Differences in carbon density and soil CH4/N2O flux among remnant and agro-ecosystems established since European settlement in the Mornington Peninsula, Australia

• We measured soil CH4 and N2O flux and ecosystem C density along a LUC sequence.
• Forest soil was a CH4 sink but pasture and viticulture soils were CH4 sources.
• Despite soil moisture differences, soil N2O emissions did not differ.
• Forest soil C density was significantly less than the agro-ecosystems.
• Changes in non-CO2 soil processes contribute to the historical GHG impacts of LUC.

National and regional C emissions from historical land use change (LUC) and fossil fuel use are proposed as a basis to ascribe ‘burden-sharing’ for global emission reduction targets. Changes in non-CO2 greenhouse gas emissions as a result of LUC have not been considered, but may be considerable. We measured soil–atmosphere exchange of methane (CH4) and nitrous oxide (N2O) in remnant forest, pasture and viticulture systems in four seasons, as well as differences in soil C density and the C density of remnant forest vegetation. This approach enabled comparative assessment of likely changes in ecosystem C density and soil non-CO2 greenhouse gas exchange along a LUC continuum since European settlement. Soil CH4 uptake was moderate in forest soil (− 27 μg C m− 2 h− 1), and significantly different to occasionally large CH4 emissions from viticulture and pasture soils. Soil N2O emissions were small and did not significantly differ. Soil C density increased significantly with conversion from forest (5 kg m− 2) to pasture (9 kg m− 2), and remained high in viticulture. However, there was a net decrease in ecosystem C density with forest conversion to pasture. Concurrently, net soil non-CO2 emissions (CH4 and N2O combined) increased with conversion from forest to pasture. Since European settlement 170 years ago, it was estimated ~ 8114 Gg CO2-e has been released from changes in ecosystem C density in the Mornington Peninsula, whereas ~ 383 Gg CO2-e may have been released from changes in soil non-CO2 exchange processes. Principally, a switch from soil CH4 uptake to soil CH4 emission after forest clearing to agro-pastoral systems provided this further ~ 5% contribution to the historical landscape CO2-e source strength. Conserving and restoring remnant forests and establishing new tree-based systems will enhance landscape C density. Similarly, minimising anaerobic, wet conditions in pasture/viticulture soils will help reduce non-CO2 greenhouse gas emissions.