Could the earth beneath our feet hold the key to climate change? According to scientists at the University of Toronto Scarborough, their NMR results show that global warming is changing the molecular structure of organic matter in soil.

"Soil contains more than twice the amount of carbon than does the atmosphere, yet, until now, scientists haven't examined this significant carbon pool closely," explains environmental chemistry professor Myrna Simpson of UTSC, "Through our research, we've sought to determine what soils are made up of at the molecular level and whether this composition will change in a warmer world."

It is the organic matter in soil, that the otherwise lifeless dirt fertile and able to support plant, microbes, and fungal life. Soil organic matter is key to life on land and, of course, for human agriculture. The organic matter also retains water in the soil and prevents erosion and ultimate desertification.

The natural processes of decomposition of the organic matter in soil provide plants and microbes with the nutrients and water they need to grow. Carbon dioxide is released into the atmosphere as a by-product of these life-giving processes. However, as global average temperature rises, so too will the rates of these biochemical processes, which will in turn increase the amount of carbon transferred to the atmosphere.

"From the perspective of agriculture, we can't afford to lose carbon from the soil because it will change soil fertility and enhance erosion," Simpson explains. "Alternatively, consider all the carbon locked up in permafrost in the Arctic. We also need to understand what will happen to the stored carbon when microbes become more active under warmer temperatures."

Until now, little detail was known about soil's molecular composition not least because soil is not a single compound but a complex mix of dozens of organic molecules. It contains humus, the base level degraded organic compounds, labile compounds such as proteins and carbohydrates, as well as lipids from leaf cuticles and roots and lignin from woody tissues. Some components can remain stable over decades or even centuries, others are at the whim of metabolism and environmental effects.

From an analytical perspective, soil is one of the most difficult systems to test due to its many components. The analysis of soil is further complicated by the presence of bacteria, fungi and an array of fresh, partially degraded or old plant material as well as the minerals from which the "dirt" component is made.

Simpson and her colleagues worked with fellow professors Dudley Williams and André Simpson together with colleagues Xiaojuan Feng, and Kevin Wilson. They carried out an outdoor field experiment in the mixed-temperature wooded valley behind the UTSC campus to ensure natural ecosystem processes were preserved. They used electrodes to warm test beds of soil between three and six degrees through both the winter and summer seasons, over a 14-month period.

With access to the only NMR facility in Canada dedicated to environmental research, the team was able to analyse soil sample molecular composition throughout that period and so gain a detailed view of the soil's molecular structure and reactivity and how it was affected by the increase in temperature.

The researchers found that the abundance of compounds derived from leaf cuticle was increased after the 14 months of soil warming. However, the levels of lignin-derived compounds fell over this period as soil fungi numbers rose.

"Future warming could alter the composition of soil organic matter at the molecular level," the researchers say. This could accelerate lignin degradation and increasing leaf-cuticle-derived carbon sequestration. The team points out that global warming is expected to increase vegetation in the northern hemisphere - more carbon dioxide feedstock for photosynthesis means more plant growth. Concomitantly, this will increase the annual litterfall of dead leaves, stems, flowers, and plants. Just a 3 degrees Celsius warming will a significant impact on this process.

The team's findings would suggest that with the rise in temperature, there will be some kind of self-regulation as carbon from litterfall will be absorbed into and locked into the soil in the form of recalcitrant organic compounds "Future warming may enhance the sequestration of cuticular carbon in soil," the researchers say.

However, there are other factors to consider, such as increased soil acidification and its effects on plant growth and this sequestration process as with growing plants, nothing is clear cut. "Long-term field studies combined with modern molecular approaches will be critical for future assessments of soil organic matter responses to climatic change," the researchers conclude.

Related links:

• Nature Geoscience, 2008, in press
• Simpson Group

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.