Effects of grassland afforestation on exchangeable soil and soil solution aluminium

The impact of land use change from grassland to conifer forest on the alumi nium (Al) concentrations in soils and soil solutions was examined. Soils fr om grassland were compared with those from adjoining 15-19-year old forest stands at 3 contrasting pairs of sites in South Island, New Zealand. The si te pairs were on a terrace [Pinus nigra/P. ponderosa, and grassland (CP)], and a hill slope [Pseudotsuga menziesii and grassland (CF)] in the Craigieb urn range, Canterbury, and a hill slope in the Lammerlaw Range, Otago [P. r adiata and grassland (LP)]. The sites had never been cultivated or fertilis ed, and for each pair the forest and grassland were similar in terms of soi l and topography. The 1 M KCl exchangeable and 0.02 M CaCl2 extractable Al levels at 0-10 cm were higher in forest than in grassland topsoil at CP and LP (P < 0.01). In soil solutions there was a trend for both 'reactive Al' and Al bound in la bile organic complexes to be higher in forest soil at all sites, but site-p air differences were only significant at LP, and only for 'reactive Al'. Th e increase in 'reactive Al' at this site was linked to the low pH and low b ase saturation. The ratios of exchangeable and soil solution Ca2+ and Mg2to 'reactive Al' were substantially lower in forest than grassland soils at all sites. Aluminium complexation capacity (Al-CC) values at all sites were higher in forest soil solutions than in grassland soil solutions. For the grassland a nd forest sites at LP, the Al-CC correlated strongly with the amount of sol uble fulvic and humic matter present, as estimated from soil solution UV ab sorbance at 250 nm. In soils with the lowest percentage base saturation and buffering capacity (LP), afforestation of pastoral grassland with Pinus radiata significantly reduced soil pH and base cation levels, while increasing both soil and soil solution Al concentrations. Under such conditions (base saturation <20%), the increase in 'reactive Al' concentrations in soil solutions under fast g rowing conifer tree species may be sufficient to affect Mg uptake.