Stand age and tree species affect N2O and CH4 exchange from afforested soils

Afforestation of former agricultural land is a means to mitigate anthropogenic greenhouse gas emissions. The objectives of this study were (1) to assess the effect of oak (Quercus robur) and Norway spruce (Picea abies [L.] Karst.) stands of different stand ages (13-17 and 40 years after afforestation, respectively) on N2O and CH4 exchange from the soil under these species and (2) identify the environmental factors responsible for the differences in gas exchange between tree species of different ages. N2O and CH4 fluxes (mean +/- SE) were measured for two years at an afforested site. No species difference was documented for N2O emission (oak: 4.2 +/- 0.7 mu g N2O-N m(-2) h(-1), spruce: 4.0 +/- 1 mu g N2O-N m(-2) h(-1)) but the youngest stands (1.9 +/- 0.3 mu g N2O-N m(-2) h(-1)) emitted significantly less N2O than older stands (6.3 +/- 1.2 mu g N2O-N m(-2) h(-1)). CH4 exchange did not differ significantly between tree species (oak: -8.9 +/- 0.9, spruce: -7.7 +/- 1) or stand age (young: -7.3 +/- 0.9 mu g CH4-C m(-2) h(-1), old: -9.4 +/- 1 mu g CH4-C m(-2) h(-1)) but interacted significantly; CH4 oxidation in the soil increased with stand age in oak and decreased with age for soils under Norway spruce. We conclude that the exchange of N2O and CH4 from the forest soil undergoes a quick and significant transition in the first four decades after planting in both oak and Norway spruce. These changes are related to (1) increased soil N availability over time as a result of less demand for N by trees in turn facilitating higher N2O production in older stands and (2) decreasing bulk density and increased gas diffusivity in the top soil over time facilitating better exchange of N2O and CH4 with the atmosphere.