THE EFFECT OF FIRE ON CARBON AND NITROGEN MINERALIZATION AND NITRIFICATION IN AN AUSTRALIAN FOREST SOIL

The causes of onset of nitrification in a soil collected under an ashbed which was produced when heaped slash was burned, and for its absence in an unburnt soil, were investigated for an acid forest soil from south-eastern Australia. The occurrence of nitrification in ashbed soils was assessed in laboratory incubations extendig to 151 days to determine if it could be attributed to (a) an increase in pH, (b) an additional supply of P, (c) the removal of chemical inhibitors, and (d) the lack of competition with heterotrophs killed during soil heating. The treatments were: percolated and unpercolated ashbed soil from 0-5 and 5-10cm depth; unburnt soil from 0-5 cm untreated and with added lime or added P; and burnt and unburnt soil from 5-10 cm depth. In addition, each treatment had an identical where the soil was inoculated with nitrifying garden soil. Compared with the unburnt surface soil (0-5 cm), ashbed soil had higher pH (3-6 units), higher mineral N (3 times) and slightly elevated NaHCO3-extractable P. During 151 days of incubation, microbial respiration in surface ashbed soil, measured as CO2 evolution, initially exceeded the values obtained in unburnt soil but then decreased to only 72% of unburnt soil at the end of the incubation period. In ashbed soil, the microbial biomass N content was low but its C/N ratio was high. Net N mineralization (N-min) in ashbed soil was not significantly different from unburnt or phosphate fertilized soils (13.1, 14.7 and 17.8 mg N-min kg-l respectively) but was lower than in limed soil (59.3 mg N-min kg(-1)). Percolation of surface ashbed soil with distilled water removed high amounts of salts and increased microbial respiration and N mineralization. Inoculation of soils with a slurry from a nitrifying garden soil induced nitrification in every treatment, regardless of their ammonium content, pH or other limiting component. Nitrification was also stimulated in unburnt surface soil on the addition of lime and P. Autotrophic nitrifiers were active only in surface ashbed soils and probably in limed soils. P addition promoted heterotrophic nitrification. It was concluded that soil heating reduced competition between autotrophs and heterotrophs for ammonium and that ash supplies nutrients, such as K and Ca which stimulate nitrification. Low pH was not a limiting factor for nitrification but a high pH may promote the establishment of autotrophic nitrifiers.