%0 Articles
%T Aluminium chemistry in ploughed podzolic forest soils
%A Tanskanen, Niina
%D 2005
%J Dissertationes Forestales
%V 2005
%N 15
%R doi:10.14214/df.15
%U http://dissertationesforestales.fi/article/1798
%X Mechanical site preparation has been used to ensure efficient reforestation of clear-felled areas in Finland since the 1960s. About 18% (3.94 million ha )of the total forest land in Finland (21.9 million ha) has been site-prepared, of which one-third has been deeply tilled. In the present study, long-term effects of deep tilling on the chemical properties of soil and soil solution chemistry were evaluated at two forest sites that had been ploughed 17 and 31 years ago and planted with Norway spruce (<em>Picea abies</em> (L.) Karst.) seedlings. Particular interest was focused on the possible mobilisation of soil aluminium in the spodic B horizons exposed upon tilling. Mechanisms controlling Al solubility in the tilled soil profiles were also examined. Decreased concentrations of soil extractable Al, mainly inorganic secondary Al, in the exposed Bs horizons of tilts and furrows and increased concentrations of organically complexed Al in the O horizons buried within the tilt indicate that some Al was mobilised at the ploughed forest sites. Accumulation of organic C on top of the tilts and furrows with the relatively high Norway spruce fine root biomass in the tilts indicated that the release of Al was mainly due to acidity produced by ecosystem internal processes such as organic acids excreted by roots or released during decomposition of above- and belowground litter. Ploughing did not affect the processes controlling activity of Al<sup>3+</sup> in soil solution. In the undisturbed and disturbed Bs horizons, Al solubility was controlled by secondary minerals while in the O and Ah/E horizons, solubility control by organically bound Al was observed. Although the soil exchangeable Al pool was small, the Al<sup>3+</sup> - Ca<sup>2+</sup> exchange based on the Gaines-Thomas equation succesfully described the relationship between these cations in soil solution and on exchange sites in the studied O and Bs horizons. In the O horizons, exchangeable Al had nonacidic properties similar to those of base cations, and the soil solution pH was explained by weak organic acid dissociation using the Henderson-Hasselbach equation. The changes observed in soil chemical properties in the ploughed soil were consistent with the processes determining the Al chemistry in soil solution, and in accordance with the theory of podzolization. It may be expected that soil formation processes will in the course of time result in the development of a new spodic horizon in the tilts and furrows. Although the time elapsed since ploughing was only a few decades in this study, the occurrence of changes observed suggest that plant-derived organic matter and plant-soil interactions are already modifying the properties of ploughed podzols.