Chemical variations in near surface drainage water for an acidic spruce forested UK upland area subjected to timber harvesting: inferences on cation exchange processes in the soil

Hydrochemical variations in the major, minor and trace element concentrations of a first order stream draining a plantation of sitka spruce (Picea sitchensis (Bong.) Carr.), subjected to felling, are described. The stream water is acidic and aluminium bearing as the area drained comprises thin, acidic, organic-rich soils. The chemical composition of the stream varies with time and the data is scattered owing to the complex hydrological, chemical and biological interactions involved. None the less, the catchment has the ability to damp down the chemical signal of the rainfall. There is no statistically significant correlation between rainfall and stream water chemistry for any of the components measured. Deforestation leads to increased concentrations of NO3, K, H+, the major elements and Al together with a decrease in alkalinity. The changes last for approximately 2-3 years: conditions then revert to pre-felling levels. These results fit well with previous findings in that deforestation leads to the disruption of the biochemical functioning of the catchment (releasing nitrate and potassium and acidifying the soil water). The major element changes are linked to increased leaching of the soil waters as a consequence of changing hydrology (evapotranspiration is probably reduced and the catchment soils wet up). However, the variations in Al, dissolved organic carbon, Br and I are less than would be anticipated based on previous results for a companion study of the main drainage area. Dissolved organic carbon, Fe, Cd, Y and La show increases, but these changes occur after the felling is complete and there has been no return to pre-felling values 3 years on. The applicability of classically used cation exchange theory for the soil, the basis for many of the conclusions derived in the soil acidification debate, is questioned and found to be highly suspect.