Catchment export of base cations: Improved mineral dissolution kinetics influence the role of water transit time

Abstract

Soil mineral weathering is one of the major sources of base cations (BC), which play a dual role for a forest ecosystem; they function both as plant nutrients, and for buffering against acidification of catchment runoff. On a long-term basis, the soil weathering rates will determine the highest sustainable forest productivity without causing acidification. It is believed that the hydrologic residence time plays a key role in determining weathering rates on a landscape scale.The weathering model PROFILE has been used for almost 30 years to calculate weathering rates in the rooting zone of forest 25 soils. However, the mineral dissolution equations in PROFILE are not adapted for the unsaturated zone, and employing these equations on a catchment scale results in a significant over-prediction of base cation release rates to surface waters. In this study we use a revised set of PROFILE equations which, among other features, include retardation from silica concentrations.Relationships between the water transit time (WTT) and soil water concentrations were derived for each base cation, by simulating the soil water chemistry along a one-dimensional flowpath, using the mineralogy from a glacial till soil. We show 30 how the revised PROFILE equations are able to reproduce patterns in BC-and Si-concentrations, as well as BC-ratios (Ca 2+ /BC, Mg 2+ /BC and Na + /BC), observed in soil water profiles and catchment runoff. As opposed to the original set of PROFILE equations, the revised set of equations could reproduce how increasing WTT led to decreasing Na + /BC, as well as increasing Ca 2+ /BC and Mg 2+ /BC. Furthermore, the total release of base cations from a hillslope was calculated using a mixing model, where water of different WTT was mixed according to an externally modelled WTT-distribution. The revised set of 35 SOIL Discuss., https://doi.equations gave a 50% lower base cation release (0.23 eq·m-2·yr-1) than the original PROFILE equations, and are in better agreement with mass balance calculations of weathering rates. The results from this study thus demonstrate that the revised mineral dissolution equations for PROFILE are a major step forward in modelling weathering rates on a catchment scale.

Reviews and syntheses: Weathering of silicate minerals in soils and watersheds: Parameterization of the weathering kinetics module in the PROFILE and ForSAFE models

“…The weathering rate was extracted from the simulations to assess the site for pollution control. In this case the ForSAFE model was used with a daily time step to estimate daily weathering rates (Sverdrup et al, 2014, McDonnel et al, 2017, Belyazid et al, 2019.…”

“…In the dynamic integrated terrestrial ecosystem assessment model ForSAFE-VEG, the system evolution over time takes account of interactions with a living biosphere, organic matter turnover and ion exchange. Further details of these models can be found in the literature (Sverdrup et al, 1987, 1996a, 2007, 2017, 2014, 2014, 2017, 2019, 2003, Zancchi et al, 2014, 2016a,b, Belyazid et al, 2017.…”

“…This study outlines our efforts to update these early mineral weathering kinetics models for watershed water chemistry and deeper groundwater. This effort is the result of preparations for, discussions at, and subsequent efforts after a workshop held at Ystad Saltsjöbad, Ystad, Sweden, April 11-14, 2016, in connection to the Swedish QWARTS research programme. Key literature to read to aid in following this text are the weathering book by Sverdrup (1990) and the articles Sverdrup and Warfvinge (1988a,b, 1992 and .…”

Improved parameterization of the weathering kinetics module in the PROFILE and ForSAFE models

“…Further details of these models can be found in the appendix and the literature (Sverdrup et al, 1987 , 2007, 2014, 2017, 2019, Wallman et al, 2002, Zancchi et al, 2014, 2016a,b, Belyazid et al, 2017.…”

“…Overlaid are the observed Bc and Si-concentrations from the soil profile, plotted against log10 of soil depth (solid lines with markers) in Figure 23c. The weathering model considers all soil processes including ion exchange, vegetation interactions, decomposition of organic matter, water transport in the catchment in both the horizontal and vertical directions (Belyazid et al, 2004, 2011a,b, 2010a, 2019, Erlandsson-Lampa et al, 2019. The model reproduces the observed field observations as a function of depth (Zanchi et al, 2016).…”