Estimating the chemical compositions of soil solutions by obtaining saturation extracts or specific 1:2 by volume extracts

The aluminium tolerance of several tree species was studied in a cloud forest in Northern Venezuela, growing on a very acid soil and rich in soluble Al. The Al-accumulator species (>1000 ppm in leaves) were compared to non-accumulator ones in relation to total Al concentration in xylem sap, pH and Al concentration in vacuoles, and rhizosphere alkalinization capacity. The Al³⁺ concentration in the soil solution and the xylem sap were also measured. The results show that in the Al-accumulator plant Richeria grandis, xylem sap is relatively rich in Al and about 35% of it is present in ionic form. In the non-accumulator plant studied (Guapira olfersiana) there is no Al detectable in xylem sap. The pH of vacuolar sap of several Al-accumulator species studied was very acidic and ranged between 2.6–4.8, but the presence of Al in vacuoles was not correlated with the acidity of the vacuolar sap. Both Al-accumulator and non accumulator plants had the capacity to reduce acidity of the rhizosphere and increased the pH of the nutrient solution by one unit within the first 24 hours. Trees growing in natural, high acidity-high Al³⁺ environment show a series of tolerance mechanisms, such as deposition of Al in vacuoles, Al chelation and rhizosphere alkalinization. These partially ameliorate the toxic effects of this element, but they probably impose a high ecological cost in terms of photosynthate allocation and growth rate.