Greenhouse gas (GHG) emissions from land-use change and agriculture are significant contributors to both Australia's and the global GHG budget. Limited data exists on CO₂, CH₄ and N₂O trace gas fluxes from subtropical or tropical soils and land-uses. To develop effective mitigation strategies a full global warming potential (GWP) accounting methodology is required that includes emissions of the three primary greenhouse gases. This study aimed to quantify GHG emissions over two consecutive years from three adjacent land-uses; a well-established, unfertilized subtropical grass-legume pasture, a 30 year old (lychee) orchard and a remnant subtropical Gallery rainforest, located in South-East Queensland, Australia. Fluxes were measured using a combination of high resolution automatic sampling and coarser spatial manual sampling.

High infiltration, drainage and subsequent soil aeration under the rainforest limited N₂O loss, as well as promoting CH₄ uptake of 11.2 g CH₄-C ha^-1 day^-1.  Interannual climatic variation resulted in significantly higher N₂O emission from the pasture during 2008 (5.7 g N₂O-N ha day) compared to 2007 (3.9 g N₂O-N ha day), despite receiving nearly 500 mm less rainfall.  Mean N₂O emissions from the lychee plantation increased from an average of 4.0 g N₂O-N ha^-1 day^-1, to 19.8 g N₂O-N ha^-1 day^-1 following a split application of nitrogen fertilizer (560 kg N ha^-1).

The conversion of rainforest to agricultural land resulted in as much as a 20 fold increase in GWP, from 126 kg CO₂ eq. ha^-1 yr^-1 in the rainforest to 848 kg CO₂ eq. ha^-1 yr^-1 in the pasture to 2424 kg CO₂ eq. ha^-1 yr^-1 in the lychee plantation. This increase resulted from altered nitrogen cycling and a reduction in the aerobic capacity of the soil in the pasture and lychee systems, enhancing denitrification and nitrification events, and reducing atmospheric CH₄ uptake in the soil. These findings demonstrate that land use change in subtropical Australia can be a significant source of GHGs and emphasises the need to develop detailed inventories from individual land-uses, particularly with increasing interest in soil carbon sequestration as a potential mitigation strategy.