Dynamic modelling of weathering rates n Is there any benefit over steady-state modelling?

Abstract. Weathering rates are of considerable importance in estimating the acidification sensitivity and recovery capacity of soil, and are thus important in the assessment of the sustainability of forestry in a time of changing climate and growing demands for forestry products. In this study, we modelled rates of weathering in mineral soil at two forested sites in southern Sweden included in the SWETHRO monitoring network using two models. The aims were to determine whether the dynamic model ForSAFE gives comparable weathering rates as the steady-state model PROFILE, and whether the ForSAFE model provided useful extra information on weathering behaviour. The average weathering rates calculated with ForSAFE were very similar to those calculated with PROFILE for the two modelled sites. The differences between the models regarding the weathering of certain soil layers seemed to be due mainly to differences in calculated soil moisture. The weathering rates provided by ForSAFE vary seasonally with temperature and soil moisture, as well as on longer time scales, depending on environmental changes. Long-term variations due to environmental changes can be seen in the ForSAFE results, for example: the weathering of silicate minerals is suppressed under acidified conditions due to elevated aluminium concentration in the soil, whereas the weathering of apatite is accelerated by acidification. The weathering of both silicates and apatite is predicted to be enhanced by increasing temperature during the 21st century. In this part of southern Sweden, precipitation is assumed to be similar to today’s level during the next forest rotation. However, in parts of Sweden with projected decreasing soil moisture, weathering might not increase despite increasing temperature. These results show that the dynamic ForSAFE model can be used for weathering rate calculations and that it gives average results comparable to those from the PROFILE model. However, dynamic modelling provides extra information on the variation in weathering rates with time, and offers much better possibilities for scenario modelling.