A Study of the Soil Properties of Tephra Profiles on the Kaingaroa Plateau, North Island, New Zealand, Under Manuka (Leptospermum Scoparium) and under Radiata Pine (Pinus Radiata D. Don)

Abstract

The soil properties of predominantly rhyolitic tephra profiles on the Kaingaroa plateau, New Zealand, under a manuka (Leptospermum scoparium) indigenous scrub stand are described and compared with the properties of similar profiles under an adjacent 45 year old radiate pine (Pinus radiate D. Don) stand. No differences in physical properties of equivalent soil horizons under the two stands were noted; differences in soil chemical properties under the two stands are accounted for by: (1) faster nutrient cycling under the radiata pine stand than under the manuka stand; (2) mineralisation of organic matter under the radiate pine stand. pH, exchangeable ca and Mg concentrations, and, at most sites, total N a organic P concentrations, are lower in the A1.1 horizon under the radiate pine stand than in this horizon under the manuka stand; titratable acidity and exchangeable A1 values are higher in the A1.1 horizon under the radiate pine stand. The mineral soil to 35 cm depth under the radiate pine stand was found to contain 30% less total N than the mineral soil to 35 cm depth under manuka. Estimates of the Ca, Mg, and K in the biomass of the radiate stand were made and it was concluded that a considerable proportion of these cations in the radiate pine stand are derived from non-exchangeable sources, i.e. from mineralisation of organic matter, and form mineral and glass weathering. Mineralisation of organic matter under the radiate pine stand is most evident in bleached soil volumes which have lower exchangeable Ca and Mg concentrations, and lower reserve Ca and Mg concentrations than surrounding soil. Results indicate that the deep rooting radiata pine has lowered exchangeable Mg concentrations in paleosols at up to 3 m depth; no effect of radiate pine on reserve Mg concentrations in paleosols was noted. The SiO2/Al2O3 mole ratio of the A1.1 horizon clay fractions is generally higher under the radiate pine stand than the SiO2/Al2O3 mole ratio of the A1.1 horizon clay fractions under the manuka stand. SiO2/Al2O3 mole ratios of Tamm-oxalate extracts increase with depth from ≤ 1.0 in the A1.1 horizon to > 1.5 in paleosols. Resilication of amorphous matter in paleosols is indicated. An effect of radiata pine on secondary clay mineral formation is evident in buried horizons: authigenic halloysite formation in the Rotongio Ash and in the paleosol developed on the Mapara Tephra is attributed to the effect of deep-rooting species, particularly radiate pine, on amorphous soil constituents. The importance of mineral and glass weathering, and of mineralisation of organic matter, as sources of nutrients for the radiate pine stand is discussed, and the nitrogen status of coniferous forests is reviewed.