The Climate Summit in Paris in December 2015 initiated the 4 per Thousand Program. The strategy of this program is to sequester carbon in soils of the world at the rate of 0.4% per year in the top 16 inches of soil.
“With the introduction of perennials like switchgrass into their crop management plans, farming carbon credits and energy may prove a valued product for farmers to diversify into.”
Fields of Potential
U.S. croplands pose great potential to make such goals realistic. A cornfield for example would take in about 8 tons of CO2 per year per acre. To reach the .4% goal, 64 pounds of CO2 per acre needs to be sequestered. However, this carbon must be stable and remain in the soil over very long periods of time.
Part of this loss comes from traditional tilling practices. Intensive tillage followed by fallow fields for the production of corn and small grains have resulted in a loss of more than half of soil’s original carbon content over time.
A study at the Northern Great Plains Research Laboratory using spring wheat cultivated with traditional tillage practices showed how differences in cropping systems affect soil structure and soil carbon. In the traditionally tilled spring wheat, the soil had 14% water-stable aggregates, and the carbon in the top 3 inches of soil measured 6.6 tons per acre. No-till continuous cropping of spring wheat/winter wheat/sunflowers increased soil quality by comparison. No-till soil had 47% water-stable aggregates, and the carbon measured 9.6 tons per acre.
This soil carbon is still not stable though. The plant litter and root structure left as the crop residue will be decomposed by bacteria, which convert only 20 – 30% of the carbon into new cells with 70-80% being released into the atmosphere. These bacteria die quickly as new bacteria replace them. The dead bacteria are consumed by nematodes and protozoa that live in the soil and release nitrogen and other nutrients the plants need in a form the plant can use.
We are progressing but the carbon content in the top 3 inches of that system is still not stable. It also does not account for the Nitrous oxide released by this decomposition and nutrient release process.
What Can Save the Prairie?
A five-year on-farm study by the Agricultural Research Service evaluated switchgrass for ethanol production. The study encompassed 10 farm fields in Nebraska, South Dakota, and North Dakota. The fields were located in marginal land areas that would have qualified for the CRP.
In the study, they found soil organic carbon increased across all sites at a rate of 980 pounds per acre per year within the top 12 inches of that soil. In Nebraska in particular, where four sites were sampled to a depth of 48 inches, carbon increased at an average rate of 2,590 pounds per acre per year.
These were average increases measured over 5 years and represent actual stable sequestration of carbon. At the end of five years, 6.5 tons of carbon was stored. The permanent deep roots of switchgrass supported fungi, which annual crop roots cannot do. Even with a cover crop, biological activity takes place primarily near the surface by bacteria. The fungi are 40-55% efficient in converting carbon to cells and as a multi-cell structure they can live a long time and sequester carbon in a stable manner.
The temporal cycle of carbon in the topsoil with bacterial dominated ecosystems is big, but fast. That is, it contains a lot of soil carbon that is converted into CO2 quickly, thereby limiting its sequestration value. Carbon in the deep subsoil dominated by fungi is not so big, but slow to be converted resulting sequestration times of many, many, many years.
It is my understanding that the Ecological Services Market Consortium will initially be assigning a credit value to a cover crop, no till practice. We will have to wait to see how they arrive at their figures. Whatever their final calculations are, they will not match the sequestration capable from a perennial, deep-rooted energy crop.
From another revenue angle, switchgrass can produce twice the biomass of corn stover per acre. After establishment, a stand can remain productive for over 10 years before it needs to be replanted.
Sequestering new carbon in the subsoil at a modest rate of .8 tons per acre for the first year (the equivalent of 2.9 tons of CO2 when released and not counting the reduction of nitrous oxide emissions), carbon credits at $54/acre ($15/ton CO2) would equal $43 and increase after that. Add to that about $35/ton value as energy at 9 tons per acre, or $315 per acre.
Harvesting costs are about $25 per acre or less with purpose designed machinery. Up front planting costs are more. Some fertilizing to get 9 ton per acre will be necessary, but after that little ongoing costs are incurred. These are conservative estimates and very favorable compared to corn economics. (Other figures have calculated far greater carbon sequestration. There is no set calculated standard.)
With the introduction of perennials like switchgrass into their crop management plans, farming carbon credits and energy may prove a valued product for farmers to diversify into.