Nitrous oxide emissions of undrained, forestry-drained, and rewetted boreal peatlands

Drained peatlands, especially nitrogen (N) -rich ones, are sources of nitrous oxide (N2O) to the atmosphere. As this emission is largely caused by N mineralization in oxic peat soil, rewetting the peat soil is likely to decrease the N2O emissions to similar levels with natural, undrained peatlands. Emissions from these water-saturated peat soils have often been assumed negligible, for example in the Intergovernmental Panel on Climate Change (IPCC) guidelines for national greenhouse gas inventories. However, the number of published data of annual N2O emissions from undrained peatlands is small and even smaller for rewetted peatlands. Thus, we still have limited understanding of the effects of land-use change such as drainage and rewetting. We analyzed N2O flux data, mostly previously unpublished, from 28 undrained, 65 forestry-drained, and 24 rewetted boreal peatland study sites in Finland and calculated annual N2O emissions for each site, including emissions from ditches at 51 sites. We classified the sites into nutrient-rich and nutrient-poor ones, similar to the IPCC guidelines. Annual N2O emissions from the undrained peatlands were consistently small, but not negligible, averaging 0.11 and 0.10 g m-2a 1 in the nutrient-rich and nutrient-poor sites, respectively. Emissions from the drained peatlands were significantly higher than from the undrained peatlands in the nutrient-rich (mean 0.23 g m(-2) a(-1)) but not in the nutrient-poor (mean 0.08 g m(-2) a(-1)) sites. Emissions from the rewetted peatlands (means 0.05 and 0.08 g m(-2) a(-1) for nutrient-rich and nutrient-poor sites, respectively) were similar or lower than from the undrained ones. Emissions from ditches were similar to those from strips and correlated positively with strip emissions at the drained sites. Nutrient-rich soil (low CN ratio) and a low water table increased emissions significantly at the drained sites, whereas the continuously higher water tables kept emissions low at all undrained and rewetted sites. In conclusion, rewetting is an effective means to decrease the N2O emissions of nutrient-rich drained peatlands back to natural levels, although this natural level is not zero, as has been suggested earlier.