Nitrogen management in Eucalyptus nitens plantations

Low nitrogen (N) availability commonly limits the growth of Eucalyptus nitens plantations on ex-forest sites in Tasmania, Australia. We summarise here a decade of mechanistic and empirical research that has improved the basis for N management of these plantations. Twenty-two fertiliser experiments were used to define relationships between tree growth and methods of N fertilisation, i.e. timing, frequency, form, rate and placement. Pools and fluxes of mineral N were monitored at five sites. Potential deficiencies of N between planting and 10 years of age were indicated by the concentrations of soil NH4, NO3, total N and total phosphorus (P). Appropriate timing of N fertilisation depended on when N demand started to exceed supply from soil reserves, and was indicated by soil total N. Plantations on lowN sites experienced N deficiency during the first few years of growth, but others experienced it later or not at all. High cumulative rates of N fertilisation (at least 500 kg N ha^-1) were needed to maximise growth at many sites, but there is no evidence to suggest that any individual application in excess of 200 kg N ha^-1 is warranted. Hence, multiple applications will be needed to maximise growth. Urea was the preferred form of N, because it was relatively cheap and at least as effective as other N forms. Limiting N application to the crown drip line of 6-year-old trees with small crowns did not increase growth compared to a totally broadcast application. Soil N availability increased for 1–2 years following most N-fertiliser applications. Tree growth rates increased significantly for the same period followed by several years of less significant responses during which re-applications promoted a more sustained response in growth. This research has fostered an increase in the use of N fertilisers to increase productivity, and it should improve the basis for evaluating the economics of options for N fertilisation. Although applications of N fertilisers at rates up to 200 kg N ha^-1 and at intervals of several years are unlikely to be a threat to water quality, especially if urea is used, research is needed to determine the environmental risks associated with long-term and higher intensity uses.