Afforestation does not necessarily reduce nitrous oxide emissions from managed boreal peat soils

Pristine peatlands have generally low nitrous oxide (N2O) emissions but drainage and management practices enhance the microbial processes and associated N2O emissions. It is assumed that leaving peat soils from intensive management, such as agriculture, will decrease their N2O emissions. In this paper we report how the annual N2O emission rates will change when agricultural peat soil is either left abandoned or afforested and also N2O emissions from afforested peat extraction sites. In addition, we evaluated a biogeochemical model (DNDC) with a view to explaining GHG emissions from peat soils under different land uses. The abandoned agricultural peat soils had lower mean annual N2O emissions (5.5 +/- A 5.4 kg N ha(-1)) than the peat soils in active agricultural use in Finland. Surprisingly, N2O emissions from afforested organic agricultural soils (12.8 +/- A 9.4 kg N ha(-1)) were similar to those from organic agricultural soils in active use. These emissions were much higher than those from the forests on nutrient rich peat soils. Abandoned and afforested peat extraction sites emitted more N2O, (2.4 +/- A 2.1 kg N ha(-1)), than the areas under active peat extraction (0.7 +/- A 0.5 kg N ha(-1)). Emissions outside the growing season contributed significantly, 40% on an average, to the annual emissions. The DNDC model overestimated N2O emission rates during the growing season and indicated no emissions during winter. The differences in the N2O emission rates were not associated with the age of the land use change, vegetation characteristics, peat depth or peat bulk density. The highest N2O emissions occurred when the soil C:N ratio was below 20 with a significant variability within the measured C:N range (13-27). Low soil pH, high nitrate availability and water table depth (50-70 cm) were also associated with high N2O emissions. Mineral soil has been added to most of the soils studied here to improve the fertility and this may have an impact on the N2O emissions. We infer from the multi-site dataset presented in this paper that afforestation is not necessarily an efficient way to reduce N2O emissions from drained boreal organic fields.