Aluminium speciation in seasonally dry high country soils, South Island, New Zealand

Abstract

Soil solutions from an altitude sequence of South Canterbury high country soils (730– 1190 m) were analysed using a recently developed technique to obtain values for ‘free Al’ ([Al ³⁺ ]+[Al(OH) ²⁺ ]+[Al(OH)2 ⁺ ]+[AlF ²⁺ ]), an ‘organic-bound Al’ fraction, and the Al- complexation capacity (Al-CC). From 1979 these soils have been fertilised, oversown, and grazed. Since 1978, topsoils (0–7 . 5 cm) have become more acid, and average pH(_{H 2 O}) (1 : 2 . 5 soil : H₂O; n = 38) has fallen from 5 . 79 in 1978 to 5 . 25 in 1996. Despite this soil acidification, the current ‘free Al’ values are low (0 . 31–0 . 75 µМ) and are unlikely to limit legume growth. This conclusion was supported by root elongation experiments using Medicago sativa (Wairau lucerne). No significant correlation was observed between measured root elongation and either soil solution pH or ‘free Al’.

Sunny aspects had higher ‘organic-bound Al’ and lower ‘free Al’ values. The values of p[Al ³⁺ ] calculated from ‘free Al’ were consistent with control of [Al ³⁺ ] by an Al(OH)₃(s) phase rather than by organic matter. ‘Organic-bound Al’ correlated strongly with the concentration of humic substances in soil solution as estimated by the UV absorbance at 250 nm. The Al-CC decreased with a decrease in soil solution pH. Relative to the total reactive Al, the capacity of soil solutions to complex Al, as may be generated by acidification, was lower for the soils at lower elevations, pointing to potential for an earlier onset of Al toxicity at these sites.