts of C in the soil.

However, some of the emerging markets are willing to pay in the range of $10 to $15 per acre, or about one credit (one ton of CO2 equivalent) to a farmer for implementing a practice such as no-till or cover crops. Perhaps more important to the farmer and society are the well documented agronomic and environmental advantages of increasing soil carbon such as: decreasing soil erosion, increasing water permeability, improving soil structure, increasing the rate of spring soil warm up, etc. We’ll continue the discussion on the ecosystem services provided by C-smart best management practices in Part 3 of this series on carbon.

For perspective, consider the atmospheric emissions of C from fuel combustion. For each gallon of gasoline combusted, 19.4 pounds of CO2 are released into the atmosphere. Nationally, the U.S. consumes approximately 140 billion gallons of gasoline per year, effectively pumping 2.7 trillion pounds of CO2 into the atmosphere annually. On average, crude oil being consumed today, represents C from plant material that was sequestered below the surface of the earth some 300 million years ago. Therefore, the annual U.S. 2.7 trillion pounds CO2 fuel emissions constitute new carbon, or at least carbon that has not been in the atmosphere for over 300 million years! Therein lies the basis of the growing climate change challenge.

On a national basis, the U.S. typically grows about 92 million acres of corn. Collectively, assuming the 300 to 1,000 pounds per acre CO2 sequestration rate, no-tilling all the nations corn crop would potentially sequester about 28 to 92 billion pounds of CO2. A respectable amount to be sure, but unfortunately, it only amounts to 1 to 3.5% of the CO2 emitted from our gasoline cars on an annual basis! Obviously, we’re not catching much lightening in our bottles as long as we continue to pump 2.7 trillion pounds of CO2 into the atmosphere annually from gasoline consumption.

However, the real power of agriculture in the C capture and storage scenario lies in the coupling of renewable fuel production from agriculturally grown feedstocks with carbon capture and storage. In other words, by using renewable energy to displace fossil fuels, we can effectively ratchet down our current 2.7 trillion pound CO2 gasoline emissions. Unlike gasoline and other fossil fuels, the CO2 emitted from bioethanol is simply carbon recycled back to the atmosphere from where it originated a year or so previous. The U.S. Environmental Protection Agency (EPA) conservatively estimates conventionally grown corn grain ethanol to be a 21% direct impact reduction in carbon footprint relative to gasoline. When cellulosic feedstocks from perennial grasses like switchgrass are used for biofuel, research at MSU and other universities has shown the reduction in carbon footprint compared to gasoline jumps to over 100%!

In the future, a combination of electric and biofuel vehicles all powered from renewable feedstocks could potentially wipe out the annual 2.7 trillion pounds CO2 emission from gasoline and provide a net gain of C sequestered into our soils where it can provide environmental and agronomic advantages. Now that’s catching some real lightening in a bottle!