Soil Microbes Accelerate Global Warming

Van Groenigen, along with colleagues from Northern Arizona University and the University of Florida, gathered all published research to date from 49 different experiments mostly from North America, Europe and Asia, and conducted in forests, grasslands, wetlands, and agricultural fields, including rice paddies. The common theme in the experiments was that they all measured how extra carbon dioxide in the atmosphere affects how soils take up or release the gases methane and nitrous oxide.

The research team used a statistical technique called meta-analysis, or quantitative data synthesis, a powerful tool for finding general patterns in a sea of conflicting results. "Until now, there was no consensus on this topic, because results varied from one study to the next," explained Professor Craig Osenberg of the University of Florida and co-author of the study. "However, two strong patterns emerged when we analysed all the data: firstly more CO2 boosted soil emissions of nitrous oxide in all the ecosystems, and secondly, in rice paddies and wetlands, extra CO2 caused soils to release more methane." Wetlands and rice fields are two major sources of methane emissions to the atmosphere.

The culprits are specialised microscopic organisms in soil, that respire the chemicals nitrate and carbon dioxide, like humans respire oxygen. The microbes also produce methane, a greenhouse gas 25 times more powerful than carbon dioxide, and nitrous oxide, 300 times more potent than carbon dioxide. Their oxygen-free habit is one of the reasons these microorganisms flourish when atmospheric carbon dioxide concentrations increase. Van Groenigen explained: "The higher CO2 concentrations reduce plant water use, making soils wetter, in turn reducing the availability of oxygen in soil, favoring these microorganisms."

The other reason these microorganisms become more active is that increasing CO2 makes plants grow faster, and the extra plant growth supplies soil microorganisms with extra energy, pumping up their metabolism. This extra plant growth is one of the main ways ecosystems could slow climate change. With more CO2, plants grow more, soaking up carbon dioxide through photosynthesis, and, the hope is that they also lock away carbon in wood and soil. But this new work shows that at least some of that extra carbon also provides fuel to microorganisms whose byproducts, nitrous oxide and methane, end up in the atmosphere and counteract the cooling effects of more plant growth.
Read more

Natural Sequence Farming Could Affect Global Climate Change

Natural Sequence Farming is a descriptor used when sustainable agriculture mimics the once highly efficient functions of the Australian landscape. NSF pioneer Peter Andrews of Denman in New South Wales and coordinator of the NSF movement, Duane Norris of Hardy's Bay, New South Wales explain how NSF techniques could re-couple environmental carbon and water cycles not only to improve farming yields but to avoid soil erosion and reduce carbon dioxide emissions.

Agricultural practices such as clearing, burning, plowing, draining, and irrigation, have become commonplace across the Australian continent, as they have elsewhere. Their effect on the organic carbon content of soil has led to a decline in soil quality across farmland on the continent with levels currently a tenth of what they were 200 years ago prior to the major European settling of Australia.

Andrews and Norris point out that this has had implications for atmospheric carbon dioxide levels and will continue to impact on global warming if farming practices are not modified. "Soils hold twice as much carbon as the atmosphere, and three times as much as vegetation," the team explains, "But carbon in soil exposed by common agricultural practices leads to the oxidation of the carbon and the release of carbon dioxide into the atmosphere." Estimates suggest that soils that once contained carbon matter 4,000 to 10,000 years old, are now holding carbon that is a mere two years old because poor management of livestock grazing leaves soil de-vegetated and in an oxidizing state.

Plants extract carbon from carbon dioxide in the air by photosynthesis, the team says. This carbon is critical to soil health and plant fertility, but it is lost when a ploughed paddock is left bare with no plant cover. More carbon is released when grassland and trees are cleared. However, when vegetation is allowed to break down, even if it is weedy cover, the carbon content of the soil is raised and growing conditions improve.
Read more