Strong pH influence on N2O and CH4 fluxes from forested organic soils

Greenhouse gas (GHG) emissions from farmed organic soils can have a major impact on national emission budgets. This investigation was conducted to evaluate whether afforestation of such soils could mitigate this problem. Over the period 1994-1997, emissions of methane (CH4) and nitrous oxide (N2O) were recorded from an organic soil site in Sweden, forested with silver birch (Betula pendula Roth), using static field chambers. The site was used for grazing prior to forestation. Soil pH and soil carbon content varied greatly across the site. The soil pH ranged from 3.6 to 5.9 and soil carbon from 34 to 42%. The mean annual N2O emission was 19.4 (+/- 6.7) kg N2O-N ha(-1) and was strongly correlated with soil pH (r = -0.93, P < 0.01) and soil carbon content (r = 0.97, P < 0.001). The N2O emissions showed large spatial and temporal variability with greatest emissions during the summer periods. The site was a sink for CH4 (i.e. -0.8 (+/- 0.5) kg CH4 ha(-1) year(-1)) and the flux correlated well with the C/N ratio (r = 0.93, P < 0.01), N2O emission (r = 0.92, P < 0.01), soil pH (r = -0.95, P < 0.01) and soil carbon (r = 0.97, P < 0.001). CH4 flux followed a seasonal pattern, with uptake dominating during the summer, and emission during winter. This study indicates that, because of the large N2O emissions, afforestation may not mitigate the GHG emissions from fertile peat soils with acidic pH, although it can reduce the net GHG because of greater CO2 assimilation by the trees compared with agricultural crops.