Discussing dissolved aluminium in soils

Studies have shown that dissolved aluminium in soils are a threat to forest health. Aluminium in soils occur naturally as aluminium is one of the most abundant metals in the earth’s crust. Upon acid deposition, aluminium is dissolved and mobilised from clays as the negative clay sites are replaced by hydrogen ions. In the soil solution, aluminium ions hydrolyse and are taken up by plants, inhibiting nutrient uptakes and plant root growth. Nutrient deficiency in trees have been found to be present at sites with high aluminium in soil solutions (Alewell et al. 2000).

Picture showing controlled experiment of plant root growth in different aluminium concentrations. Taken from http://www.summitfertz.com.au/research-and-agronomy/soil-ph.html

However, there are also healthy trees found under similar aluminium concentrations (Huber et al. 2004). There is no direct relationship between forest/tree health with acid deposition or dissolved aluminium toxicity in soils. Some researchers have thus used bases such as calcium, magnesium and potassium to aluminium ratio as a predictor of forest vitality (De Vries et al. 2003). However, as different types of vegetation, soils and ecological environments have different sensitivities to dissolves aluminium in soils, it is difficult to determine a threshold of base to aluminium ratio. Laboratory experiments of aluminium toxicity have been disputed by whole-ecosystem experiments as well. Laboratory experiments have been criticised for using potentially toxic levels of aluminium in controls and the amount of nitrogen added will have influence on root growth. Whole-ecosystem experiments are inconclusive about the role of aluminium in reducing root growth and nutrient uptake due to the presence of multiple variables (De Wit et al. 2001).

There is a need to reconcile both methods, in order to prove a direct, unambiguous relationship of nutrient uptake and root growth with aluminium toxicity.

References

Alewell, C., Manderscheid, B., Gerstberger, P., Matzner, E., 2000. Effects of reduced atmospheric deposition on soil solution chemistry and elemental contents of spruce needles in NE-Bavaria, Germany. Journal of Plant Nutrition and Soil Science—Zeitschrift Fur Pflanzenernahrung Und Bodenkunde 163, 509– 516.

De Vries, W., et al. (2003) Intensive monitoring of forest ecosystems in Europe2: atmospheric deposition and its impacts on soil solution chemistry. Forest Ecology and Management 174, 97–115.

De Wit, H.A. et al. (2001) Aluminium: the need for a re-evaluation of its toxicity and solubility in mature spruce stands. Water Air and Soil Pollution: Focus 1, 103–118.

Huber, C., et al. (2004) Response of artificial acid irrigation, liming, and N-fertilisation on elemental concentrations in needles, litter fluxes, volume increment, and crown transparency of a N saturated Norway spruce stand. Forest Ecology and Management 200, 3–21.