Recovery from acidification in the Tillingbourne catchment, southern England: catchment description and preliminary...

Measurements of acid deposition and streamwater chemistry made in 1979-1982 and 1999-2000 are compared for a small, acid-sensitive catchment in Southeast England. The location, geology, soils, vegetation and hydrology of the catchment are described. The catchment is located on an acidic cretaceous sandstone with a low permeability clay sub-stratum. Soils are predominantly podzol and gley, with some mesotrophic peat. The catchment is forested. Mean volume-weighted concentrations in precipitation have changed approximately in proportion to emission changes. SO4(2-) has declined by 61%, H+ by 75%, both NO- and NH4+ by 37% and Cl- by 26%. Changes in wet deposition are greater, sulfate deposition declined by 69%, non-marine SO4(2-) by 73%, H+ deposition by 75%, NO3- and NH4+ by 50% and Cl- by 41%. Sulfate deposition in throughfall, a surrogate for total deposition measurement, has declined by 82% and non-marine SO4(2-) by 86%. Some of these changes are due to alterations in the tree cover and location of the collectors. In 1979-1982, the flux of NO3- and NH4+ in throughfall was less than in rainfall, 7.5 compared with 11.3 kg N ha(-1) year(-1), showing that N uptake by the canopy was greater than dry deposition of these species. However, in 1999-2000, the throughfall flux of N was greater than rainfall, 19.6 compared to 5.7 kg N ha year(-1), indicating that canopy uptake is not occurring to the same extent. Surface water was sampled at the same locations in the catchment during the two periods. At the catchment exit, mean pH increased, from 3.93 to 4.21 mg l(-1), and SO4(2-) declined from 20.2 to 16.7 mg l(-1) (18%). The decrease in SO4(2-) is much less than the reduction in deposition, suggesting that the predicted recovery is being delayed by release of sulfur from the soil. In contrast, NO3- concentrations in the catchment waters increased from 0.22 to 0.52 mg N l(-1) (133%) despite the reduction in N deposition. NH4+ concentrations were low during both study periods. It is concluded that recovery from acidification is probably occurring, but is possibly being delayed by desorption of soil S. The catchment is also showing signs of increasing N saturation, despite a reduction in N inputs.