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 ashb ed which was produced when heaped slash was burned, and for its absenc e in an unburnt soil, were investigated for an acid forest soil from s outh-eastern Australia. The occurrence of nitrification in ashbed soil s was assessed in laboratory incubations extendig to 151 days to deter mine if it could be attributed to (a) an increase in pH, (b) an additi onal 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 o r 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 ni trifying 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 evolut ion, initially exceeded the values obtained in unburnt soil but then d ecreased to only 72% of unburnt soil at the end of the incubation peri od. 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 li med soil (59.3 mg N-min kg(-1)). Percolation of surface ashbed soil wi th distilled water removed high amounts of salts and increased microbi al respiration and N mineralization. Inoculation of soils with a slurr y from a nitrifying garden soil induced nitrification in every treatme nt, regardless of their ammonium content, pH or other limiting compone nt. Nitrification was also stimulated in unburnt surface soil on the a ddition of lime and P. Autotrophic nitrifiers were active only in surf ace ashbed soils and probably in limed soils. P addition promoted hete rotrophic nitrification. It was concluded that soil heating reduced co mpetition between autotrophs and heterotrophs for ammonium and that as h supplies nutrients, such as K and Ca which stimulate nitrification. Low pH was not a limiting factor for nitrification but a high pH may p romote the establishment of autotrophic nitrifiers.