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Global Warming Hits Bottom

Impact of global warming on soil and soil inhabitants.


worm in pan
Earthworm in trouble.

By Erin Volheim

It’s not easy being dirt. For one, you get walked on all the time. And if that weren’t bad enough there’s erosion, salinisation, contamination and that whole global warming that suddenly is “really” happening.

Only the tiniest percentage of the Earth’s subsurface environment has been explored. To scientists, it is “the final frontier in ecological research.” Still, it is widely known that subsoil ecosystems are one of the most species-rich environments on the planet. Importantly, many of the underground critters significantly modify key biogeochemical processes by providing critical links between the terrestrial, aquatic and atmospheric realms. Therefore, diverse losses in the Earth’s basement can bring about changes in top floor biodiversity, as well as changes in various other rooms of our global house.  Although the main culprit of global warming is our insatiable drive for coal, oil, and natural gas – the release of soil carbon into the atmosphere is gaining even more attention.


Good soil is black because it contains carbon, and carbon is also something that we need to keep in the soil, rather than let it flit about the atmosphere. Soil microbes eat the soil carbon, and they tend to eat even faster when the soil is warmer. Global warming feeds back upon itself by making soil microbes work faster, releasing the greenhouse gas, CO2, into the atmosphere at greater rate than it is being replenished, further accelerating global warming. More than twice as much carbon exists in soil as in the atmosphere or in living plants.

Soil carbon sinks appear to be a counterbalance to global warming. In order to see significant soil-carbon increases, soil must be protected. This means shielding it from oxidation. Oxidation occurs when a soil is sifted and broken up, such as when it is tilled or dried out. Plowing exposes a lot of the subsoil humus to the atmosphere, and the increased oxygen levels allow for a quicker breakdown of organic material, thus more CO2 output. As the soil food web decomposes organic material, it releases carbon into the atmosphere as CO2 or converts it to a variety of forms of soil organic matter. Labile, or active fractions of organic matter stay in the soil for a few years. Stable forms reside in the soil for decades or hundreds of years. Humified organic matter is stable because bacteria and fungi have helped form molecules that are too complex and large for soil organisms to decompose.

Some scientists suggest national policies should emphasize conservation tillage or no-plow, to increase the levels of carbon stored in soil and combat the rise of CO2 in the atmosphere. If large swaths of soil are protected, it is possible that atmospheric CO2 levels could be stabilized or even reduced in the short term. Ideally this dovetails with permanent alternative energy technologies as our consumption goes down.

Ironically, in addition to causing global warming, the extra CO2 now in the atmosphere is helping some plants to grow faster, which increases carbon inputs to the soil and removes CO2 from the atmosphere. However, the current evidence is that even these soil carbon storage increases aren’t enough to counter the potentially huge atmospheric carbon releases that result when permafrost thaws and peatlands dry out. A new study out of the U.K. suggests that terrestrial sinks across the planet are mopping up much less carbon than predicted, and so the planet may warm at an even faster rate than expected. The study focused on the carbon content in soil at 6000 sites in the U.K. between 1978 and 2003 and found that the soil released the equivalent of eight percent of the U.K.’s total 1990 CO2 emissions. This was more than the entire reduction in emissions the U.K. achieved between 1990 and 2002 as part of its commitment to the Kyoto Protocol. This would effectively cancel out the England’s recent successes in reducing greenhouse gas emissions, and could have wider global implications as well.

Our understanding of what makes soil carbon change is incomplete, but we now know enough to be worried that huge stocks of soil carbon in the Arctic are at risk of being lost as the Earth rapidly warms, making the situation worse. The sensitivity of functional groups of soil organisms to disturbances and local extinctions is also likely to vary. The dispersal rate of larger “ecosystem engineers,” like worms, is often slower than that of the smaller biota, making larger fauna more sensitive to local extinctions. Following this reasoning, a loss of three species from a pool of 300 may not be dramatic, unless they belong to the same functionally important, but species-poor functional group.

Underground residents

Bacteria alone fall into four functional groups. Most are decomposers that consume simple carbon compounds, like fresh plant litter. By this process, bacteria create energy useful to the rest of the organisms in the soil food web. A second group of bacteria are the mutualists that form partnerships with plants, (such as nitrogen-fixing bacteria). The third group of bacteria are the pathogens. A fourth group, called lithotrophs or chemoautotrophs, obtains its energy from compounds of nitrogen, sulfur, iron or hydrogen instead of from carbon compounds. Some of these species are important to nitrogen cycling and degradation of pollutants.

Like bacteria, fungi are important for retaining nutrients in the soil. In addition, many of the secondary metabolites of fungi are organic acids, so they help increase the accumulation of humic-acid rich organic matter that is resistant to degradation, and may stay in the soil for hundreds of years.

Soil fungi can be grouped into three general functional groups based on how they get their energy. Decomposers – that convert dead organic material into fungal biomass, CO2 and small molecules, such as organic acids. Mutualists – the mycorrhizal fungi – colonize plant roots. In exchange for carbon from the plant, mycorrhizal fungi help solubolize phosphorus and bring soil nutrients to the plant. The third group of fungi, pathogens or parasites, cause reduced production or death when they colonize roots and other organisms.

That’s just the beginning of the roster of underground residents. Let me introduce you to the protozoa that eat bacteria and release the excess nitrogen to plants and other members of the underground community. Meet the nematodes, non-segmented worms that function at several levels of the soil food web. Let’s not forget the many bugs, also known as arthropods, that make their home in the soil. They include insects, such as springtails, beetles, and ants; crustaceans such as sowbugs; arachnids such as spiders and mites; myriapods, such as centipedes and millipedes; and scorpions. Nearly every type of soil is home to many different arthropod species. Certain row-crop soils contain several dozen species of arthropods in a square mile several thousand different species may live in a square mile of forest soil.

From this perspective, it becomes clear that the priority of conserving and restoring functional group diversity can exceed the need to conserve individual taxonomic diversity. One way to do this is by restoring soil biology. Compost is ideal for that purpose. The compost should have a huge functional group species diversity, not just bacteria but fungi, protozoa, nematodes and microarthropods, as well as organic matter for them to feed on. The compost should be made locally, so that its soil biology is similar to the soil on which it is applied. Also, compared to a field with a two-year crop rotation, a field with a four crops grown in rotation may have a greater variety of food sources and therefore is likely to have more types of bacteria, fungi, and other organisms. A cleanly-tilled field with few vegetated edges may have fewer habitats for arthropods than a field broken up by grassed waterways, terraces, or fence rows.

It should be public knowledge that the soil and aboveground life are undivided entities. Sustainable land management, particularly in the era of global warming, requires approaches that protect all the realms of life that are supporting each other, including us.

Erin Volheim is a dirty fingered writer living on the warm soil in Southern Oregon.
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