Influence of parent material on the aluminium fractions in acidic soils under Pinus pinaster in Galicia (NW Spain)

Study of the influence of the parent rock on Al fractionation in acidic soils that are rich in organic matter can provide useful information related to forest productivity in areas of high geological diversity. We studied how parent material affects several chemical parameters, mainly focusing on the total soluble aluminium and aluminium forms in solid fractions in different acidic soils in Galicia (NW Spain). The soils were developed over slate, biotitic schist, mica schist and granite and all were under Pinus pinaster. Six soil samples, three samples of rhizospheric and three samples of non-rhizospheric soil, were collected in each study plot. Aluminium was extracted from the solid phase with the following solutions: ammonium oxalate (Alo), sodium pyrophosphate (Alp), copper chloride (Alcu), lanthanum chloride (Alla) and ammonium chloride (AlNH4). The total Al in the liquid phase (Alt) was also determined. There was no significant difference in any parameter between rhizospheric and non-rhizospheric soils. In all soils, organo-aluminium complexes predominated over inorganic compounds of low crystallinity (Alp always represented more than 96% of the Alo). Overall, the parent material had a significant effect on chemical soil parameters. In the soil over biotitic schist, characterised by low acidity (pHwater=5.4) and organic matter with relatively high evolution (C/N=12.7), the concentrations of Alo and Alp were higher and tended to form highly stable organic–aluminium complexes, whereas the concentrations of exchangeable AlNH4 and Alt were significantly lower than in the other soils. Nevertheless, in soils developed from slate, the higher acidity (pH=4.7) and the organic matter with relatively low evolution (C/N=20.7) favoured the formation of moderate and low stability organic–aluminium complexes (Alcu, Alla) and higher concentrations of exchangeable Al and Al in solution. The characteristics of the soils over granite and mica schist were intermediate. In more acidic conditions, the destabilisation of organo-aluminium complexes and the release of Al to the soil solution seem the most likely mechanisms of pH buffering.