The impact of land use change from grassland to conifer forest on the
aluminium (Al) concentrations in soils and soil solutions was examined. Soils
from 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 site pairs were on a terrace
[Pinus nigra/P. ponderosa,
and grassland (CP)], and a hill slope
[Pseudotsuga menziesii and grassland
(CF)] in the Craigieburn range, Canterbury, and a hill slope in the
Lammerlaw Range, Otago [P. radiata and
grassland (LP)]. The sites had never been
cultivated or fertilised, and for each pair the forest and grassland were
similar in terms of soil and topography.
The 1 M KCl exchangeable and 0.02 M CaCl 2 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 labile organic
complexes to be higher in forest soil at all sites, but site-pair differences
were only significant at LP, and only for ‘reactive Al’. The
increase in ‘reactive Al’ at this site was linked to the low pH
and low base saturation. The ratios of exchangeable and soil solution Ca
2+ and Mg 2+ to
‘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 and
forest sites at LP, the Al-CC correlated strongly with the amount of soluble
fulvic and humic matter present, as estimated from soil solution UV absorbance
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
growing conifer tree species may be sufficient to affect Mg uptake.
Analysis of ammonia-oxidizing bacteria populations in acid forest soil during conditions of moisture limitation
