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Rising Atmospheric CO2 May Stimulate Microbes to Release Carbon and Offset Gains in Carbon Uptake by Plants
2 Apr, 2007 11:21 pm
Changes in the amount of carbon in soils can affect the amount of CO2 in the atmosphere. In a four year forest experiment, elevated CO2 caused a loss carbon from the soil. The amount of soil carbon lost was enough to offset about half of the carbon gained by the forest, which grew faster at elevated CO2 as expected. We showed that the loss of soil carbon was due to changes in the types and activity of soil microorganisms, which was triggered by the increased availability of fresh plant detritus. The microbes where stimulated to consume soil organic matter at higher rates. 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 a soil from a potential carbon sink into a carbon source. This could offset some portion of the gains in carbon storage in plant biomass due to increased growth at elevated CO2.
Our research team recently reported the results of a six year experiment in a scrub oak forest ecosystem in which doubling the greenhouse gas carbon dioxide (CO2) caused a reduction in carbon storage in the soil (1). Surprisingly, this occurred even though the amount carbon stored as plant biomass increased due to more rapid plant growth in the elevated CO2 atmosphere. This result adds a new perspective on the capacity of Earth’s soils to store carbon and suggests that a measure of caution is required when predicting the potential of the soil to store carbon. In this particular case, a potential carbon sink – the soil -- became a carbon source.
Previous studies (including the present study) have shown that most plants will respond to elevated CO2 with increased growth, thereby taking up a portion of the excess CO2 that is accumulating in the atmosphere due to human activities. This has led scientists to speculate that plants may be able to reduce the pace at which atmospheric CO2 is rising, and that soils, which represent the largest and most stable terrestrial carbon pool, also may serve as a sink for excess carbon. As predicted, we found that the amount of carbon in the ecosystem (i.e., plants + soils) increased in the elevated CO2 atmosphere. However, we also found there was a consistent loss in soil carbon under elevated CO2 conditions. The carbon lost from soils offset about half of the extra carbon that had accumulated in the shoots and roots of trees over the same period of time. Thus, the net effect of elevated CO2 was to remove CO2 from the atmosphere and store it as plant biomass, but the forest was only about half as effective as it might have been because of simultaneous losses of carbon from the soil.
But how could soils loose organic carbon when carbon inputs from plant growth increased? We explain this as a change in the types and activities of soil microorganisms, triggered by the increased input of fresh plant biomass to soils. Most soil microbes, including both bacteria and fungi, are heterotrophic and depend on freshly dead plant biomass for energy and nutrients. We propose that the increased supply of fresh plant biomass at elevated CO2 caused a so-called “priming effect,” in which certain microbes were stimulated to increase their soil carbon decomposition activity. Soils exposed to elevated CO2-grown plants supported relatively more fungi and higher activities of a soil carbon-degrading enzyme. As the fungi, and enzymes they produced, decomposed the organic matter in the soil, they freed up stored carbon and released it back to the atmosphere as CO2. Thus, faster growing plants led to more actively decomposing microbes, which led to less soil carbon. This offset some of the gains in carbon storage in plant biomass due to increased growth at elevated CO2.
The scrub oak forest site where the study took place is an ideal model site to observe this affect of plants on microbial activity -- an increase in decomposition of soil carbon. The sandy soils at this site do not clump together to form clods that can protect carbon against microbial attack. In soils with more clay and silt particles, such miniature clods may provide enough physical protection to prevent a loss of soil carbon such as we observed. Nonetheless, we suspect that the changes in microbial community composition and activity help explain why previous studies have observed a wide variety of responsess to elevated CO2, ranging from increases to decreases in soil carbon (2).
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 a 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.
1. J.P. Megonigal et. al. 2007. Altered soil microbial community at elevated CO2 leads to loss of soil carbon. Proceedings of the National Academy of Sciences 104:4990-4995.
2. 2005. Elevated atmospheric carbon dioxide increases soil carbon. Global Change Biology 11:2057-2064.