Operative controls on dissolved aluminium activity in acid forest soils of southern and southwestern China: mineral equilibrium, cation exchange, organic complexation, or kinetics?

Monitoring of five acid forest catchments in southern and southwestern China (Guo et al., 2007, Applied Geochemistry) showed that none of the classical mineral dissolution equilibria — gibbsite, jurbanite, kaolinite or imogolite — could explain the observed dissolved $Al^{3+}$ activity across the pH range 3.6–5.6: $pAl$ correlated with pH with horizon-specific slopes of 1.28 (upper) and 2.00 (lower), soil acidity and ionic strength were the strongest statistical predictors, cation exchange dominated the upper horizon and proton-driven dissolution the lower, and soil organic matter played no clearly significant role. Follow-up monitoring of throughfall, soil water and streamwater at the Tieshanping catchment over 2001–2013 (Yu et al., 2017) documented delayed recovery from acidification, with soil-water pH increasing only recently, but did not identify the processes controlling dissolved aluminium. The problem is to determine which thermodynamic and kinetic processes actually govern dissolved $Al^{3+}$ activity in these highly weathered subtropical acid soils — equilibrium with specific solid phases, cation exchange, complexation by soil organic matter, ionic-strength effects, or kinetic/non-equilibrium constraints — and to establish a quantitative, field-validated geochemical model that predicts aluminium solubility across pH and soil depth, with the process attribution discriminated against competing control hypotheses.

Background

Acid deposition — the input of sulfuric and nitric acids from fossil-fuel emissions — accelerates the acidification of forest soils, and its most damaging proximate effect is the mobilization of aluminium (Al), the third most abundant element of the continental crust. In soils below about pH 5.5, Al-bearing solids and exchange surfaces release dissolved Al into the soil solution (the water held in soil pores), where the free ion $Al^{3+}$ is toxic to fine roots and, after export to streams, to aquatic biota. Because toxicity and transport are governed by the activity (the thermodynamically effective concentration) of $Al^{3+}$ rather than by total dissolved Al, predicting $Al^{3+}$ activity has been a central problem of soil and catchment acidification chemistry since the 1980s.

Three families of control have been proposed. First, equilibrium dissolution of a solid Al phase fixes $Al^{3+}$ activity relative to pH and other solutes: gibbsite ($Al(OH)_3$) implies that $pAl$ (defined as $-\log$ of the $Al^{3+}$ activity) is linear in pH with slope 3; jurbanite (a basic aluminum sulfate, $AlSO_4OH\cdot 5H_2O$) implies a slope near 1 at fixed sulfate; kaolinite (a clay aluminosilicate) and imogolite (a short-range-order aluminosilicate) additionally couple Al to dissolved silica. Second, cation exchange — the reversible competition of $Al^{3+}$, $H^+$ and base cations ($Ca^{2+}$, $Mg^{2+}$, $K^+$, $Na^+$) for negatively charged colloid surfaces, conventionally described by Gaines–Thomas or Gapon selectivity equations — can buffer solution Al without any dissolving phase, and is modulated by ionic strength (the charge-weighted measure of dissolved salt load). Third, complexation of Al by soil organic matter (SOM), both dissolved and solid, represented for example by the Wesselink-type $Al_{org}/C$ models and the WHAM (Windermere Humic Aqueous Model, a humic-ion-binding framework) speciation model. Which control dominates is known to depend on horizon and region: European podzol Bs horizons often track imogolite-type solubility (Gustafsson et al., 2001), while organic horizons of northeastern US soils are well described by competition of $Al^{3+}$ and $H^+$ for organic binding sites (Li and Johnson, 2016).

