Effects of radiata pine on soil fertility

Abstract

The objective of this research project was to determine the effects of radiata pine on the nature, distribution and availability of organic matter and nutrients in hill country soils developed under improved pasture at Mahurangi Forest (9,000 ha) in Northland. Following extensive field investigations, a suite of four field sites was selected for this study (designated Barkers, Dibbles, Kraaks and Wreaks). Each of the field sites consisted of adjacent plots (20 x 20 m, split into 10 x 10 m quadrants [replicates]) on the same soil type under radiata pine (16 - 19 years old) and pasture. In addition to soil profile descriptions and associated chemical analyses under each land use, replicate soil samples were taken under trees and pasture to a depth of 30 cm for detailed chemical analysis. Tree biomass measurements were made at each site, and forest litter samples were also taken. Differences in soil type at the various sites were related to slope position and associated drainage. Thus, soils at the Barkers and Kraaks sites were moderately well drained compared with soils at the Dibbles and Wreaks sites which were imperfectly drained. Detailed profile analysis confirmed that the soil type was essentially the same under trees and adjacent pasture at each site. Results demonstrated that concentrations of sulphate sulphur were significantly higher under trees compared with pasture whereas mineralisable nitrogen was significantly lower under trees compared with pasture. The magnitude and direction of the apparent changes in soil fertility under trees were influenced by two main factors, initial soil fertility and soil drainage. At three sites amounts of organic carbon, total nitrogen, total sulphur were markedly lower under trees compared with pasture. These results indicated that radiata pine enhanced mineralisation of soil organic matter and associated nutrients (nitrogen, sulphur) although significant quantities of organic carbon and nutrients were present in the forest litter. Whereas at the other site, trees reduced soil total S, but the magnitude of the reduction was not as great, while soil organic C and total N remained relatively unchanged, compared to pasture. As expected, soil pH and exchangeable cations (and hence base saturation) were lower under trees compared with pasture.