Nitrous oxide and methane emissions from soil are reduced following afforestation of pasture lands in three contrasting climatic zones

Land use change from agriculture to forestry offers potential opportunities for carbon (C) sequestration and thus partial mitigation of increasing levels of carbon dioxide (CO2) in the atmosphere. The effects of land use change of grazed pastures on in situ fluxes of nitrous oxide (N2O) and methane (CH4) from soil were examined across 3 forest types in Australian temperate, Mediterranean, and subtropical regions, using a network of paired pasture-forest sites, representing 3 key stages of forest stand development: establishment, canopy-closure, and mid to late rotation. During the 12-month study, soil temperature ranged from -6 degrees to 40 degrees C and total rainfall from 487 to 676 mm. Rates of N2O flux ranged between 1 and 100 mu g/m(2).h in pasture soils and from -5 to 50 mu g/m(2). h in forest soils; magnitudes were generally similar across the 3 climate zones. Rates of CH4 flux varied from -1 to -50 mu g/m(2). h in forest soil and from +10 to -30 mu g/m(2). h in pasture soils; CH4 flux was highest at the subtropics sites and lowest at the Mediterranean sites. In general, N2O emissions were lower, and CH4 consumption was higher, under forest than pasture soils, suggesting that land use change from pasture to forest can have a positive effect on mitigation of non-CO2 greenhouse gas (GHG) emissions from soil as stands become established. The information derived from this study can be used to improve the capacity of models for GHG accounting (e.g. FullCAM, which underpins Australia's National Carbon Accounting System) to estimate N2O and CH4 fluxes resulting from land use change from pasture to forest in Australia. There is still, however, a need to test model outputs against continuous N2O and CH4 measurements over extended periods of time and across a range of sites with similar land use, to increase confidence in spatial and temporal estimates at regional levels.