Effect of Simulated Fire on Organic Matter and Selected Microbiological Properties of Two Contrasting Soils

The effect of heating and ash deposition on microbiological properties was studied in two arid soils in an incubation experiment, simulating the effects of bushfire. Top soil (0-15 cm) was heated to 150°C, 250°C and 500°C for 30 minutes; unheated soil was taken as a control (25°C). Samples of the soil heated to 250°C were mixed with black ash (1%, w/w) for analysis of their effect on microbial population and activity. Soils were incubated for nine months and their basal respiration was assessed three times a month. Normalized and specific respiration, biomass-C, bacteria, and fungi numbers were analyzed one month after heating and in the final month of incubation to assess short-term and long-term effects of fire on soil microbiology. At the start of the incubation period and for intermediate heating treatments (150°C and 250°C), basal and specific soil respiration and biomass-C were enhanced in calcareous soil but depleted in gypsiferous soil. At the highest temperature (500°C), these biological properties, as well as specific respiration and colony forming units, were significantly reduced in both soil types and for each period of sampling. At the end of the incubation period, all biological properties maintained the differences between treatments but with lower values than at the beginning of the incubation. Black ash addition increased basal respiration in both soils but did not affect other biological properties. These results demonstrate the existence of both labile and permanent effects of soil burning and a differential response on C dynamics as a function of soil properties.