Nature defies logic on soil carbon

The journal Nature has made the headlines today with a story about emissions from soils that was misunderstood by journalists reporting on it. The study concluded that increased levels of CO2 in the atmosphere cause certain soil microbes to release methane and nitrous oxide. The scientists and journalists all assumed that this was bad news for soil carbon sequestration. Logically, however, it is the opposite. It is not the CO2 in soils that causes the additional emissions of methane and nitrous oxide, but the CO2 in the atmosphere. If there is more CO2 in the atmosphere, soils will release more Greenhouse Gases. Soils can take CO2 out of the atmosphere. Therefore we need to get soils working hard as soon as possible. If we don’t, soils will be 20% less effective at taking CO2 out of the atmosphere.

Increasing CO₂ in the atmosphere makes plants grow faster. 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 depositing carbon in wood and soil. But 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. "It's an ecological point and counterpoint: the more the plants soak up CO₂, the more microbes release these more potent greenhouse gases," said Bruce Hungate, Professor at Northern Arizona University and co-author on the study. "The microbial counterpoint is only partial, reducing the cooling effect of plants by about 20%."

Professor Alex McBratney of Sydney University - who estimated that soil carbon sequestration could absorb between 10% and 30% of Australia's emissions prior to the study - now estimates CO2 absorption could remove between 8% and 30%.