Southern and southwestern China hosts extensive acid forest soils under a subtropical monsoon climate with historically severe acid deposition. These soils are highly weathered — desilication and allitization, i.e. pedogenic loss of silica with relative enrichment of Al and Fe oxides — and their climate, mineralogy, organic-matter status and deposition loading differ substantially from the European and North American soils from which most Al-solubility concepts derive, so those concepts cannot simply be transferred. Guo et al. (2007) monitored soils and soil solutions of five forest catchments in the region, fractionated dissolved Al into organic and inorganic monomeric forms, tested equilibrium control against four mineral phases, and applied partial least squares (PLS) regression, a multivariate technique suited to strongly covarying predictors, to identify the dominant explanatory variables. They found: inorganic Al dominated (organic Al usually below 10% of monomeric Al); $pAl$ correlated closely with pH over 3.6–5.6 with regression slopes of 1.28 (upper horizons) and 2.00 (lower horizons); none of gibbsite, jurbanite, kaolinite or imogolite equilibrium matched the observed $pAl$–pH and $Al^{3+}$–Si relations, especially in the upper horizon and at low pH; soil acidity and solution ionic strength were the main explanatory variables; upper-horizon Al solubility was strongly regulated by cation exchange and drawn from both organic and inorganic solid Al pools, while lower-horizon Al was dominantly of inorganic origin and controlled mainly by proton-promoted dissolution; and organic matter played no clearly significant role. The paper thus rejected the simple mineral-equilibrium framework for these soils without establishing what does control $Al^{3+}$ activity — the balance among specific solid phases, exchange reactions, organic complexation, ionic strength and kinetic constraints was left unresolved, as was the transferability of the horizon-specific relations across the wider acid-rain control zone.

Problem Statement

Determine which thermodynamic and kinetic processes actually govern dissolved $Al^{3+}$ activity in the highly weathered acid forest soils of southern and southwestern China — equilibrium with specific Al-bearing solid phases (and if so, which phases, without restricting the candidate set to a fixed list), cation exchange, complexation by soil organic matter, ionic-strength effects, or non-equilibrium (kinetic or transport-mixing) constraints — and establish a quantitative, field-validated geochemical model that predicts $Al^{3+}$ activity as a function of pH, solution composition and soil depth for these soils. The model must be consistent with the observed horizon-specific $pAl$–pH relations in the five-catchment monitoring data (regression slopes of 1.28 in upper and 2.00 in lower horizons over the pH range 3.6–5.6) as descriptive observational constraints rather than fittable targets, and the process attribution must be discriminated against the competing single-control hypotheses — mineral dissolution equilibrium (including the phases whose equilibria failed to explain the observations in Guo et al., 2007), cation exchange alone, organic complexation alone, and kinetic limitation — understood as representative rather than exhaustive alternatives, using appropriate observational, experimental or modelling evidence, so that the operative conditions controlling aluminium solubility during acidification of these subtropical soils are identified mechanistically rather than merely correlated.

