Influence of environmental conditions and some soil chemical properties on the carbon and nitrogen contents of some tropical Australian rainforest soils

Abstract

Concentrations of carbon, total nitrogen, total phosphorus, soil pH, the sum of the exchangeable basic cations, clay and dithionite-extractable iron were examined in the surface soils of 72 rainforest sites from north-eastern tropical Queensland. Soils derived from basalt had higher levels of most of the above properties than those formed on other parent materials. Differences between the carbon status of soils formed from basalt and those formed from other parent materials may be related to the higher free iron levels and phosphorus status of the former group. Excluding the soils of basalt origin, both carbon and nitrogen are positively related to clay content. In contrast, within the soils of basaltic origin, carbon concentrations are negatively related to clay contents through interactions with free iron oxides. Separate univariate regression relationships were established between carbon and nitrogen concentrations and site temperature and precipitation for soils derived from basaltic and combined granitic and acid volcanic parent materials. Coefficients relating both nitrogen and carbon to temperature differed between these groupings. The importance of topography in controlling carbon levels is demonstrated in soils that are poorly drained or formed on ridges, while the importance of soil age is demonstrated by a juvenile soil formed on weakly weathered basalt.