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Abstract
Acidic forest soils with two dominant understories, bilberry (Vaccinium myrtillus L.) or alpine lady fern (Athyrium distentifolium Tausch ex Opiz), have been investigated to evaluate the influence of the understory on sorption properties and aluminium behaviour. We investigated acidity, cation exchange properties, organic matter content (SOM) and quality as well as Al bonding to organic matter in humus horizons of soils taken from two different locations. SOM was the major contributor to total cation exchange capacity (CECt) and effective cation exchange capacity (CECe), and a strong relationship was found between SOM and both CECt and CECe, independently of the understory and location. SOM differed in quality, having more fulvic acids in soils under bilberry than in soils under alpine lady fern. For that reason, SOM in soils under bilberry complexed more Al in the exchangeable form in the soluble fulvic acid fraction such that they could be easily washed out, while in soils under alpine lady fern aluminium was mainly bound more stably. We also found that the chemical character of Al depended on the understory type. In soils under bilberry the relation between base saturation and pH could be successfully modelled by a modified Henderson–Hasselbalch equation when Al was considered as a base cation, which is typical for forest podzols. However, in soils under alpine lady fern, this model was efficient only when Al was treated as an acid cation. We concluded that alpine lady fern as the main understory species reduces solubility and potential toxicity of aluminium, thus significantly affecting soil biogeochemistry.
Highlights
Acidic forest soils with two dominant understories, bilberry (Vaccinium myrtillus L.) or alpine lady fern (Athyrium distentifolium Tausch ex Opiz), have been investigated to evaluate the influence of the understory on sorption properties and aluminium behaviour
The threshold value of pH ranged between 4.61 in fern soils in Gorce (FG), 4.66 in reference soils (Ref) soils, 4.70 in fern soils in Beskid Z_ywiecki (FB) and 4.74 in bilberry soils both in Gorce (BG) and Beskid Z_ywiecki (BB); the pH of all soils was below pHthreshold
For most of the parameters investigated in this study both bilberry and fern soils were different than the reference soils which in many cases were intermediate
Summary
Acidic forest soils with two dominant understories, bilberry (Vaccinium myrtillus L.) or alpine lady fern (Athyrium distentifolium Tausch ex Opiz), have been investigated to evaluate the influence of the understory on sorption properties and aluminium behaviour. Al cations interact with SOM by neutralizing the electrical charge on organic molecular surfaces through both weak and strong binding processes (Huang 1988; Li et al 2016) In such conditions the most effective SOM compounds taking part in cation exchange reactions (weak binding) are fulvic acids (FAs), which exhibit higher total acidity values than humic acids (HAs) (Tan 2005). Organic matter–rich forest soils the value of pH solution can be described by the Henderson–Hasselbach equation, which after the modification of Bloom and Grigal (1985) takes the following form: pH 1⁄4 pKapp þ nLog[ðBSe=1 À BSeÞ; ð2Þ where pKapp stands for the negative logarithm of the apparent dissociation constant, n is an empirical stoichiometric constant of the reaction, BSe is the Biogeochemistry (2019) 143:55–66 effective base saturation, i.e. the fraction of the effective cation exchange capacity (CECe) neutralized by base cations (Ca2?, Mg2?, K?, Na?). 1 - BSe refers to the fraction of exchangeable acidity (He ? Ale)
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