Effect of rainfall on pedogenesis in a climosequence of soils near Lake Pukaki, New Zealand

Abstract

Abstract Four soils were sampled along a 42 km transect near Lake Pukaki, South Island, New Zealand, to study the physical, chemical, and mineralogical changes that have occurred under mean annual rainfalls of 640 mm at profile 1 to 2000 mm at profile 4. Sampling sites were chosen to minimise difference in soil-forming factors other than rainfall. All soils were formed in loess, ranging in thickness from 40 to 76 cm, over till. The marked differences found in soil properties of the four profiles illustrated the major effect rainfall has in determining the rates of leaching, weathering, and podzolisation. Leaching of bases, acidity, and exchangeable aluminium were found to increase markedly with rainfall. The main differences occurred between profiles 1 and 2, which reflected the marked soil moisture deficit and low soil water surplus characteristic of profile 1. A progressive increase in weathering was shown by increasing profile development and clay content with rainfall, by increasing conversion of total aluminium and iron to readily extractable forms, and by changes in clay mineralogy. Soil profiles showed progressive development of podzol morphology with increasing rainfall. Horizons identified in profile 1 mainly reflected accumulation of organic matter and weak weathering whereas profile 4 had a distinct eluvial horizon and evidence of accumulation of sesquioxides and humus in B horizons. This trend of increasing podzolisation was substantiated by chemical evidence.