In a new paper published online in Nature Geoscience, researchers from UC Irvine and Yale have found that as global temperatures increase, soil microbes become less efficient over time at converting soil carbon into carbon dioxide.
Soil microbes are extremely abundant in most ecosystems around the world. The most numerous of these microbes are bacteria followed by actinomycetes then fungi, soil algae, cyanobacteria, and protozoa. For the purposes of this post I am not including the diverse set of animals that live in soils such as nematodes, microarthropods, earthworms, larger insects, etc. Soil microbes are extremely important in the cycling of the soil nutrients carbon (C), nitrogen (N), phosphorus (P), and sulfur (S). They can regulate the quantities of nitrogen available to plants (think natural fertilizer), and are hugely important in recycling nutrients tied up in organic materials. Many of the soil microbes require organic carbon compounds to oxidize for energy (microbial or soil respiration, usually expired as CO2) and the building materials for their cells. Some microbes get this carbon from CO2, but much of this carbon is collected through the decomposition of organic materials. Soil respiration is known to be affected by such factors as temperature, soil moisture, and nutrient availability.
Naturally, the high abundance of soil microbes and their release of CO2 through respiration has caused many scientists to take a special interest in this system and how it relates to global climate change. It stands to reason that even a few degrees of warming will shift these microfauna into overdrive, increasing the atmospheric CO2. However, this new research shows that, in a warmer environment, soil respiration increases for a short period of time, but if the higher temperatures remain constant, the less efficient use of carbon causes the microbes to decrease in number. This decrease in number decreases soil respiration (decrease in CO2 "exhaled" from soils). The microbes get overheated and "burned out," if you will.
These results contradict some of the results in older models which assume microbes will stay at constant numbers or increase; older models usually do not include enzyme production/activity which is sensitive to temperature and important in the reactions that break down organic carbon. When you get down to the mechanism, its all about how efficiently the microbes can use soil organic carbon. If they are less efficient then populations decrease. If they stay efficient, or adapt to remain as efficient or more so (and remember that microbes can adapt very quickly), then respiration/CO2 emission will stay the same or increase.
Read more here:
Allison Steven D., Matthew D. Wallenstein and Mark A. Bradford (2010) Soil-carbon response to warming dependent on microbial physiology Nature Geoscience: published online (DOI: 10.1038/ngeo846)
(image from www.sustainabilityninja.com)