Soil chronosequences in subalpine superhumid Cropp Basin, western Southern Alps, New Zealand

Soil chronosequences are described from Cropp River, western Southern Alps, New Zealand. In this area of high rainfall and bedrock of quartzofeldspathic schist, soils are developed in alluvium on gentle slopes and colluvium on steeper slopes, and form under indigenous grassland or scrubland, respectively. Soil ages ranging from 70 to c. 10 000 yr were determined from dendrochronology, radiocarbon dating, and stratigraphic relationships. Soil morphological and chemical changes with time reveal a similar soil development sequence on both gentle and steep slopes. Soil development is rapid and progressively forms Fluvial or Orthic Raw Soils (< 200 yr), Fluvial or Orthic Recent Soils, Acid Brown Soils and Orthic Podzols (500-1500 yr), and Perch-gley Podzols (7000-10 000 yr). Strong leaching is indicated by rapid acidification of surface horizons in Recent Soils. Morphological and chemical evidence shows that older soils (> 1000 yr) are podzolised and gleyed. Eluvial/illuvial coefficients calculated from total element analyses, using Zr as an internal standard, indicate large relative losses of all elements from A and E horizons, with the greatest losses in the older soils. The formation of low-permeability silt loam E horizons, as a result of strong weathering of alluvium and colluvium, causes the development of perch-gley conditions in soils on very steep slopes and in relatively young soils that do not have iron pans. As a result of the very high rainfall, soil development is more rapid than described from anywhere else in Westland.