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No-till, cover crops don’t ensure net soil carbon gains; integrated crop-livestock systems should be incentivized

By Nicole Tautges

Interest in buying and selling carbon credits from agricultural lands is rapidly ballooning among the farm and business community. Farmers are looking to carbon credits as a new opportunity to increase farm revenues and profitability, to subsidize the sustainable practices (like cover cropping and perennial cropping) that aren’t as profitable or cost more than conventional row cropping with corn and soybeans. But perhaps most of all, farmers are looking for ways they can be financially compensated by society for performing land management practices that protect and enhance ecosystem services—in other words, the ways that natural or ecological functions benefit humans, like water filtration, clean air, biodiversity, and, importantly for climate change, soil carbon storage.

On the other end of the value chain, large corporations in the U.S. are centering their short- and long-term goals around sustainability and climate change mitigation, with most pledging to achieve “net zero carbon emissions” in one to two decades. These companies have earmarked significant dollars to achieve these goals and, while they are unlikely to cut their fossil fuel emissions entirely, they are looking to purchasing carbon credits to offset the carbon emissions associated with their activities. This has led to formation of carbon markets that “find” carbon storage mechanisms and sell them to carbon buyers.

Nori1 and IndigoAg2 are the two most recent and notable startups buying carbon credits from farmers (surviving for the meantime on investor funding), as farmland is seen as having large potential for greenhouse gas emission offsetting and atmospheric CO2 drawdown. On IndigoAg’s platform, farmers can earn up to $30/acre/year and $50/acre/year on Nori’s platform for adopting practices like no-tillage, cover cropping, nitrogen fertilizer usage reduction, and/or increasing farm rotational diversity. 

However, Nori is not accepting applications at this time, and it is unclear when new opportunities will open up for growers on their platform. Furthermore, equity concerns abound with Nori’s program, as they only enroll growers operating 1,000 acres or more. Farmers at this scale are often the most well off to begin with, perpetuating a system where the “rich get richer” while the “ag of the middle” and small-scale growers are excluded from being able to access the benefits of carbon markets. Nori also requires startup/enrollment fees of at least $3,000 for each farm, so getting a farm enrolled certainly represents an up-front cost. Both platforms are requiring third-party verification of net carbon gain and, in IndigoAg’s case, after enrollment, farmers only get paid if their soils actually gain in carbon. 

A business executive very interested in these carbon markets recently said to me, “Farmers don’t have anything to lose; they get to earn money for carbon credits on top of receiving the benefits of doing the practices.” (The corporate business community she works with is investing heavily in climate change-mitigation efforts.) I’m not so sure. Given the substantial amounts of time (and dollars, in Nori’s case) farmers invest in getting enrolled, there seems to be a substantial risk of not getting paid if the prescribed practices (cover cropping, no-till, rotation diversification) don’t result in net carbon gain. 

That’s where the importance of understanding the “state of the science” of soil carbon sequestration comes in.

The world’s soils were being openly discussed as a potentially important “sink” for atmospheric carbon in the early 2000s. Given that around 30% of the planet’s soils are used for farming, focus turned to agricultural management practices as a means to increase soil carbon. No-till was a practice reported early on to create a net gain in soil carbon. As crop residue was thought to be a major carbon retention mechanism in no-till systems, some called for increases in fertilizer use to increase crop residue inputs to soils, a practice that researchers rightly cautioned against, due to the disproportionately high greenhouse gases associated with the manufacture and transport of nitrogen fertilizer. 

However, globally, soil carbon gains achieved with no-till have been mixed as more studies were conducted at a broader geographic range, with some crop rotations even resulting in losses of soil carbon with the adoption of no-till3. Policymakers (and these most recent carbon markets) have largely ignored the complicating factors and persist in promoting no-till4  as a panacea practice for soil carbon storage and other beneficial conservation outcomes. It is crucial to note here that there is strong scientific consensus around no-tillage being proven to be beneficial for soil retention and the prevention of erosion, a hugely important consideration for choosing management practices that contribute to environmental and agricultural sustainability.

Other practices identified early on, like managed perennial grass establishment (e.g., pasture) and manure/compost application, have been largely confirmed by the majority of studies to result in meaningful increases in soil carbon. Researchers at the University of Wisconsin-Madison, in one of few head-on comparisons of annual row-cropped systems compared to well-managed perennial grass-based pastures, found that perennial grasses were an essential component of cropping systems that achieved gains in soil carbon, whereas losses occurred in all annual systems.

Managed perennial pastures have been observed to increase soil carbon in tropical regions5 as well. (Well-managed pastures include good soil fertility management with either chemical or organic fertilizers.) Long-term application of manure not only improves soil carbon levels, but also benefits soil health indicators6. Some counter that manure or compost application simply recycles agricultural carbon and doesn’t represent net removal of carbon from the atmosphere. However, keeping organic wastes from emitting methane is an important step for mitigating agricultural sources of greenhouse gases, of which methane is the most potent. The carbon footprint of manure/compost transport is important to consider, but greater integration of crop and livestock production across the landscape could decrease transport distances and the carbon cost. The carbon markets make little mention of perennial cropping and manure application in their lists of incentivized management practices.

