Land can be either a carbon sink or a carbon source. That is, it can take in and hold carbon dioxide and other greenhouse gases (carbon sink) or release them back into the atmosphere (carbon source).
Forests and grasslands occupy a large but shrinking portion of the earth’s landmass.
Trees have a prominent role in sequestering carbon. Forests hold some 39% of the carbon in land-based ecosystems. It’s no surprise that environmental groups hope to plant a trillion trees in the next ten years. But did you know that grasslands hold about 34% of that same carbon? The loss of grasslands is every bit as serious as deforestation. So restoring grasslands is as important for sequestering carbon as restoring forests. Maybe even more.
Consider the North American prairies. Not much prairie land is left. It has been dug up for farms, suburban housing, railroads, and highways. Every other iconic grassland in the world is also under threat, including the Serengeti in Tanzania, the Cerrado in Brazil, and the steppes of Mongolia.
Biomass (specifically, the total weight of plants) and soil carbon don’t increase together. That is, when the environment uses the carbon to increase biomass, it usually causes a decrease in the carbon stored in the soil. When a plant dies, it releases some of its stored carbon into the atmosphere. But when the soil sequesters carbon, it basically stays there permanently.
In fact, the amount of carbon stored in soil exceeds the carbon in all the plant biomass in the world. Grasslands, with less biomass than forests, makes a very effective carbon sink.
One of the worst mistakes we could make in controlling carbon is to plant forests in existing grasslands.
Different kinds of grasslands
Grasslands offer other environmental benefits besides storing carbon. They prevent soil erosion and generally serve as a buffer against extreme weather. For example, they hold stormwater, and reduce flooding. Then, in dry periods, they release the water gradually. Pollinators such as bees and butterflies depend on grasslands.
A lot of the publicity surrounding deforestation and forest restoration has come at the expense of ignoring grasslands as a strategy for sustainability. One recent study concluded that restoring grasslands in California might have a greater impact than restoring forests.
For one thing, they are less vulnerable to fires. Trees lock carbon in their trunks and leaves. Grasses store most of theirs in their roots. Therefore, even when they burn, they release less carbon into the atmosphere.
Grasslands are not all the same. Fine distinctions can be arbitrary, but broadly speaking, it helps to differentiate between temperate and tropical grasslands. I am only looking at temperate grasslands. It also helps to differentiate between more or less natural grasslands and grasslands made or extensively altered by human activity.
The North American prairies are a little of both. As a natural grassland, the rainfall is too low to support the growth of forests. Therefore, the dominant species has always been various kinds of grass.
Of course, much of the American prairie landscape has been converted to farmland. Some of it serves for the cultivation of airable crops and is no longer considered grassland. Some of it serves as grazing land for cattle.
All plants remove carbon dioxide from the atmosphere and give back oxygen. Carbon sequestration refers to the various ways in which plants deposit carbon in the soil. Grazing land sequesters more carbon than cultivated land. Both kinds of agricultural activity release carbon back into the atmosphere. In fact, it appears that managed grasslands used for heavy grazing are carbon sources. But most temperate grasslands still act as carbon sinks.
Management practices that minimize or eliminate soil disturbance enhance the carbon sequestration. These include sowing legumes along with grasses and introducing earthworms. Grasslands that have been depleted by poor management have the greatest potential to increase their storage of carbon in the soil.
Vegans love to complain about the environmental impact of raising cattle, but it turns out that properly managed grazing can actually increase soil carbon. All the variables that determine whether a particular parcel of land will be more of a carbon sink or more of a carbon source are too complicated to try to describe here.
Turfgrass—lawns, parkland, golf courses, etc.—has become a potent carbon source. Lawnmowers, aerators, and other lawn maintenance equipment emit greenhouse gases. Manufacture of petroleum-based fertilizers and pesticides emits greenhouse gases.
And our lawns need plenty of fertilizer as long as we insist on growing monocultures of only one species of plant on the same ground year after year.
Yet even turfgrass, if better managed, can potentially become a carbon sink. Thus, it is a kind of grassland that can contribute to reducing greenhouse gas emissions, or at least cease to be part of the problem.
Restoration of grasslands
Although cultivated grasslands can be effective carbon sinks when well managed, it’s still important to restore grasslands to a more nearly natural condition.
British meadows have suffered the same kinds of degradation as American prairies. More than 2 million acres have disappeared over the last century. At the same time, woodlands, urban development, and infrastructure have expanded. Grasslands that could have remained carbon sinks have been converted to uses that make them carbon sources.
The British conservation group Plantlife has joined other groups to advocate for worldwide grassland preservation to be part of any agreement that comes out of the Glasgow climate talks in November 2021. It also has projects to restore meadowland in the UK.
One of them, the Ranscombe Farm Nature Reserve in Kent, nurtures plant life much like other nature preserves serve as animal sanctuaries. Comprising both grasslands and woodlands, it takes care of rare plants in various small sections. It hopes to supply them to other, larger habitats.
Ranscombe Farm has been in operation since 2010 and has not yet been able to reverse all the damage done to the land before then.
Technology offers all kinds of new ways to capture and store carbon. Some of them will doubtless prove very valuable. But nature can do a better job if we let it.
Related posts in Sustaining Our World:
Carbon capture and storage and direct air capture
Preserving and restoring our wetlands
Reforestation: regreening deserts in Asia and Africa
Regenerative agriculture: old practices become new
Steps to a sustainable lawn and garden
Carbon sequestration in temperate grassland ecosystems and the influence of management, climate and elevated CO2 / M.B. Jones and Allison Donnelley, New Phytologist. October 20, 2004
The potential of turfgrass to sequester carbon and offset greenhouse gas emissions / Madeline Leslie, Turfgrass Science, University of Minnesota. 
Soils or plants will absorb more CO2 as carbon levels rise – but not both, Stanford study finds / Josie Garthwaite, Stanford News. March 24, 2021
This might just look like grass, but it has the power to absorb a load of our carbon emissions / Lauren Kent, CNN. October 22, 2021