Collaborators: F. Jay Breidt, Marlen Eve, and Keith Paustian

The concentration of carbon dioxide (CO2) has been steadily increasing in the atmosphere over the past century due to a variety of activities, particularly fossil fuel combustion. Terrestrial soils are the largest reservoir of carbon in the biosphere, and can act as a sink or source for atmospheric CO2 depending on a several factors, including land use and management. For example, past agricultural management has typically reduced the amount of carbon stored in soils, thus releasing CO2. However, it is possible to reverse such trends. In fact, a variety of alternative agricultural practices have been implemented, which were originally intended to reduce soil erosion and increase crop production, but have an ancillary benefit of sequestering CO2 in soils. Some of the more notable practices include: no-till or reduced tillage management, intensification of cropping rotations with winter cover crops or removing fallow from rotations, and setting-aside lands from agricultural production.

No-till agricultural can increase carbon storage in soils (Photo Courtesy of K. Paustian)

In order to evaluate the effect of agricultural land use and management between 1990 and 2001, we quantified the change in soil organic carbon stocks for U.S. agroecosystems, using a carbon accounting approach developed by the Intergovernmental Panel on Climate Change (IPCC). For this method, a reference carbon stock is applied to agricultural lands based on soil type and climate. Management factors are then used to modify the reference carbon stocks based on impacts of land use and management on carbon storage. As part of this calculation, we provided uncertainty in estimates so that policy makers could assess the risk of using agriculture soil sequestration to mitigate greenhouse gas emissions. To estimate uncertainty, we determined the variability in the numerical values for components of the IPCC equations, including reference carbons stocks, management factors, and land use/management history; then combined this information, using a Monte Carlo Approach, to simulate 50,000 estimates of carbon change across U.S. agroecosystems. From these results, we generated a 95% confidence interval for the final estimates.

Agricultural land use and management sequestered CO2 from the atmosphere at a modest rate of 0.7 to 1.5 Tg CO2/yr, with uncertainty ranging from losses to gains of about 20 Tg CO2/yr (uncertainty declines in the middle to latter part of the 1990s). Agricultural land on mineral soils sequestered CO2 at a rate of 35 to 36 Tg CO2/yr, with uncertainty ranging from 21 to 50 Tg CO2/yr. Most of these gains were due to setting-aside land from agricultural production in the Conservation Reserve Program. In fact, without carbon sequestration due to enrollment in this program, agricultural production on mineral soils would have generated a net flux of CO2 to the atmosphere. In contrast to mineral soils, agricultural lands on organic soils lost an average of 34 to 35 Tg CO2/yr, with uncertainty ranging from about 24 to 49 Tg CO2/yr. Interestingly, organic soils only comprise about 1 million of the 385 million hectares of U.S. agricultural land, and consequently drainage of these soils is having a disproportionate effect on the net CO2 flux relative to the amount of area that they comprise.

Applying organic amendments (i.e., livestock manure and sewage sludge) to grazing and croplands enhances soil fertility and crop/forage production, and also increases carbon storage. Currently, U.S. databases do not track applications with enough detail to estimate the effects using the Monte Carlo Approach. However, we did compute the change in carbon storage due to this practice in a separate calculation, with estimates ranging from 21 to 23 Tg CO2/yr over the inventory period.

Overall, agricultural land use and management sequestered atmospheric CO2 in U.S. agroecosystems at rates from 23 to 24 Tg CO2/yr between 1990 and 2001. Even though current sequestration is small relative to CO2 emitted through fossil fuel burning, more widespread adoption of conservation management practices could lead to sequestration at higher rates, ranging from 300 to 700 Tg CO2/yr. Rates in the near future, however, will likely depend on policy, particularly with regards to mitigation of greenhouse gas emissions. This research provides baseline information that will aid policy makers as they deal with unfortunate side-effects of fossil fuel combustion.

No-till agricultural can increase carbon storage in soils (Photo Courtesy of R. Conant)