Weathering rates in catchments calculated by different methods and their relationship to acidic inputs

The sensitivity of catchments to acidification is often assessed by calculation of weathering rates for comparison of the rates of release of base cations with the measured acidic inputs. Methods of calculation of weathering rates include (1) long-term rates from elemental depletion in soil profiles; (2) current rates from input-output budgets; (3) strontium isotope ratios to modify current rates for calcium; (4) modelling using PROFILE or MAGIC; (5) laboratory experimental methods. Not all these methods can be applied in any one situation and when more than one method can be used, there are often discrepancies in the resulting figures. Comparison of long-term and current rates with acidic inputs are often consistent with the known acidification status of some Scottish catchments, but in others it is often difficult to establish a relationship. In some catchments where acidification only occurs under high-flow conditions, for example, long-term rates (12-24 meg m(-2)a(-1)) are an order of magnitude lower than current rates (185-340 meg m(-2)n(-1)). In seven Scottish catchments on four rock types, weathering rates calculated by PROFILE are of the same order of magnitude as long-term rates calculated for the same soil profiles. Current rates, on the other hand, are always higher than the long-term rates, sometimes by a factor as high as 22. and although this could indicate that release of base cations from these soils has increased in recent times, possibly due to anthropogenic inputs, the comparisons may not be valid.