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° to 40°C and total rainfall from 487 to 676 mm. Rates of N₂O flux ranged between 1 and 100 μg/m².h in pasture soils and from -5 to 50 μg/m².h in forest soils; magnitudes were generally similar across the 3 climate zones. Rates of CH₄ flux varied from -1 to -50 μg/m².h in forest soil and from +10 to -30 μg/m2.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.