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 mu g N m(-2) h(-1), significantly greater than in the natural and managed forests (< 2.0 mu g N m(-2) h(-1)). N2O emission from pasture soil increased after rainfall events (up to 100 mu g 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 N-15 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 mu g C m(-2) h(-1) in the native woodland. Pasture soil was an occasional CH4 source, but weak CH4 sink overall (-3 mu g 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.