That brings us to cover crops, a long-incentivized practice by the USDA Natural Resources Conservation Service (NRCS) and others for a variety of conservation benefits, with some of the most important being soil retention and conservation, and reduced fertilizer losses and resultant pollution of ground and surface waters. However, the state of the science on cover crops resulting in meaningful soil carbon gains is far from consensus. An analysis of studies conducted around the globe found that cover crops resulted in modest soil carbon gains7, but the majority of these studies only measured soil carbon in the top foot of soil. Studies that have measured soil carbon deeper in the profile actually observed net losses in soil carbon over 19 years with the addition of cover cropping to a chemically fertilized system8 (compared to the same crop rotation with winter fallow), but large gains in soil carbon were observed with cover cropping plus composted manure application (full disclosure: I co-authored that paper). Cover crops seemed to alter soil microbial activity in the winter relative to a winter fallow system, resulting in greater soil microbial activity and respiration (i.e., microbial emission of soil carbon from intermediate depths in the soil profile). We urged caution with those results as the study was conducted in a semiarid region of California, and soil bio-physico-chemical cycles can differ significantly between semiarid and humid temperate climates. However, notably, a study in Iowa also observed that cover crops significantly altered soil biological cycles and did not promote soil carbon storage9.

It is important to state here, again, that even if additional studies confirm the limited benefit or negative effect of cover crops on soil carbon, their benefits for other environmental conservation goals remain important. Not all agricultural management practices need to be beneficial for every single environmental indicator to merit incentivization. However, when we incentivize a management practice, with either public or private dollars, it is crucial that we understand what outcome we are getting for our investment. Misrepresentation of outcomes will lead to mistrust of climate change-mitigation efforts as a whole, and will waste crucial time in the limited window we have to enact meaningful changes to alter the current trajectory of atmospheric carbon accumulation.

So, if there is greater scientific consensus around managed/fertilized perennial systems and integrated crop-livestock systems (leading to manure application and recycling of organic wastes), why aren’t those the practices being incentivized? 

Probably because they represent the greatest change, especially in terms of economics. Keeping annual row crop rotations and cash crops the way they are now, with their unceasing reliance on corn and soybean in much of the U.S., could be largely preserved and continued, with some (not altogether small) management tweaks, in terms of changing over to no-till equipment and lots of extra work to get cover crops into the ground in fall. Crop marketing and processing could remain the same. Commodity price structures wouldn’t have to change too much. And, as the science suggests, our carbon situation likely wouldn’t change much either.

Conversely, conversion to perennial and integrated crop-livestock systems that result in net carbon sequestration in the soils would necessitate a fundamental restructuring of not just the farm landscape, but also our food system, with many of the intermediate steps between farm and fork needing to be reconfigured. A larger percentage of farms would have to keep livestock and hundreds of miles of fencing would need to be (re)erected across America. New marketing structures would have to be developed for meat and other livestock products; we would have to find ways to NOT reject meat and livestock product consumption, but rather find a way to make livestock a sustainable part of our agroecological landscapes and diets (or fiber systems). New perennial crops may need to be developed and adopted10, and food product innovations would have to occur that don’t stick soy and corn byproducts into everything under the sun. 

This level of change sounds really hard. No wonder many prefer the quick supposed fixes of winter cover crops and no-till. I wish they decisively worked for carbon offsetting, too. (Again, let’s not forget that they are beneficial for other conservation goals.)

These new carbon markets are pretending they’re a new frontier for sustainable and regenerative agriculture, but they’re not embracing the hard, complicated work that needs to be done to transform our carbon, financial, and human farm/food economies. Let’s work together, urban to rural, corporations to grassroots, federal and local, to construct new financing structures that do support the hard, system-wide changes that need to happen in our agricultural and food system landscapes.

Dr. Nicole Tautges also provided this brief reminder:  Earth’s atmosphere is mostly made up of atmospheric nitrogen (luckily for Rhizobia bacteria and legumes), followed by oxygen and argon, in gaseous form. Carbon dioxide, nitrous oxides, and methane make up less than 0.5% of the atmosphere. While nitrogen and oxygen gases do not interact with infrared radiation (the “heatcausing” portion of solar radiation), carbon dioxide and the other trace gases mentioned above do, first absorbing heat-creating infrared waves and then re-emitting about half of them into our atmosphere. That’s why an increasing portion of our atmosphere being carbon dioxide, nitrous oxides, and methane (the worst!) causes warming of our planet.

Nicole Tautges is an agroecologist with Michael Fields Agricultural Institute.

References:

1 https://nori.com 2 https://www.indigoag.com 3 Govaerts et al. 2009. Conservation Agriculture and Soil Carbon Sequestration: Between Myth and Farmer Reality. Critical Reviews in Plant Sciences 28, 97-122. 4 Graham et al. 2021. Modest capacity of no-till farming to offset emissions over 21st century. 5 Thomas et al. 2012. Whole farm net greenhouse gas abatement from establishing kikuyu-based perennial pastures in south-western Australia. Animals 2, 316-330. 6 Su et al. 2006. Long-term effect of fertilizer and manure application on soilcarbon sequestration and soil fertility under the wheat–wheat–maize cropping system in northwest China. Nutrient Cycling in Agroecosystems 75, 285-295. 7 Poeplau and Don. 2015. Carbon sequestration in agricultural soils via cultivation of cover crops–A meta-analysis. Agriculture, Ecosystems & Environment 200, 33-41. 8 Tautges and Chiartas et al. 2019. Deep soil inventories reveal that impacts of cover crops and compost on soil carbon sequestration differ in surface and subsurface soils. Global Change Biology https://doi.org/10.1111/gcb.14762. 9 Ye and Hall. 2020. Mechanisms underlying limited soil carbon gains in perennial and cover‐cropped bioenergy systems revealed by stable isotopes. GCB Bioenergy 12, 101-117. 10 https://www.forevergreen.umn.edu

 

From the March | April 2021 Issue

 

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