The Role of Soil: Carbon Sink, Water Sponge

We may see the soil under our feet as simply dirt, with all the associated folk expressions – common as dirt, say. But farmers and scientists alike have long known that a healthy soil system is not just the key to our agricultural cycles; soil is one of the major building blocks of farm viability, floodwater management, carbon sequestration, ecosystem productivity, and a clean atmosphere. All this is especially true as we move forward into an era of increased confrontation with anthropogenic climate change.

For one thing, soil is not a homogenous entity, and it is dynamic in the way that it absorbs and releases carbon. All told, the earth’s soil holds 75 percent of the planet’s land-based carbon. 

Art by Erok; Courtesy of RUral Vermont

Art by Erok; Courtesy of RUral Vermont

At its most basic level, carbon is added to the soil through the growth and decay of plant roots. Through photosynthesis, plants assimilate carbon from the atmosphere into their tissue. As carbon-rich plant material decays, its carbon is incorporated and stored as soil organic matter. More growing plant material corresponds to an increase in carbon sequestration in plants and soil. Meanwhile, healthy microbial communities enhance this sequestration.

In reverse, carbon can be released from soil into the atmosphere, especially after a topsoil layer has been tilled, cultivated, or otherwise disturbed. When microorganisms in the soil meet open air, they react with oxygen, forming carbon dioxide through respiration and decomposition. Once a field has been cultivated, it is estimated that the soil will lose more than half of the carbon that has been stored safely underground. Large carbon releases go hand in hand with other environmental problems associated with agriculture: soil erosion and greater nutrient runoff into waterways.

Agriculture in and of itself does not stand in the way of using soil to store carbon. In the process of planting, farmers have a number of choices to make in how they manage the carbon in their soils. “No-till” cultivation leaves topsoil in place instead of degrading it through plowing or the use of heavy machinery, which hardens soil surfaces. Leaving mulch on top of soil or using cover crops can reduce erosion in an agricultural area while allowing it to retain more water. What’s more, raising animals on pasture, whether for dairy or meat, avoids the carbon release of soil tillage, since soils sown with grass are protected from erosion and aren’t tilled in the way of row crops.

While the carbon cycle may seem abstract to non-chemists, soil’s important place in the water cycle will be evident to any Vermonter who has ever lived through mud season. Healthy soil that has seen the least disturbance will retain the most water, both during periods of drought and periods of flooding. Well-managed agricultural lands, with healthy root systems that hold onto water, play a similar role. A 2016 report by the Composting Association of Vermont, supported by High Meadows, notes that soil’s ability to act as a sponge is what allows land to store water rather than letting it run off into rivers and streams. In light of future floods and severe storms, water storage is more important than ever.

The more grass-covered farmland we have in Vermont, the more possibility there is for topsoil to be both a carbon sink and a water sponge, a resource that holds carbon out of the atmosphere and keeps floodwaters at bay. By treating soil with respect, rather than as dirt, land managers in our state can use this resource to help mitigate climate change, through carbon storage, and adapt to its impacts, by slowing down and storing water.