Stand age and tree species affect N 2O and CH 4 exchange from afforested soils

Abstract

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 N 2O and CH 4 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. N 2O and CH 4 fluxes (mean ± SE) were measured for two years at an afforested site. No species difference was documented for N 2O emission (oak: 4.2 ± 0.7 μg N 2O-N m -2 h -1, spruce: 4.0 ± 1 μg N 2O-N m -2 h -1) but the youngest stands (1.9 ± 0.3 μg N 2O-N m -2 h -1) emitted significantly less N 2O than older stands (6.3 ± 1.2 μg N 2O-N m -2 h -1). CH 4 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 μg CH 4-C m -2 h -1, old: -9.4 ± 1 μg CH 4-C m -2 h -1) but interacted significantly; CH 4 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 N 2O and CH 4 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 N 2O production in older stands and (2) decreasing bulk density and increased gas diffusivity in the top soil over time facilitating better exchange of N 2O and CH 4 with the atmosphere. © Author(s) 2011.