Abstract

Nitrogen (N) fluxes in surface soils (0–10 cm) were measured by an in situ soil-core technique at 5 sites supporting 1- to 4-year-old Eucalyptus nitens plantations in Tasmania. Rates of net N mineralisation (NNM) ranged from 13 to 188 kg N/ha.year. The highest rate was from an ex-pasture site, which was almost double the highest rate of NNM measured on an ex-forest site. Although there was a tendency towards lower rates of NNM in uncultivated versus cultivated soils, and in the final versus previous year or two of measurement, these differences were not significant. Concentrations of mineral N, however, decreased at all study sites during the first 3–4 years, significantly at 4 of these sites. Within the errors of measurement, most N that was mineralised was also nitrified and leached. Seasonal patterns in NNM and concentrations of nitrate (NO₃^–) occurred only at one site, which also had the highest annual rate of NNM. The balance of mineral N remaining in surface soil each October was 2–17 kg N/ha. We concluded that there was little evidence of a decrease in rates of NNM between 1 and 4 years after planting, and that high rates of NNM at some sites were accompanied by high rates of leaching that limited the accumulation of mineral N. Sitespecific management of N fertiliser may be warranted to compliment the wide range of N availability in eucalypt plantations.