NITROGEN FLUXES IN SURFACE SOILS OF 1-2-YEAR-OLD EUCALYPT PLANTATIONSIN TASMANIA

To improve our understanding of nitrogen (N) supply in eucalypt planta tions in Tasmania, N fluxes were determined in surface soils (0-10 cm) at 4 sites supporting 1-2-year-old plantations of E. nitens. Net N mi neralisation, nitrification, leaching, and uptake were measured by an in situ soil-core technique. Soils were derived from basalt (3 sites) or mudstone (1 site). Average rates of net N mineralisation ranged fro m 18 to 91 kg N/ha . year, and most mineralised N was nitrified and le ached. There were significant linear relationships among net N mineral isation, nitrification, and leaching (r = 0.61-0.83). Annual rates of net N mineralisation varied as much within sites as between them, and rates in individual plots were significantly correlated with anaerobic ally mineralisable N (r = 0.82) or total N (r = 0.66), but were not co rrelated or only weakly correlated with C:N ratio, loss-on-ignition, o rganic C, water content, or temperature. Leaching was weakly correlate d with effective rainfall (rainfall minus evaporation, r = 0.39). Soil contained most mineral N during February-April (i.e. late summer-earl y autumn) and least during October-November (i.e. late spring). We con cluded that available N at these sites was highly variable spatially a nd temporally, and at a plot scale was closely related to concentratio ns of mineralisable substrate and not to soil water or temperature.