“Carbon farming,” in my world as a scientist who studies soils and crop production, seems to be all the rage these days in the media. The idea is to build up carbon in soil while drawing down carbon in the atmosphere through improved soil management. This matters because building healthy soils requires adding carbon, while stabilizing the climate requires taking carbon out of the atmosphere. I want to dig—pun intended!—deeper into the nitty-gritty and discuss what we already know about a few agricultural practices that commonly come up in this discussion.
Let’s take a quick trip back to high school science class for a “carbon farming” primer. All plants pull carbon out of the atmosphere through what might seem like the magic process of photosynthesis. In plants, carbon dioxide plus water and sunlight are converted to sugars, with carbon as the key backbone of these chemical compounds. Some plant material is removed from farm fields and rapidly consumed by humans, animals and other organisms but much of it is not (think about plant roots or plant leaves versus the fruits or seeds we normally eat). Carbon farming is about ensuring that the carbon that stays behind in farm fields is effectively captured and banked in soil for the long term.
Much of the carbon farming conversation revolves around a series of well-established conservation practices that protect the soil, such as keeping continuous soil cover (cover crops) and minimizing soil disturbance (reduced- or no-tillage). I’ll delve into just those practices here and will also discuss more ecological practices in a separate post. For a more thorough look at the science of soil carbon sequestration (long-term capture), the National Sustainable Agriculture Coalition recently released a report on the topic and a paper out last month in the journal Nature details the idea as well.
How much do these commonly discussed practices make a difference? The science is on!
I specifically mention cover cropping and reduced plowing because of the wide use and general acceptance of these solid conservation agriculture practices by scientists and industry. But, what do we know from actually measuring their effect on soil carbon?
Cover cropping refers to the practice of producing a crop that is not primarily intended for harvest. This is particularly apt when the soil would otherwise be bare, and in principle this has the potential to increase carbon in the soil. From a carbon “budget” standpoint, adding a plant that you do not remove can be a potential net carbon input. A synthesis of field research found that cover cropping practices increased topsoil carbon to an average level that—if applied to 25% of current cropland globally—could offset approximately 8% of agricultural emissions. The authors of that analysis recognize this to be a rough estimate, but it does represent a start on soil-based climate mitigation. Agricultural emissions, keep in mind, make up approximately 11% of overall global emissions, so even if this one practice could make a difference within the agricultural sector, a lot more work would be needed to offset as much as possible of the remainder.
Soil tillage (plowing)
Disturbing soil, as with plowing (to prepare seedbeds) or cultivation (to control weeds), is the main purpose of soil tillage. This disturbance can expose carbon to decomposition, with a significant fraction of this eventually emitted as greenhouse gases. However, there are planting and weed control techniques that do not require these modes of soil disturbance. Just how much can the reduction (or elimination) of soil disturbance increase soil carbon?
Scientists have found that such “no-till” practices may increase topsoil carbon while simultaneously reducing it in the subsoil, where plowing would normally move plant residue from the soil surface to a deeper layer (see here, here and here: these are “meta-analyses,” which summarize the outcomes of numerous field studies, and are a powerful way to understand separate experiments). For this reason there is much scientific debate about tillage and carbon sequestration; many studies do not measure carbon beyond the topsoil (soil surface layer.)
It has also been found that just one year of plowing after multiple years of not plowing can negate years of carbon accumulation. Such a practice is often necessary after a sequence of years without plowing or cultivation. As a result, some researchers believe the overall carbon storage potential from reduced tillage alone may be overstated. It is also worth noting that on many farms, reduced- or no-till practices are made feasible by replacing tillage with herbicides, most commonly paired with genetically engineered herbicide-resistant crops.
Farmers constantly need to evaluate trade-offs like these. This calls for the further development of agroecological techniques that suppress weeds without forcing farmers into the false choice between disturbing soil (and the consequent greenhouse gas emissions) or contributing to the development of herbicide-resistant weeds.
These are useful practices—but we need to dig deeper
The bottom line is that the best soil management techniques in an annual cropping system optimized to remove large amounts of carbon as grain or forage can have at best a minor role in reversing climate change. Ultimately, an agricultural system redesign with more complexity and featuring perennial vegetation will likely be necessary for soil and crop management to reverse net carbon losses.
However, there are additional sound agricultural reasons to reduce tillage and increase soil cover. Plowing the soil can degrade soil structure and reduce its ability to maintain healthy nutrient and water cycling (as is vividly seen in this demonstration). Further, given the many co-benefits of topsoil carbon, such as greater ability to store water, small increases of soil carbon can have large impacts.
In a companion post, I look at a series of other agricultural practices that might offer more on the carbon storage front.