The scientists said these findings add a new perspective on the capacity of Earth's soils to store carbon, and a measure of caution suggesting that elevated CO2, by altering microbial communities, may turn the soil from a potential carbon sink into a carbon source. This could offset some of the gains in carbon storage in plant biomass due to increased growth at elevated CO2.

Previous studies (including the present study) have shown that plants will respond to higher CO2 by increasing growth and taking up much of the excess carbon. This has led some to speculate that plants may be able to mitigate increases in atmospheric CO2 and that soils, which represent the largest and most stable terrestrial carbon pool, also may serve as a sink for excess carbon.

During the course of their study, Smithsonian scientists found that the amount of carbon in the ecosystem as a whole increased. However, they also saw a consistent loss in soil carbon under high CO2 conditions. The CO2 loss from soils offset about 52 percent of the additional carbon that had accumulated in the plants above ground and in the roots.

"We were surprised to find that these soils were losing soil carbon despite the fact that there was more plant growth," said Patrick Megonigal, a microbial ecologist at SERC and one of the study's authors. "We thought that higher plant growth at elevated CO2 would either add more carbon to soils, or at least leave it the same. We now need to consider a third possibility-the carbon already in soils will end up back in the atmosphere as a greenhouse gas."

Working at a long-term Smithsonian experimental CO2 site in a Florida scrub oak ecosystem, the researchers compared core samples from test plots that had been exposed to six years of elevated CO2 and core samples from plots exposed to ambient CO2. They also performed laboratory experiments on soils from both elevated and ambient plots to understand microbial composition and activity within each type of soil.

Their study reveals that added CO2 has a so-called "priming effect," stimulating certain microbes and increasing decomposition. Soils exposed to the elevated CO2 had higher relative abundances of fungi and higher activities of a soil carbon-degrading enzyme.

As the fungi and enzymes decompose the organic matter in the soil, they free up stored carbon and release it through respiration as CO2. With the priming effect of added CO2, more soil decomposition results in higher respiration rates, an overall loss of carbon and an increase in the release of CO2 from the soil.

earlier related report
Color Analysis Rapidly Predicts Carbon Content Of Soil
Ames IA (SPX) Mar 14 - Scientists report in the Soil Science Society of America Journal that soil color can be as accurate as the lab for carbon content Scientists at Iowa Sate University recently discovered that simply looking at soil color is reasonably as accurate as time-consuming and expensive laboratory tests. Soil color can be used as a simple, inexpensive method to predict measurements of soil organic content (SOC).

These measurements provide a lens through which researchers can assess soil quality and better understand global carbon cycles. Proper modeling of global carbon cycles and monitoring of carbon sequestration require wide-spread, accurate assessments of soil carbon contents.

The researchers compared field and laboratory measurements to determine the color and the organic content of soil samples from cultivated and native land in northeast Iowa.

"Soil color is one of the most obvious features of soil and organic matter has long been known as one of the primary pigmenting agents in soil," said Skye Willis, lead author of the Iowa State study that was published in the March-April issue of the Soil Science Society of America Journal.

Soil field descriptions made in the U.S. are based upon the Munsell color system - field scientists match soil color to standardized color chips based upon hue, chroma, and value. Additional laboratory tests, such as the chroma meter, offer rapid quantification of soil color. In general, darker soil colors indicate more SOC is present.

To test the efficiency of color analysis as a measure of SOC content on different landscapes, scientists collected soil samples from an agricultural field and an adjacent native prairie in northeast Iowa. Scientists analyzed the color of soil samples using three tests:

Soil cores were split in half and matched to a color chip in a Munsell Soil Color Book

The matrix color of soil layers were described according to Munsell Soil Book

Soil was ground and analyzed by a chroma meter, an instrument used to digitally record the color reflectance of soil sample.

From these three assessments, scientists determined the soil color (represented by hue, value and chroma) and predicted the SOC content.

According to Willis, "We found that typical description colors done by a soil scientist were nearly as effective in predicting SOC as the more expensive and tedious method of deriving colors by a chroma meter."

Color analysis is capable of predicting SOC values more accurately in common land areas (agricultural fields) in comparison to less common land areas (native prairies). Additional studies are needed to better predict SOC under native soil conditions.

This rapid SOC measurement will increase the understanding, prediction, and modeling efficiency of carbon distribution across fields, watersheds, and larger regions as the current methods of characterizing SOC are costly and time-consuming. As an alternative to direct measurements of SOC, soil color can be used as an efficient predictor of SOC soil contents.

Study authors (first report) are: Karen Carney and Bruce Huntgate, SERC post-doctoral fellows at the time of the study, who now work at the U.S. Agency for International Development and Northern Arizona University, respectively; Bert Drake, SERC plant physiologist; and Patrick Megonigal, SERC microbial ecologist. The study will be published this week in Proceedings of the National Academy of Sciences.

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