I read an interesting paper recently talking about soil carbon restoration and the practices that are needed to do it right.
I would submit that it’s difficult or impossible that clean tilling your fields will accomplish much of anything except vanquishing your profitability to the Heavens. But it’s still interesting to explore how no-till, cover crops and high-biomass systems get the job done.
“Bare soil oxidizes carbon, while plants protect it. Green plants form a barrier between air and soil, slowing the process of carbon emission, says Jack Kittredge, policy director for North East Organic Farming in Australia.
He says organic cropping systems with the highest levels of carbon restoration are those with no-till because they are adding organic matter to the soil. “Cover crops are essential in any organic strategy to reduce or eliminate tillage, control weeds, and build soil carbon,” he adds.
Although some environmentalists raise concerns about raising large numbers of ruminant animals because they give off greenhouse gas in the process of digestion, “it is only when ruminants are away from biologically active soil or water, such as in feedlots, that ruminant emissions can be of concern,” Kittredge says.
He touches on some interesting points about soil organic matter and how it’s stored in no-tilled fields. There are some studies that report that the soil carbon gains of no-till are not distributed deeply through the soil profile, but rather occur mostly near the surface.
“This is a problem, they suggest, because the best chance for humus formation and long-term carbon stabilization seems to be deeper in the soil, closer to clay and minerals to which the carbon can bond to resist oxidation,” Kittredge says. “They also argue that the kind of soil organic matter produced under no-till management is only incorporated in the sand/soil fraction of the soil near the surface and is easily oxidized upon eventual disturbance.”
Some studies also point to the shallowness of organic matter build-up under no-till. But they also report a slow deepening of soil organic matter after 10-15 years under the system, presumably because of both decreased organic matter decomposition and long term soil mixing by larger soil organisms.
Cover crops, crop rotation, mulch and equipment innovations are emerging that give organic no-till practices a boost for those wanting to move away from tillage. For many of you who already no-till those aren’t things that need constant review.
And here’s more about how grazing ruminants can help with organic matter: “The grazing itself promotes the growth, then sloughing off, of grass roots — which provides carbon to feed hungry soil microbes,” Kittredge says. “Pastures and perennial systems, if properly managed, can show rapid increases in organic matter. Animal manure is one of the most valuable products of the small mixed farm, rich as it is in both carbon and the living microbes that inoculate soil with biological diversity.”
Pasturing is a highly effective method of agriculture to restore soil carbon. A recent study of land converted from row cropping to management intensive grazing showed a remarkable carbon accumulation of 3.24 tons per acre per year, he says. That’s in the range of deep-rooted African grasses planted to savannas in South America that achieved rates of 2.87 tons of carbon an acre per year.
Kittredge says part of the efficiency of pastures at fixing carbon is probably related to the fact that several grasses use the C4 photosynthetic chemical pathway, which evolved separately from the more usual C3 pathway.
Particularly adapted to situations of low water, high light and high temperature, C4 photosynthesis is responsible for some 25%-30% of all carbon fixation on land, despite being used by only 3% of the flowering plants. (Muller)
“If we want to survive,” Kittredge says, “we really have no alternative but to restore carbon to the soil. That this can be done through biology, using a method that has worked for millions of years, is exciting.”
I don’t think I could say it any better myself.