Modeling Aluminum and Organic Matter Solubility in the Forest Floor Using WHAM

Abstract

Several studies suggest that solution concentrations of Al in organic surface soils are controlled by complexation with organic matter. We applied the mechanistic Windermere humic aqueous model (WHAM) to describe the solubility of Al and organic matter as observed in a batch equilibrium study with a forest floor Oe horizon. WHAM is unique in that it considers interactions of soil organic matter with protons and metals. We also compared WHAM with a previously proposed linear regression model that describes Al solubility. A range of soil Al contents was established by adding different amounts of Al in batch prior to titration with acid or base. The soil Al content was described by the bound Al ratio (BAR), defined as the equivalent ratio of organically bound Al and carboxyl groups. The bound Al ratio and pH ranged from 0.1 to 3, and from 1.7 to 6.3, respectively. Solutions were undersaturated with respect to Al(OH)3, except at BAR ≥ 2 and pH ≥ 4.5. Aluminum solubility increased with increasing BAR. Organic matter solubility was greatest at low BAR and high pH. WHAM reproduced the observed pH and Al concentrations, using parameters derived from experimental data. At BAR < 0.7, pH–pAl relationships were approximately linear. At BAR ≥ 0.7, there was a nonlinear increase in ΔpAl/ΔpH with pH. WHAM simulated the changing slope of the pH–pAl curves satisfactorily and reproduced observed trends in dissolved organic C (DOC) concentrations. This supports the hypotheses and assumptions concerning mechanisms for binding Al to soil organic matter in a forest floor, as embodied in WHAM.

Aquatic DOC export from subarctic Atlantic blanket bog in Norway is controlled by seasalt deposition, temperature and precipitation

“…Physico-chemical models provide a mechanistic explanation for this behavior, i.e., increased solubility of organic matter under conditions of lower conductivity and lower acidity, changing ion-binding and increasing negative charge of humic compounds, leading to a tendency to overcoming of inherent hydrophobicity of humic compounds (Tipping 1994). The WHAM model was successfully tested on soil-solution interactions in laboratory conditions (De Wit et al 1999), and the concept of chemical controls on desorption behavior of DOC was successfully implemented in the INCA-C model (Futter and de Wit 2008).…”

Humic Ion-Binding Model VII: a revised parameterisation of cation-binding by humic substances

“…Examples include 71 the acidification of soils [7][8][9][10][11][12][13][14] and surface waters [15] , trace metal behaviour in soils [16][17][18][19][20][21][22] , surface 72 waters [23][24][25][26][27][28][29][30][31] and groundwaters [32] , lake sediment diagenesis [33,34] , rare earth geochemistry [35-73 37] , iron and manganese geochemistry [38][39][40][41] , radionclide geochemistry [42][43][44][45] , organic matter 74 solubility in soils [46,47] , catchment modelling [48,49] , interactions of metals with biota [50,51] , 75 ecotoxicology [52][53][54][55][56][57][58][59] and Critical Loads …”

“…Proton dissociation is represented by postulating 8 groups with different acid strengths, the 125 reactions being characterised by intrinsic equilibrium constants, the negative logarithms of 126 which are denoted by pK 1 -pK 8 . The four most strongly-acid groups (groups [1][2][3][4] The maximum number of parameters that can be optimised to describe metal binding is six 158…”

Modelling the effects of climate on long-term patterns of dissolved organic carbon concentrations in the surface waters of a boreal catchment

“…Estimates of the effects of anthropogenic acid deposition on DOC dynamics in soils and surface waters would improve model predictions. More information on the controls of sorption/desorption dynamics of organic carbon in boreal soils would help in assessing the adequacy of the model of in-soil processes in INCA-C. Chloride, SO 4 (Kalbitz et al, 2000) and Al (de Wit et al, 1999) can all influence organic carbon sorption dynamics in soils. Measured data on soil solution chemistry are available for some years but high quality modelled data would be useful as they would fill-in the time periods for which measured data were not available and would permit forecasting of future conditions.…”

“…This effect has been shown experimentally by F. Moldan (personal communication) and through modelling of long-term time series (Futter and de Wit, 2008). Greater ionic strength as well as increased hydrogen ion and aluminium (Al) concentrations in soil solution all can decrease organic matter solubility (de Wit et al, 1999;Kalbitz et al, 2000). Neff and Asner (2001) state that the two most important mechanisms controlling DOC flux from forested catchments are hydrologic flow paths and the sorption dynamics of organic carbon in soils.…”