Using compositional change within soil profiles for modelling base cation transport and chemical weathering

The pedological mass balance (PMB) model is still among the best quantitative geochemical approaches to estimate chemical weathering and pedogenesis. This one-dimensional vertical approach permits estimation of the enrichment factor for a given element in a soil layer, based on changes in 1) residual enrichment, 2) strain, and 3) mass transport. The first two components represent the closed-system contributions to the enrichment factor since they occur without movement of the element under consideration. The last component represents the open-system contribution that results from net element mass movement across soil layer boundaries. Modelling the mass transport function represents a crucial step in soil pedogenesis model definitions and in the estimation of soil chemical weathering rates. We present here a method to empirically model this mass transport function based on the change in soil horizon composition within a soil profile, taking base cations (Ca, Mg, K, and Na) and Ti composition in three northern forest soil profiles as showcase. The empirically modelled compositional trends explained between 81 and 94% of the change in element composition within the soil profiles. Among the studied elements, Ti was the most recalcitrant one, except at one site where K tended to accumulate (relatively to Ti). The results suggest that changes in composition within soil profiles are mostly linear. The calculated degree of weathering (k) of a given soil horizon was linearly related to the calculated gain/loss of base cations in the three soils using the PMB model, and was correlated to observed soil organic matter, carbon and N concentrations and its texture, pH, and bulk density. Although the modelled trend can successfully predict mass transport changes with depth as the composition of soil horizons changes upon weathering, the question of time function for this process to operate remains to be quantified. (C) 2008 Elsevier B.V. All rights reserved.