SURVIVAL OF THE SOIL MICROBIAL BIOMASS AT ELEVATED-TEMPERATURES

A grassland soil was incubated for 240 days at either 15. 25 or 35°C. Changes in soil microbial biomass were monitored throughout the incubations in three ways: from soil ATP content, by the fumigation-extraction method and by a new version of the fumigation-extraction method in which carbohydrate released by fumigation is measured, rather than total C. All three methods of measuring biomass concur in showing that the biomass drifted downwards very slowly at 15° (with a mean decline of 0.11% per day) and 25°C (0.21% per day), whereas it fell precipitously during the first 50 days of incubation at 35°C (1.72% per day over the 0-50 day period). Both CO2 output and biomass C remained remarkably constant over the 150-240 day period in all the incubations. The specific death rates of the biomass over this period were 0.0072, 0.016 and 0.25 day-1 at 15. 25 and 35°C respectively. The corresponding biomass turnover-times were 139, 62 and 4 days. Raising the incubation temperature had two effects: the rate of release of substrate increased but the specific death rate increased even more. The activation energy of death was 130kJ mol-1, similar to values for thermal death in bacteria, suggesting that thermal denaturation was the process responsible for the enhanced death rate at 35°C. Although some of the CO2 liberated during these prolonged incubations must have come from the initial microbial biomass, through cryptic growth and consumption of intracellular C reserves, most can only have come from soil organic matter. Some chemical or biochemical process (or processes) must operate on soil organic mailer to release a steady flow of substrate to the waiting biomass. The measurement of carhohydrate released by fumigation may provide a convenient way of measuring soil microbial biomass and deserves further testing. © 1990.