Abstract
Prescribed burning, the planned application of fire to reduce the hazards of wildland fuels, is coming into wider use in southern California chaparral. Soil was heated to various temperatures in factorial combination with various soil moistures to determine the direct effects of fire and soil moisture on the survival of microorganisms in chaparral soil. For fungi, mild heating increased germination of dormant forms yielding significantly higher counts than those in unheated soil. With increasing temperatures, microbe populations showed an exponential decrease. For heterotrophic soil bacteria, this decrease was summarized as a function of soil moisture and temperature. Microbial groups differed significantly in sensitivity to temperature: fungi > nitrite oxidizers > heterotrophic bacteria. Physiologically-active populations in moist soil were significantly more sensitive than were dormant populations in dry soil. The mathematical model presented shows qualitatively that more of the microbial biomass will be killed when the soil is moderately moist—as during prescribed burning—than when it is dry. Mineralization of killed microbial biomass in soil and release of plant nutrients may partially explain the increased plant growth and reduced response to fertilizer at burned sites.
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