The USDA’s Natural Resources Conservation Service evaluated 160 conservation standards, and identified 35 as positively reducing greenhouse gas emissions and increasing carbon sequestration. They provide an on-line tool, COMET Planner, that estimates the potential reductions at the county level for each of these practices, and for some combinations of them.
We would presumably get the most sequestration through converting the cleared land back to mature forest. If that’s correct, then anything we do about agricultural sequestration would be well below the forest estimates. Most of the NRCS estimates are. Here are a few examples:
Converting 1,000 acres of pasture to silvopasture by adding trees and shrubs sequesters 1,330 metric tons CO2e a year (about half the iTree County estimate for mature forest, which makes sense.) Converting to prescribed grazing on 1,000 acres of non-irrigated pasture sequesters 26 metric tons CO2e a year. Adding a seasonal legume cover crop to 1,000 acres of irrigated cropland sequesters 124 metric tons CO2e a year. (They estimate this emits 46 tonnes of nitrous oxide a year, which reduces some of the benefits of the 170 tonnes of CO2e sequestered.) Shifting 1,000 acres of irrigated cropland from intensive tillage to no-till or strip-till cultivation sequesters 234 metric tons CO2e a year. Shifting 1,000 acres of non-irrigated croplands to no-till or strip till, using a seasonal legume cover crop, and replacing synthetic nitrogen fertilizer on them with compost sequesters 324 metric tons CO2e a year.
Project Drawdown’s first global estimates for conservation agriculture were somewhat higher. The book says that “conservation agriculture” – a combination of minimizing soil disturbance, maintaining soil cover (leaving residues or planting cover crops), and managing crop rotations “sequesters a relatively small amount of carbon—an average of half a ton per acre. But given the prevalence of annual cropping around the world, those tons add up.” The updated estimates in the technical summary on the new version of their website are more specific. They now estimate sequestration in temperate/boreal areas at .38 tons of carbon per hectare per year, and the emissions reductions (presumably from reduced nitrous oxide emissions) at .23 tons of CO2e per hectare per year. (The sequestration estimates for tropical areas are twice that, which presumably accounts for the higher global estimate in the text.) That looks like US tons; if so the sequestration would work out to .35 metric tons of carbon per hectare and the emissions reductions would be .21 metric tons CO2e per hectare. Since a ton of carbon is the equivalent of 3.67 tons of CO2, the sequestration is the equivalent of 1.3 metric tons of CO2e per hectare, and our total offset using their estimates would be 1.5 metric tons CO2e per year per hectare or .61 metric ton CO2e per acre. Our 1,000 acres would sequester around 610 metric tons a year using the new estimates. (Their sequestration estimate, by itself, is .52 tonnes/acre, just over 20% of the iTree County estimate for sequestration by areas with mature canopy, which is 2.5 tonnes/acre.)
The only apparent differences between “conservation agriculture” and “regenerative agriculture” in the original Drawdown book’s categories were that “regenerative agriculture” added a few things to “conservation agriculture” – “diverse” cover crops instead of just cover crops, in-farm fertility (no external nutrients), and no pesticides or synthetic fertilizers. The new website changes the name of this category to Regenerative Annual Cropping, and shifts the definition slightly. It now consists of any four practices selected from the ones in conservation agriculture and these additional ones.
The new Technical Summary for this section estimates the annual sequestration from these practices in temperate/boreal areas at .6 tons carbon per hectare, or .24 metric tons per acre. This is the equivalent of .88 metric tons of CO2e/acre. They estimate the annual emissions reductions (presumably from reductions in nitrous oxide emissions) at .23 tons/hectare per year, or .09 metric ton per acre. Together, this would make the annual reduction from our 1,000 acres 970 metric tons, about 36% of the iTree forest sequestration estimate.
We can use this estimate to get some sense of the maximum possible benefits of steps like these. By way of scale, we have about 22,000 acres of cropland in the county. At Drawdown’s estimate, “regenerative agriculture” on all of that might sequester something like 21,400 tonnes a year. Our inventoried 2016 emissions were 2,965,754 metric tons/year so that would offset about 0.72% of them. Through a somewhat different perspective on relative scale, an average gasoline car emits 4.6 tonnes/year, so that’s the equivalent of taking 4,652 cars off the road. It is a lot, yet it still isn’t much in terms of the scale of our inventoried emissions.
There seems to be a considerable gap between what conventional agronomists think is possible and what the advocates for regenerative agriculture think is possible… A recent piece by a dubious soil agronomist at WSU gives an overview of some of the issues, with a lot of links to blog posts about related issues and to research articles. I think the comments at the bottom of it give you a look at what comes up about the differences in a civil conversation between these two camps.
Even at the reported gains Drawdown uses for “regenerative agriculture”, it doesn’t look to me as if this can be a major factor in reducing our emissions.