Text Size

Soil Carbon Banks

How much carbon do farm and grazing land soils are currently store?:


Depositing Carbon in the Bank: The Soil Bank, That Is

Who would have thought that while industrial plants and vehicles spew greenhouse gases, thought to be causing global warming, that  farms could be removing some of the excess of one of these gases, carbon dioxide (CO2), from the air? Most of us remember at least some of it from elementary school: Plants take in carbon dioxide and use the carbon to grow. When they die, the carbon in them is returned to the soil as they decompose. Now, U.S.

Estimating the Storage Potential How to figure out national carbon storage figures for 6 different climate regions, 6 soil types, and 22 land-use types. Interestingly, while atmospheric carbon, in the forms of CO2 and CH4 (methane) is a component of these potential greenhouse gases, soil carbon is extremely beneficial to the environment because it is key to soil fertility and stability. The interest comes from international agreement discussions on whether countries should be allowed to offset CO2 emissions with "credits" for carbon stored in soil and trees on farms, grasslands, and in forests. These carbon credits would be traded as pollution credits currently are.

For this, NRCS's long-standing National Resources Inventory (NRI), a survey of changes in land use and farm practices done every 5 years on 800,000 fields, proved invaluable. A computer program was developed that uses procedures of the Intergovernmental Panel on Climate Change to calculate estimated changes in soil carbon from the NRI data. The calculations yielded the first numbers consistent with the assessment of ARS soil scientists that farm soils become a net carbon sink as farmers started using conservation tillage techniques, all but abandoning the moldboard plow that opened up the black prairies and perpetuates a carbon drain.

CQESTR (pronounced "sequester," after the term used synonymously with carbon storage), a new and very detailed computer model created by ARS scientists in Pendleton, Oregon, allows farmers "to determine short-term carbon gain or loss each year, based on specific management practices," says soil scientist Ronald Rickman. "Farmers can also put together sequences—such as 5 years of no-till, 1 year of conventional till, then 3 more years of no-till—to look at the consequence of changing a practice. This model lends itself to current specific, individual applications on one farm at a time, for the current season.  The Century model, developed by William J. Parton at Colorado State University in collaboration with ARS, is a more general, long-term plant-soil-nutrient model that links the carbon, phosphorus, and nitrogen cycles and can be used to calculate carbon storage on grass, crop, and forest lands. The important thing about Century, Parton says, is that it gives a comprehensive simulation of carbon dynamics across an entire ecosystem over months and years so it can be used for accurate, long-term assessments of carbon storage under various practices on a regional, national, or global scale. "All of the drivers of the carbon cycle are there—temperature, precipitation, and carbon dioxide levels," says Parton. This model considers plant responses to soil nitrogen and management practices such as no-till to predict crop yields and levels of soil carbon.

Both models measure carbon losses and gains in a much shorter time frame—in hours and days. In the lab, and automatically sample CO2 emissions from 60 soil samples at a time, measuring the soil's potential emissions from microbes eating organic matter," 

Keeping CO2 Down on the Farm Tillage releases carbon into the air in sudden rushes of CO2 gas that escape as soil is opened up. This can be measured with a large portable chamber placed on the soil shortly after plowing.  The potential amounts of carbon that can be stored by farms and grazing lands in a country or region can be calculated.  Pasture and rangelands cover large areas globally, so they are a vital part of the carbon storage puzzle.  Researchers conclude that 3 acres of plain or grassland can store up to 1 metric ton of carbon a year for 25 years. Turning erodible cropland into grasslands that produce biofuel crops like native prairie switchgrass grown for ethanol production or direct burning in power plants. Biofuel crops recycle carbon rather than add more carbon to the atmosphere,"  Switchgrass could yield 500 gallons of ethanol per acre there.  Crop varieties that either store more carbon in the soil or work better with farming methods that promote carbon storage are being investigated. These methods include strip tillage and other forms of conservation tillage. Strip tillage is a compromise between two extremes—no-till and plowing. Farmers till just the part of each crop row where seeds will be planted. This is becoming an increasingly popular technique. Allmaras says that strip tillage is just one example of how American farmers have compromised to turn their land into carbon storage banks.  However, as new tillage methods, such as ridge and strip tillage, are improved, farmers will be able to conserve soil, increase soil carbon, and improve productivity.

soil carbon is both a priceless key to the planet's health and an agricultural commodity with a promising price tag.  Not only do soil carbon credits buy us time, but they also buy us improved soil, water, and air quality. Carbon-rich organic matter does this by reducing soil erosion while helping soil retain and break down pesticides and excess nutrients. Organic matter also contributes to agricultural productivity by providing plant nutrients and by increasing the soil's ability to hold water. In fact, 

"Depositing Carbon in the Bank: The Soil Bank, That Is" was published in the February 2001 issue of Agricultural Research magazine.