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

Abstract The influence of forest stand age in a Picea sitchensis plantation on (1) soil fluxes of three greenhouse gases (GHGs – CO 2 , CH 4 and N 2 O) 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 CO 2 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 CH 4 and N 2 O 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 CO 2 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. CH 4 efflux was highest at the CF site, with peak flux 491±54.5 μg m −2 h −1 and maximum annual flux 18.0±1.1 kg CH 4 ha −1 yr −1 . No consistent uptake of CH 4 was noted at any site. A linear relationship was found between log CH 4 flux and the closeness of the water table to the soil surface across all sites. N 2 O efflux was highest in the 30y site, reaching 108±38.3 μg N 2 O‐N m −2 h −1 (171 μg N 2 O m −2 h −1 ) in midsummer and a maximum annual flux of 4.7±1.2 kg N 2 O ha −1 yr −1 in 2001. Automatic chamber data showed a positive exponential relationship between N 2 O flux and soil temperature at this site. The relationship between N 2 O emission and soil volumetric moisture indicated an optimum moisture content for N 2 O flux of 40–50% by volume. The relationship between C : N ratio data and integrated N 2 O flux was consistent with a pattern previously noted across temperate and boreal forest soils.