Minnesota Science
Vol. 48, No. 2
Carbon Measurements Affect Climate Forecasting
by Larry Etkin and Dave Hansen
Global warming is related to increased atmospheric "greenhouse" gases, say most researchers. A major component of those gasses is carbon dioxide. But all that carbon doesn't just go into the air and stay there. Much of it cycles between terrestrial and atmospheric forms, and predictions of climate change lack information about where and how that cycle operates.
Buildup of atmospheric C02 is moderated by "sinks" on the earth's surface that use some C02 and store much of the carbon in living organisms, organic matter and carbonate minerals, says soil scientist H.H. Cheng. These carbon sinks include the oceans that cover more than 70 percent of the earth surface, forests and other vegetation covering the land, and organic matter in the soil.
In the carbon cycle, plants use C02 for photosynthesis. Some of the carbon goes into animals that feed on plants. Much of it eventually becomes soil organic carbon organic matter, through decay of plants and animals. Some returns to the atmosphere through the decay process. Cheng and colleagues Jean Alex Molina and William Koskinen are studying both how these processes take place, and how climate change may affect them.
"Though the ocean is by far the largest reservoir for carbon, the exchange between the carbon species in the ocean and those in the atmosphere is extremely slow, and biological carbon fixation is limited to the ocean surface," says Cheng. Estimating the amount of carbon in standing forests is fairly straightforward. It is soil organic matter – grasslands, cropland, swamps and others – where Cheng says much is unknown about the carbon exchange. "It is imperative that the carbon budget of the vast land areas covered by prairies and agricultural crops, and the carbon cycling processes affecting these systems, also be accurately assessed," he says. "Most global estimates of stored carbon are based on data from very few samples."
Collecting better data is vital because carbon stored in terrestrial systems is two or more times greater than that in the atmosphere. With more than 10 percent of atmospheric carbon being cycled each year between the atmosphere and the forests and crops on land, change in the terrestrial system quickly affects the atmosphere. However, only a part of the carbon on earth is readily accessible for conversion to C02. Cheng says this is usually overlooked in global predictions of climate change.
"Total soil organic carbon is not always an accurate indicator of climate changes, especially in the short term." Cheng says researchers need to identify specific soil components that are actively involved in carbon cycling.
Soil carbon exists in many forms, from plant residues and microbial biomass to highly stable humus. "Not all soil organic carbons participate equally in the transformation process," Cheng says.
Molina is reviewing theoretical models and laboratory techniques used to estimate soil carbon losses. New laboratory methods for isolating soil organic fractions actively involved in the carbon cycle biological process, such as supercritical fluid extraction, have been developed in collaboration with Koskinen of the USDA Agricultural Research Service.
The team's research promises the development of more reliable methods for evaluating biologically active soil organic carbon. This will become a valuable index for determining whether soil is a source or a sink for atmospheric C02 under a given climatic condition. The index will also be useful in assessing the impact of various organic materials on water quality.
PHOTO CUTLINE: Understanding the carbon cycle both affects our estimates of
global climate change and leads to more accurate soils tests. Experiment station
scientists are refining our knowledge of carbon as it is transformed from plant
tissue to soil to atmosphere to plant.