Fluxes of N2O, CH4 and CO2 on afforested boreal agricultural soils

After drainage of natural boreal peatlands, the decomposition of organic matter increases and peat soil may turn into a net source of CO2 and N2O, whereas CH4 emission is known to decrease. Afforestation is a potential mitigation strategy to reduce greenhouse gas emission from organic agricultural soils. A static chamber technique was used to evaluate the fluxes of CH4, N2O and CO2 from three boreal organic agricultural soils in western Finland, afforested 1, 6 or 23 years before this study. The mean emissions of CH4 and N2O during the growing seasons did not correlate with the age of the tree stand. All sites were sources of N2O. The highest daily N2O emission during the growing season, measured in the oldest site, was as high as 29 mg N2O m(-2)d(-1). In general, organic agricultural soils are sinks for methane. Here, the oldest site acted as a small sink for methane, whereas the two youngest afforested organic soils were sources for methane with maximum emission rates (up to 154 mg m(-2)d(-1)) similar to those reported for minerogenous natural peatlands. Soil respiration rates decreased with the age of the forest. The high soil respiration in the younger sites, probably resulted from the high biomass production of herbs, could create soil anaerobiosis and increase methane production. Our results show that afforestation of agricultural peat soils does not abruptly terminate the N2O emissions during the first two decades, and afforestation can even enhance methane emission for a few years. The carbon accumulation in the developing tree stand can partly compensate the carbon loss from soil.