Effect of stand age on greenhouse gas fluxes from a Sitka spruce [Picea sitchensis (Bong.) Carr.] chronosequence on a peaty gley soil

The influence of forest stand age in a Picea sitchensis plantation on (1) soil fluxes of three greenhouse gases (GHGs - CO2, CH4 and N2O) and (2) overall net ecosystem global warming potential (GWP), was investigated in a 2-year study. The objective was to isolate the effect of forest stand age on soil edaphic characteristics (temperature, water table and volumetric moisture) and the consequent influence of these characteristics on the GHG fluxes. Fluxes were measured in a chronosequence in Harwood, England, with sites comprising 30- and 20-year-old second rotation forest and a site clearfelled (CF) some 18 months before measurement. Adjoining unforested grassland (UN) acted as a control. Comparisons were made between flux data, soil temperature and moisture data and, at the 30-year-old and CF sites, eddy covariance data for net ecosystem carbon (C) exchange (NEE). The main findings were: firstly, integrated CO2 efflux was the dominant influence on the GHG budget, contributing 93-94% of the total GHG flux across the chronosequence compared with 6-7% from CH4 and N2O combined. Secondly, there were clear links between the trends in edaphic factors as the forest matured, or after clearfelling, and the emission of GHGs. In the chronosequence sites, annual fluxes of CO2 were lower at the 20-year-old (20y) site than at the 30-year-old (30y) and CF sites, with soil temperature the dominant control. CH4 efflux was highest at the CF site, with peak flux 491 +/- 54.5 mu g m(-2) h(-1) and maximum annual flux 18.0 +/- 1.1 kg CH4 ha(-1) yr(-1). No consistent uptake of CH4 was noted at any site. A linear relationship was found between log CH4 flux and the closeness of the water table to the soil surface across all sites. N2O efflux was highest in the 30y site, reaching 108 +/- 38.3 mu g N2O-N m(-2) h(-1) (171 mu g N2O m(-2) h(-1)) in midsummer and a maximum annual flux of 4.7 +/- 1.2 kg N2O ha(-1) yr(-1) in 2001. Automatic chamber data showed a positive exponential relationship between N2O flux and soil temperature at this site. The relationship between N2O emission and soil volumetric moisture indicated an optimum moisture content for N2O flux of 40-50% by volume. The relationship between C : N ratio data and integrated N2O flux was consistent with a pattern previously noted across temperate and boreal forest soils.