Reference
Livesley, S.J., Kiese, R., Miehle, P., Weston, C.J., Butterbach-Bahl, K. and Arndt, S.K. 2009. Soil-atmosphere exchange of greenhouse gases in a Eucalyptus marginata woodland, a clover-grass pasture and Pinus radiata and Eucalyptus globulus plantations. Global Change Biology 15: 425-440.
Background
The authors write that "soils provide the largest terrestrial carbon store, the largest atmospheric CO2 [carbon dioxide] source, the largest terrestrial N2O [nitrous oxide] source and the largest terrestrial CH4 [methane] sink, as mediated through root and soil microbial process," and that "a change in land use or management can alter these soil processes such that net greenhouse gas exchange may increase or decrease."
What was done
Seeking to determine how the emission and absorption of these three greenhouse gases differ between forests and pastures, Livesley et al. "measured soil-atmosphere exchange of CO2, N2O and CH4 in four adjacent land-use systems (native eucalypt woodland, clover-grass pasture, Pinus radiata and Eucalyptus globulus plantation) for short, but continuous, periods between October 2005 and June 2006 using an automated trace gas measurement system near Albany in southwest Western Australia."
What was learned
The six scientists discovered that soil N2O emissions were more than an order of magnitude greater in the pasture than in the natural and managed forests, while soil CH4 uptake was greatest in the native woodland, even though the measured rates of uptake there were, as they describe it, "still small when compared with many other studies on native forest soils worldwide." At the other end of the spectrum, they determined that the pasture soil had the least soil CH4 uptake (a mean of 17% of that in the native woodland) and that it was an occasional CH4 source, "possibly related to its greater soil water status, greater soil nitrogen status and possible differences in soil microbial community structure." In between these two extremes, they found that "afforestation with pines or eucalypts increased CH4 uptake to 32% and 43% of that in the native woodland, respectively." As for CO2, they noted the obvious fact that "it is widely accepted that afforestation leads to an increase in carbon sequestration through tree biomass growth."
What it means
Putting these several observations together, Livesley et al. say their study confirmed "there is a triple greenhouse-gas benefit from afforestation of pasture systems," noting that in addition to carbon sequestration via tree biomass, "there is a decrease in N2O emissions because of lower nitrogen inputs and a tighter nutrient cycling, and an increase in CH4 uptake by forest soils."