Soil–atmosphere exchange of greenhouse gases in a Eucalyptus marginata woodland, a clover-grass pasture, and Pinus radiata and Eucalyptus globulus plantations

Soils provide the largest terrestrial carbon store, the largest atmospheric CO₂ source, the largest terrestrial N₂O source and the largest terrestrial CH₄ sink, as mediated through root and soil microbial processes. A change in land use or management can alter these soil processes such that net greenhouse gas exchange may increase or decrease. We 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. Mean N₂O emission in the pasture was 26.6 μg N m^?2 h^?1, significantly greater than in the natural and managed forests (< 2.0 μg N m^?2 h^?1). N₂O emission from pasture soil increased after rainfall events (up to 100 μg N m^?2 h^?1) and as soil water content increased into winter, whereas no soil water response was detected in the forest systems. Gross nitrification through ¹⁵N isotope dilution in all land‐use systems was small at water holding capacity < 30%, and under optimum soil water conditions gross nitrification ranged between < 0.1 and 1.0 mg N kg^?1 h^?1, being least in the native woodland/eucalypt plantation < pine plantation < pasture. Forest soils were a constant CH₄ sink, up to ?20 μg C m^?2 h^?1 in the native woodland. Pasture soil was an occasional CH₄ source, but weak CH₄ sink overall (?3 μg C m^?2 h^?1). There were no strong correlations (R < 0.4) between CH₄ flux and soil moisture or temperature. Soil CO₂ emissions (35–55 mg C m^?2 h^?1) correlated with soil water content (R < 0.5) in all but the E. globulus plantation. Soil N₂O emissions from improved pastures can be considerable and comparable with intensively managed, irrigated and fertilised dairy pastures. In all land uses, soil N₂O emissions exceeded soil CH₄ uptake on a carbon dioxide equivalent basis. Overall, afforestation of improved pastures (i) decreases soil N₂O emissions and (ii) increases soil CH₄ uptake.