Recovery of sulfate saturated soils in the Plynlimon catchments, mid-Wales following reductions in atmospheric S inputs from the 1980s to 2011

Sulfate adsorption capacity of B-horizons of base-poor, predominantly stagnopodzol, soils from the Plynlimon catchments, mid-Wales was determined by combination of laboratory adsorption and desorption isotherms. Results show that sulfate adsorption capacity of a range of stagnopodzol (Histic-stagno-podzol (Leptic), WRB), brown podzolic soil (Histic-umbrisol (Leptic), WRB) and stagnohumic gley (Histic-stagno-gleysol, WRB) B-horizons was positively related to the amounts of extractable (pyrophosphate and oxalate) Fe + Al, with the stagnopodzol and brown podzolic soil Bs horizon having the largest adsorption capacity and stagnohumic gley Bg horizon the smallest adsorption capacity. Results show that dissolved organic carbon (DOC) has a negative but limited effect on sulfate adsorption in these soils. Results obtained from a set of historical soil samples revealed that the grassland brown podzolic soil Bs horizon and afforested stagnopodzol Bs horizon were highly saturated with sulfate in the 1980s, at 63% and 89% respectively, whereas data from some recently sampled soil from two sites revisited in 2010-11 indicates that percentage sulfate adsorption saturation has since fallen substantially, to 41% and 50% respectively. Between 1984 and 2009 the annual rainfall-weighted mean excess SO(4)-S concentration in bulk precipitation declined linearly from 0.37 mg S l(-1) to 0.17 mg S l(-1). Over the same period, flow weighted annual mean stream water SO(4)-S concentrations decreased approximately linearly from 1.47 mg S l(-1) to 0.97 mg S l(-1) in the plantation afforested Hafren catchment compared to a drop from 1.25 to 0.69 mg S l(-1) in the adjacent moorland catchment of the Afon Gwy. In flux terms, the mean decrease in annual stream water SO(4)-S flux has been approximately 0.4 kg S ha(-1) yr(-1), whilst the recovery in stream water quality in the Afon Cyff grassland catchment has been partly offset by loss of SO(4)-S by desorption from the soil sulfur pool of approximately 0.2 kg S ha(-1) yr(-1).