Not Just Dirt – Soil as a Carbon Sink

Emma L. Locke, Ph.D.

What if we could simultaneously sequester huge amounts of climate-changing carbon dioxide and help farmers and ranchers improve their land’s productivity, all while alleviating the effects of drought? We need to put carbon into the soil, rather than the atmosphere.

When it comes to climate change, the carbon cycle is both the problem and the solution. Photosynthesis captures energy from sunlight, converting carbon dioxide (a gas) into carbohydrates (for my purposes, solids). While still in plants, carbohydrates can be turned into cellulose, amino acids, fatty acids, etc., which can be consumed by animals or microbes. Since they are solids, they don’t contribute to global warming. Both respiration of carbohydrates by living organisms and combustion of fossil fuels release energy held in carbon bonds and give off carbon dioxide as a waste product. That second part of the cycle – where carbon dioxide is released – is where we have a problem.

However, if we focus on the first part of the cycle, on photosynthesis, we might have hope. Humans have used products made from fast-growing plants, like bamboo, to utilize more carbon in its solid form. But we can only use so many bamboo products. We have a carbon storage problem.

2015 was the Year of Soil, but that didn’t make headlines. Soil can be a huge carbon sink. Worldwide, under best management practices, soils could take up as much as 4-5 gigatons of carbon dioxide per year. Adoption of new technologies could increase that number even more.

Soil holds carbon both as carbon dioxide and as solid carbon. Solid carbon, which soil scientists refer to as soil organic matter (SOM), can stay in the soil as dead matter and as microorganisms for hundreds of years before being turned into carbon dioxide. Now – here’s the best part: in undisturbed soil, higher levels of carbon dioxide depress microorganism activity, causing them to break down SOM slowly.

When the opposite happens, through soil tillage, microorganisms exposed to a burst of oxygen complete their life cycles quickly and SOM is broken down much faster. Therefore, carbon dioxide is released into the atmosphere both from the carbon dioxide gas being released as the soil’s physical surface is disrupted and from the breakdown of SOM.

Unfortunately, this was part of the reason that the Great Plains were tilled up leading to the Dust Bowl. Not understanding the process, agronomists in the early 1900s recommended soil tillage because that breakdown of SOM also allowed mineral nutrients held in the SOM, such as nitrogen, to be released, essentially becoming fertilizer through the tillage process.
So, if soil tillage is bad, why do farmers still practice it? With the loss of organic matter from repeated tillage, the fertilizing effect is gone. There are three reasons. Weeds, compaction, and surface litter.

First, crops don’t grow well with weed competition for sunlight, water, and nutrients. Tillage is one way to control weeds.

The second reason is compaction. Tillage is a vicious cycle. When a soil has been tilled, it will have low SOM. In clay soils, like we have in most of Colorado, the depletion of SOM makes the clay hard, and that makes it difficult to plant and for seeds to germinate. Water runs off a hard soil very fast, so farmers till to make a nice seedbed where water will soak in.

The third reason, surface litter, is related to the second, but it’s harder to understand. In untilled systems, leaves and plant debris on the soil shelter it, leaving it moist and cool. Have you ever sunk your fingers down through the grass to the soil level while waiting for fireworks to start on the Fourth of July? Even during summer, the soil under the thatch will be cool and moist. Microorganisms can thrive in that environment, so the plant litter left on the soil surface after harvest decomposes quickly. The problem is, in irrigated and tilled systems, the plant litter left on the soil surface after harvest can’t be broken down quickly because the soil isn’t protected, and the microorganisms have starved. When a farmer irrigates, plant litter from the previous crop impedes water flow and creates a big mess, like log jams in a river.

We absolutely must enable farmers to implement no-till systems and adopt other soil conservation strategies. We must provide incentives and support for ranchers and other land managers to improve soil health. We need to prioritize the USDA in the same way that we have prioritized the National Institutes of Health because of cancer and lifestyle diseases. USDA funded grants for soil conservation research and implementation not only allow us to work on climate change, they make land more productive, help us improve water conservation and prevent climate disasters like the Dust Bowl. The money could not be better spent.

Emma L. Locke, Ph.D.
Agricultural Policy Advisor
McCorkle for Colorado
Paid for by McCorkle for Congress
(720) 675-2753
PO Box 6664, Longmont, CO 80501