Abstract
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 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 μg N2O-N m−2 h−1, spruce: 4.0 ± 1 μg N2O-N m−2 h−1) but the youngest stands (1.9 ± 0.3 μg N2O-N m−2 h−1) emitted significantly less N2O than older stands (6.3 ± 1.2 μg 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 μg CH4-C m−2 h−1, old: −9.4 ± 1 μg 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.
Highlights
Afforestation is viewed as a means to mitigate rising atmospheric concentrations of CO2 because forest ecosystems can store atmospheric carbon in the long term
Diffusion of CH4 into the soil has been proposed as a primary constraint on CH4 oxidation rate in soils (Ball et al, 1997) and it is generally accepted that the porosity of forest soils is larger than in agricultural soils, thereby providing a soil environment more suited for CH4 uptake
As we did not observe any consistent effect of tree species on N2O emissions we suggest that the tree species affect the N2O emissions indirectly by influencing the soil N status differently, because N availability (extractable N (Fig. 4) and subroot zone NO3-N concentrations (Table 1)) in general was higher in the soil under Norway spruce than oak
Summary
Afforestation is viewed as a means to mitigate rising atmospheric concentrations of CO2 because forest ecosystems can store atmospheric carbon in the long term. The larger CH4 uptake rates in forest soils have been attributed to both physical and biogeochemical changes in the soil environment following afforestation that favour methanotrophic bacteria. It has been documented that the methanotrophic community responsible for consumption of CH4 is sensitive to disturbance of the soil; the CH4 uptake capacity recovers only slowly to pre-cultivation levels in afforested soils (Prieme et al, 1997). This has recently been demonstrated in a field based study of GHG exchange in a Canadian afforestation chronosequence (Peichl et al, 2010). It is recognised that CH4 uptake in soils is further suppressed by high levels of available mineral nitrogen (N) in the soil as well as atmospheric deposition of N (Butterbach-Bahl et al, 1998; Reay and Nedwell, 2004; Steudler et al, 1989)
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