Disrupting the Environment of Soil Microbes Might Worsen Climate Change
Soils represent the largest carbon reservoir within terrestrial ecosystems. The mechanisms controlling the amount of carbon stored and its feedback to the climate system, however, remain poorly resolved. Global carbon models assume that carbon cycling in upland soils is entirely driven by aerobic respiration; the impact of anaerobic microsites prevalent even within well-drained soils is missed within this conception.
And now, anaerobic microsites – microscopic habitats that lack oxygen and impede the ability of soil microbes to metabolize carbon into climate-active CO2 – are an important unrecognized factor in climate change say researchers at the Stanford University.
The study led by Stanford’s Scott Fendorf and former postdoc Marco Keiluweit, finds that these oxygen-free pockets of soil are vulnerable to disruption from climate change and some farming practices. The scientists said this work could help in modeling future carbon emissions by giving better predictions of how much CO2 might be released from the soil.
“Without recognizing the importance of anaerobic microsites in stabilizing soil carbon, models are likely to underestimate the vulnerability of the soil carbon reservoir to disturbance induced by climate or land use change,
” Keiluweit said. “Our research suggests that [these changes] may release greater amounts of carbon from soils than we expected.”Keiluweit explained that soil organic matter stores three times more carbon than the atmosphere, but the presence of these large carbon stocks was attributed to other stabilization mechanisms that are much less vulnerable to disturbance than anaerobic microsites. When microbial metabolism shifts into less efficient anaerobic mode, the organisms cannot convert carbon to CO2 or decompose organic compounds, such as lipids and waxes, as easily. Shifting back to aerobic respiration produces a tenfold increase in volume-specific mineralization.
In the course of this study, the team created anaerobic microsites in the lab by painstakingly manipulating the flow of oxygen to soil samples and then measured their CO2 output as well as their lipid and wax concentrations. They found that as oxygen became scarce, the soil microbes shifted from aerobic to increasingly less efficient anaerobic respiration. As a result, fewer carbon-rich lipid and wax molecules were decomposed and CO2 production dropped by a factor of 10.
As a real-world check on their results, the researchers also examined soil from agricultural field sites in Oregon. Both the lab and field results showed remarkably consistent trends, indicating that, contrary to conventional wisdom, upland soils do in fact contain high volumes of anaerobic microsites that protect specific types of carbon molecules.
“Based on our lab results, we would expect that soils rich in anaerobic microsites would have lots of lipids and waxes left over, and that’s what we found in the fields,” Keiluweit said.
“Changes in soil moisture arising from irrigation or from climatic patterns will therefore alter the distribution of microbial metabolisms and the rate of CO2 production,” Fendorf said.
In addition, frequent loosening, or tilling, of soils aerates the soil, turning anaerobic microsites aerobic and increasing release of CO2. “Our findings highlight a benefit of low till practices and other land use practices that limit increased soil aeration,” Fendorf said.
