CHANGES IN HEATED AND AUTOCLAVED FOREST SOILS OF SE AUSTRALIA .1. CARBON AND NITROGEN

The effect of heating and autoclaving on extractable nitrogen, N miner alisation and C metabolism was studied by heating five forest soils in the laboratory, simulating the range of effects of heat due to bushfi re. Top soil (0-5 cm) was heated to 60 degrees C, 120 degrees C and 25 0 degrees C for 30 minutes; unheated soil was taken as a control. Samp les of the soil heated to 250 degrees C were also inoculated with fres h soil to accelerate the recovery of the microbial population. Soil au toclaving was carried out as another heat treatment (moist heat). Soil s were analysed immediately after heating and 3 rimes during seven mon ths of incubation to assess immediate and longer-term effects of heati ng. Extractable N (organic and mineral forms) increased after heating to 120 degrees C, but decreased with further heating to 250 degrees C suggesting the volatilisation of N. N associated with microbial biomas s diminished with heating and was barely detectable after the 250 degr ees C treatment. Microbial biomass was an important source of soluble N in heated soils, and only partly recovered during subsequent long in cubation. The amount of N mineralised during incubation depended on bo th soil and temperature. Nitrification did not occur when soils were h eated to 250 degrees C (with or without inoculum), or after autoclavin g, demonstrating the high sensitivity of nitrifiers to heat. At the be ginning of soil incubation, respiration was enhanced in heated soils ( 250 degrees C, 250 degrees C inoculated) and autoclaved soils, but aft er 30 days of incubation respiration decreased to values either simila r to or lower than those in control. This respiration pattern indicate d that a fraction of labile C was released by heating, which was quick ly mineralised within 30 days of incubation. These results demonstrate some effects of soil heating on C and N dynamics in forest soils.