J. Bauhus et al., THE EFFECT OF FIRE ON CARBON AND NITROGEN MINERALIZATION AND NITRIFICATION IN AN AUSTRALIAN FOREST SOIL, Australian Journal of Soil Research, 31(5), 1993, pp. 621-639
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.