Plant and Soil 92,341-362 (1986). 1986. Martinus Ni/hoffPublishers, Dordrecht. Printed in the Netherlands. Ms. 6323 Nutrient cycling and nitrogen mineralization in eucalypt forests of south-eastern Australia H. Indices of nitrogen mineralization M. A. ADAMS and P. M. ATTIWILL School o f Botany, University o f Melbourne, Parkville, 3052, Victoria, Australia Received 20 March 1985. Revised August 1985 Key words Eucalypt forests Fire Immobilization Nitrogen mineralization Nitrogen turnover Nitrification Nitrogen conservation Resilience Summary Nitrogen mineralization was measured in the laboratory and in situ in eight eucalypt forests covering a wide range of climates and soil types. Aerobic and anaerobic incubations as well as chemical indices showed consistently higher rates of mineralization and nitrification and higher mineralization potentials in the wetter, high productivity forests. Nitrification was not confined to these forests and appears best related to soil C/N ratios and the rate of N turnover. Immobilization is recognized as a major process in eucalypt forest soils; in two forests which were burnt by bushfire during this study immobilization prevented over-accumulation of inorganic-N and possible leaching of NOg-N. Calculated from in situ incubations, annual uptake of inorganic-N ranged from 27 to 160 kg N ha-1 ; in two of four forests nitrate uptake was not apparent, in a third forest nitrate accounted for 10% of total uptake and in the remaining forest nitrate comprised about one-third of inorganic-N taken up. Ammonium is thus the dominant inorganic form taken up in eucalypt forests of this region. There was general agreement between laboratory and in situ incubations as to the occurrence of nitrification and the difference in mineralization rates between forests. In conjunction with previous studies, the turnover and maintenance of N-pools in eucalypt forests is discussed; forests with low N-capital appear to be resistant to possible N-loss after perturbation, forests with higher N-capital are more susceptible to loss but recover quickly. Immobilization of inorganic-N is central to the recovery process. These results agree with recent hypotheses proposed by Vitousek and Boerner. Introduction Forest productivity and N-turnover rates may be closely related and have been suggested as strong controls, both together and individually, on nitrification and net mineralization ''32'36 in forest soils. Examination of the above-ground cycle of nitrogen in forests suggests that such relationships 'ought to exist', and several studies have sought indices of N-availability (e.g. the Waring-Bremner anaerobic incubation test 34) which correlate with productivity 24' 27 341