Concentrations and fluxes of dissolved organic carbon in UK topsoils

Dissolved organic carbon (DOC) concentrations in soil water samples collected from depths of 5 to 20 cm at 10 moorland and 11 forest sites during the period 2000-2006 were obtained from new measurements and from the monitoring programmes of the UK Environmental Change Network and the International Cooperative Programme (ICP) on Forests. Data on soil properties and vegetation type were also assembled. Considering data from Prenart tension collectors, which were used at nearly all the sites, mean annual concentrations ranged from 1.3 to 97.5 g m(-3) with means of 19.5 (standard deviation 15.2) and 27.6 (SD 23.3) g m(-3) for moorland and forest sites respectively. Interannual mean DOC concentration at an individual site varied by only 1.5-fold, averaged over all sites with at least three years' data. Concentrations during summer months (April to September) were on average 17% greater than those in winter (October to March). If data from two sites (the single peatland and an unusual forest site) were ignored, DOC concentrations were strongly inversely related to water flux, estimated from rainfall and evaporation data. Fluxes of DOC, calculated by combining concentration with water flux, ranged from 2.2 to 71.9 gC m(-2) yr(-1) over all sites and years, with overall means of 19.2 (SD 13.6) and 12.2 (SD 13.9) gC m(-2) yr(-1) for the moorland and forest sites respectively. However, if the two exceptional sites were omitted, the overall mean was 9.1 gC m(-)(2) yr(-1) with a standard deviation of only 4.9 gC m(-2) yr(-1). Annual DOC flux was strongly dependent upon annual water flux, varying by 3.5-fold between years when averaged over all sites. On average, 75.5% of the DOC was exported during the winter period (October to March).