NUTRIENT BALANCE IN FORESTS OF NORTHERN TASMANIA .2. ALTERATION OF NUTRIENT AVAILABILITY AND SOIL-WATER CHEMISTRY AS A RESULT OF LOGGING, SLASH-BURNING AND FERTILIZER APPLICATION

In situ measurements of nitrogen mineralization and nutrient mobility in severely perturbed forest soils (the forest was clear-felled, slash-burnt and then fertilizer amended) were compared with those in the undisturbed forest. Nitrification was absent in all soils. Nitrate concentrations in soil-water were low and probably not responsible for cation movement. Cation concentrations in soil-water were immediately increased by fire and by fertilizer additions and decreased continuously after these perturbations. Addition of strong-acid anions (SO42-) in fertilizer increased cation movement. Soil-water sampled at 10 cm depth was often coloured and contained high concentrations of oxidizable organic carbon; lower concentrations were found in samples collected from 30 cm depth. Uptake and mineralization of N were increased by logging and slash-burning. N-mineralization was promoted further by the addition of fertilizer-N and was reduced by the addition of P, or N and P together. Nutrient losses are exacerbated by fertilizer additions and restricted by the lack of nitrification, by nutrient uptake by soil microorganisms and vegetation, and probably by the formation of complexes between organic anions and cations in the soil profile.