Some future implications for greenhouse gas mitigation policy frameworks: the example of methane oxidation by soils in a temperate oak woodland

At the end of the 1990s, atmospheric concentrations of methane, a contributor to global warming, approached 1.8 parts per million by volume—nearly double pre-industrial levels. This is due not only to increasing emissions, but also to inhibition of natural sinks. One of these sinks occurs in soils. Two distinct groups of soil bacteria, the methanotrophs and the nitrifiers, are capable of methane oxidation. The highest rates of methane uptake occur in soils inhabited by methanotrophs, while the lowest rates are characteristic of nitrifying bacteria: ammonium fertilization tends to encourage dominance by nitrifiers. Short-term chamber experiments were carried out in a variety of different terrestrial environments in Victoria, British Columbia, Canada. Results were consistent with those obtained by other investigators for temperate forest sites elsewhere. Uptake rates of 0.059–0.082 mg·m−2 ·h−1 were measured at the Garry oak ( Quercus garryana) woodland, while the closed-canopy mixed forest (Acer macrophyllum and Pseudotsuga menziesii) had values in the 0.032–0.042 mg·m−2·h −1 range. Modified environments such as lawns had significantly lower uptake rates. An abandoned hayfield sampled for this study showed intermediate values. Other researchers have shown that it can take years or decades for environments to recover after reversion to low-nitrogen regimes, which is consistent with a long-term shift in bacterial community composition. Given that changes in land use affect soil processes which are intimately linked to atmospheric trace gas regimes, these issues will likely grow in importance over this century. Although current international legislation emphasizes the sequestration of atmospheric carbon dioxide in biomass, not all greenhouse gases follow this model. If mitigative policies are to be extended to other compounds such as methane and nitrous oxide, better understanding of non-sequestration sinks, e.g. soil uptake of CH4, and the processes regulating them is essential. More flexible “adaptive management” strategies are desirable, to accommodate changes in environmental conditions and scientific knowledge.

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