References

1. Guo, J., Zhang, X., Vogt, R.D., Xiao, J., Zhao, D., Xiang, R., Luo, J. (2007). Evaluating main factors controlling aluminum solubility in acid forest soils, southern and southwestern China. Applied Geochemistry 22, 388–396. DOI: 10.1016/j.apgeochem.2006.11.003. https://doi.org/10.1016/j.apgeochem.2006.11.003
2. Guo, J., Vogt, R.D., Zhang, X., Zhang, Y., Seip, H.M., Xiao, J., Tang, H. (2006). Aluminium Mobilization from Acidic Forest Soils in Leigongshan Area, Southwestern China: Laboratory and Field Study. Archives of Environmental Contamination and Toxicology 51, 321–328. DOI: 10.1007/s00244-003-0151-0. https://doi.org/10.1007/s00244-003-0151-0
3. Yu, Q., Zhang, T., Ma, X., Kang, R., Mulder, J., Larssen, T., Duan, L. (2017). Monitoring Effect of SO2 Emission Abatement on Recovery of Acidified Soil and Streamwater in Southwest China. Environmental Science & Technology 51, 9498–9506. DOI: 10.1021/acs.est.7b01147. https://doi.org/10.1021/acs.est.7b01147
4. Huang, Y., Kang, R., Mulder, J., Zhang, T., Duan, L. (2015). Nitrogen saturation, soil acidification, and ecological effects in a subtropical pine forest on acid soil in southwest China. Journal of Geophysical Research: Biogeosciences 120, 2457–2472. DOI: 10.1002/2015JG003048. https://doi.org/10.1002/2015JG003048
5. Gustafsson, J.P., Belyazid, S., McGivney, E., Löfgren, S. (2018). Aluminium and base cation chemistry in dynamic acidification models – need for a reappraisal? SOIL 4, 237–250. DOI: 10.5194/soil-4-237-2018. https://doi.org/10.5194/soil-4-237-2018
6. Li, W., Johnson, C.E. (2016). Relationships among pH, aluminum solubility and aluminum complexation with organic matter in acid forest soils of the Northeastern United States. Geoderma 271, 234–242. DOI: 10.1016/j.geoderma.2016.02.030. https://doi.org/10.1016/j.geoderma.2016.02.030
7. Jiang, J., Wang, Y., Yu, M., Cao, N., Yan, J. (2018). Soil organic matter is important for acid buffering and reducing aluminum leaching from acidic forest soils. Chemical Geology 501, 86–94. DOI: 10.1016/j.chemgeo.2018.10.009. https://doi.org/10.1016/j.chemgeo.2018.10.009
8. Wesselink, L., van Breemen, N., Mulder, J., Janssen, P. (1996). A simple model of soil organic matter complexation to predict the solubility of aluminium in acid forest soils. European Journal of Soil Science 47, 373–384. DOI: 10.1111/j.1365-2389.1996.tb01411.x. https://doi.org/10.1111/j.1365-2389.1996.tb01411.x
9. Skyllberg, U. (1999). pH and solubility of aluminium in acidic forest soils: a consequence of reactions between organic acidity and aluminium alkalinity. European Journal of Soil Science 50, 95–106. DOI: 10.1046/j.1365-2389.1999.00205.x. https://doi.org/10.1046/j.1365-2389.1999.00205.x
10. Lofts, S., Woof, C., Tipping, E., Clarke, N., Mulder, J. (2001). Modelling pH buffering and aluminium solubility in European forest soils. European Journal of Soil Science 52, 189–204. DOI: 10.1046/j.1365-2389.2001.00358.x. https://doi.org/10.1046/j.1365-2389.2001.00358.x
11. Gustafsson, J., Berggren, D., Simonsson, M., Zysset, M., Mulder, J. (2001). Aluminium solubility mechanisms in moderately acid Bs horizons of podzolized soils. European Journal of Soil Science 52, 655–665. DOI: 10.1046/j.1365-2389.2001.00400.x. https://doi.org/10.1046/j.1365-2389.2001.00400.x
12. Gruba, P., Mulder, J. (2008). Relationship between Aluminum in Soils and Soil Water in Mineral Horizons of a Range of Acid Forest Soils. Soil Science Society of America Journal 72, 1150–1157. DOI: 10.2136/sssaj2007.0041. https://doi.org/10.2136/sssaj2007.0041
13. Gruba, P., Mulder, J., Brożek, S. (2013). Modelling the pH dependency of dissolved calcium and aluminium in O, A and B horizons of acid forest soils. Geoderma 206, 85–91. DOI: 10.1016/j.geoderma.2013.04.036. https://doi.org/10.1016/j.geoderma.2013.04.036
14. Reuss, J.O., Hopper, R.W.E., Walthall, P.M., Roswall, E.C. (1990). Aluminum Solubility, Calcium-Aluminum Exchange, and pH in Acid Forest Soils. Soil Science Society of America Journal 54, 374–380. DOI: 10.2136/sssaj1990.03615995005400020013x. https://doi.org/10.2136/sssaj1990.03615995005400020013x