Fire - Agroecology: The Ecology of Sustainable Food Systems - page 132

FIGURE 10.1 Chaparral fire in the Santa Ynez mountains near Santa Barbara, CA. Periodic fires are part of the evolutionary
history of chaparral; humans have only recently disrupted the natural pattern of burning.

EFFECTS OF FIRE ON SOIL

minutes (Raison, 1979). These temperatures are high
enough to modify the soil environment in ways that can
be useful for agroecosystem management.
The complete burning of above-ground organic matter
combusts most nitrogen and organic acid components,
returning inorganic cations to the soil (mainly K + and
Ca 2+ ) which then have an alkalizing effect. The strength
of this effect depends on the intensity of the fire and the
thoroughness of the combustion of plant biomass, but
increases in soil pH during the first several days following
fire, especially once the soil is moistened by precipitation,
are commonly 3 or more pH units.
Following the fire, the blackened soil surface will tend
to have more solar gain; however, if the standing biomass
was considerable before the fire and burn temperatures
were very high, enough white ash may be present at the
surface to actually have the opposite effect for a short
period of time. The higher albedo of the white surface will
reflect solar energy and limit soil heating.
The hot temperatures caused by fire can greatly reduce
the amount of organic matter in the upper layers of the
soil. At a temperature of 200 to 300°C for 20 to 30 min
there is an 85% reduction in organic matter, with an

Much of the ecological significance of fire revolves
around its effects on the soil. Fire has very noticeable
impacts on a range of abiotic and biotic components of
the soil ecosystem, and knowledge of these impacts is
important in employing fire as a tool for agroecosystem
management. It must be pointed out, however, that the
effects of fire will vary widely depending on the type and
stage of development of the vegetation, the type of soil,
the season of burning, the prevailing weather conditions,
the amount of time since the last fire, and other conditions.

A BIOTIC F ACTORS

When a fire occurs, the temperature of the surface layers
of the soil is raised. The actual heating rate and depth
depends on the amount of moisture in the soil and the type
of fire. Temperatures during a burn at the surface of the
soil almost always exceed 100°C and can reach as high
as 720°C for brief periods of time. Increases in tempera-
ture below the surface are usually restricted to the upper
3 to 4 cm of soil, where they rise 50 to 80°C above the
temperature present before the fire, usually for only a few