By Janet Fang

Soils around the globe have increased their emissions of carbon dioxide over the past few decades, according to an analysis of 439 studies.

The findings, published in the March 25 issue of Nature, match predictions that increasing temperatures will cause a net release of carbon dioxide from soils by triggering microbes to speed up their consumption of plant debris and other organic matter.

Ben Bond-Lamberty and Allison Thomson, terrestrial carbon research scientists at the University of Maryland's Joint Global Change Research Institute in College Park, conducted the study by stitching together almost 50 years of soil-emissions data--1,434 data points--from 439 studies around the world. To compare measurements, the researchers accounted for differences between the studies, such as mean annual temperatures and techniques used to gauge carbon dioxide levels. They totaled the data for each year to create a global estimate of soil respiration--the flux of carbon dioxide from the ground into the atmosphere.

The researchers found that soil respiration had increased by about 0.1 percent per year between 1989 and 2008, the span when soil measurement techniques had become standardized. In 2008, the global total reached roughly 98 billion tons, about 10 times more carbon than humans are now putting into the atmosphere each year. The change within soils "is a slow increase, but the absolute number is so large, even a small percentage increase is quite a bit," says Bond-Lamberty.

"There are a few plausible explanations for this trend, but the most tempting, and perhaps most likely explanation is that increasing temperatures have increased rates of decomposition of soil organic matter, which has increased the flow of CO2," says Eric Davidson, a biogeochemist at the Woods Hole Research Center in Falmouth, Massachusetts. "If true, this is an important finding: that a positive feedback to climate change is already occurring at a detectable level in soils."

Unknown cause

The extra soil emissions could come from two types of sources: microbes and plants. If plant roots are emitting more carbon dioxide, the additional flux could be balanced by increasing rates of photosynthesis, resulting in no net increase in atmospheric carbon dioxide.

In contrast, warming soils could prompt microbes to break down old sources of carbon that have been locked away for a long time. This would cause a net increase in the atmosphere's store of carbon dioxide.

Although the study shows an increase in respiration, it can't distinguish between the two potential causes.

Researchers who study soil carbon say they are impressed with the huge undertaking. "It's extremely difficult to compare soil respiration measurements between different experiments, let alone different regions of the world," says Dustin Bronson from the University of Wyoming in Laramie.

William Schlesinger, president of the Cary Institute of Ecosystem Studies in Millbrook, N.Y., says the new study should motivate further work on the response of the carbon cycle to a warmer world.

Thomson and Bond-Lamberty agree that their work is just a beginning. They have started an online global soil respiration database to which researchers can contribute additional studies as they are completed. Over time, says Thomson, "we can see if this relationship holds up."