Vegetation impacts ditch methane emissions from boreal forestry-drained peatlands-Moss-free ditches have an order-of-magnitude higher emissions than moss-covered ditches

Ditches of forestry-drained peatlands are an important source of methane (CH4) to the atmosphere. These CH4 emissions are currently estimated using the IPCC Tier 1 emission factor (21.7 g CH4 m(-2) y(-1)), which is based on a limited number of observations (11 study sites) and does not take into account that the emissions are affected by the condition and age of the ditches. Furthermore, the total area of different kinds of ditches remains insufficiently estimated. To constructmore advanced ditch CH4 emission factors for Finland, we measured CH4 emissions in ditches of 3 forestry-drained peatland areas (manual chamber technique) and amended this dataset with previously measured unpublished and published data from 18 study areas. In a predetermined 2type ditch classification scheme, the mean CH4 emissions (+/- standard error) were 2.6 +/- 0.8 g CH4 m(-2) y(-1) and 20.6 +/- 7.0 g CH4 m(-2) y(-1) in moss-covered and moss-free ditches, respectively. In a more detailed 4-type classification scheme, the yearly emissions were 0.6 +/- 0.3, 3.8 +/- 1.1, 8.8 +/- 3.2, and 25.1 +/- 9.7 g CH4 m(-2) y(-1) in Sphagnum-covered, Sphagnum-and vascular plant-covered, moss-free and vascular plant-covered, and plant -free ditches, respectively. Hence, we found that Tier 1 emission factor may overestimate ditch CH4 emissions through overestimation of the emissions of moss-covered ditches, irrespective of whether they harbor potentially CH4 conducing vascular plants. Based on the areal estimates and the CH4 emission factors for moss-covered and moss-free ditches, CH4 emissions of ditches of forestry-drained peatlands in Finland were 8,600 t a(-1), which is 63% lower than the current greenhouse gas inventory estimates for ditch CH4 emissions (23,200 t a(-1)). We suggest that the Tier 1 emission factor should be replaced with more advanced emission factors in the estimation of ditch CH4 emissions of boreal forestry-drained peatlands also in other countries than in Finland. Furthermore, our results suggest that the current practice in Finland to minimize ditch-network maintenance by ditch cleaning will likely decrease CH4 emissions from ditches, since old moss-covered ditches have very low emissions.