Fire - Agroecology: The Ecology of Sustainable Food Systems - page 140

more permanently into soil humus). Burning, however,
makes the nitrogen readily available for uptake by plants.
Even though most nitrogen is lost through volatilization
during burning, the C-to-N ratio of the ash is lower relative
to that of unburned residue, making the nitrogen that
remains more readily available and reducing the need for
external nitrogen amendments.
Another benefit of residue burning is reduction in the
amount of tillage needed. Also, in many parts of the devel-
oping world, residue is burned not to eliminate the residue,
but as fuel for home heating or cooking. Sometimes the
ash is collected and returned to fields as a soil amendment.
Rice production is often associated with fire. In any
part of the world where rice is grown, the straw and stubble
left following harvest can amount to as much as 0.95 to
1.0 kg/m 2 . Traditionally, this straw has been used as ani-
mal feed, fuel, or construction material, or as raw material
for compost. In many present-day rice systems, however,
the increasing need to get another crop into the ground as
soon as possible following the rice harvest has led to the
use of fire to quickly reduce the straw to ash. Burning
does reduce stubble-borne diseases and insects, and also
reduces the potential of methane being produced during
decay under flooded conditions in amounts that might
become toxic to some following crops. But due to the
perceived impact of the smoke on atmospheric quality,
regulations increasingly limit burning and force farmers
to deal with the reincorporation of the straw into the
soil, or to find alternative uses for harvested straw
(Blank et al., 1993). In Amazonian slash and burn systems,
including rice, researchers found that nutrient losses from
burning exceeded inputs for N, K, Ca and Mg, but not for
P. Thus, this group is currently searching for alternatives
that do not use fire (Denich et al., 2005).
From the standpoint of sustainability, the many advan-
tages of residue burning must be weighed against disad-
vantages that include loss of nutrients through volatiliza-
tion or leaching, air pollution, exposure of soil surface,
and loss of organic matter inputs to the soil.

be moved rapidly over the soil surface to destroy weed
seedlings. Both backpack- and tractor-mounted flame
weeders are available. Specially shaped nozzles and an
assortment of deflectors and shields protect any crop seed-
lings while desiccating the weeds. Weed seedlings must
be very small to be effectively controlled with this tech-
nology, or the seedlings of the crop must be at a stage of
development that gives them greater resistance than the
weeds to the heat. Under some field conditions, a crop
such as corn in its first and second leaf stage has a structure
and moisture content that will keep it from suffering dam-
age while most surrounding weed seedlings are killed. The
necessary equipment can be expensive to purchase and
use, and depends greatly on the use of fossil fuel, but in
some very weed-prone crops like carrots and onions, flame
weeders are a very cost-effective means of weed control.
But fire must be used on weeds with care. Perennial
weeds and those with fire-resistant roots, rhizomes,
crowns, or other structures that resist burning may actually
be stimulated by fire. Bracken ( Pteridium aquilinum ), for
example, is a very aggressive plant that can act as a weed
in deforested or pasture areas, and is favored by fire in
two ways (Gliessman, 1978d). Its deep underground rhi-
zomes permit it to survive fire, and there is some evidence
that removal of above-ground litter of bracken actually
promotes more vigorous regrowth of the fern. At the same
time, spores of the fern are favored by the soil conditions
created by fire and ash, allowing for initial establishment
of the fern where it did not occur before and the potential
for its aggressive vegetative growth from then on. In shift-
ing cultivation systems, where fire is used to help clear
the fallow, fire can begin to have negative effects if the
fallow period is too short. These effects can include leach-
ing of nutrients and invasion of fire-resistant weeds. In
general, use of fire for weed control requires careful con-
sideration of its potential impacts, based on the unique
characteristics of the system.

Management of Arthropods

Fire is a very effective means of eliminating damaging
arthropods, such as insects and mites, from an agroecosys-
tem. Heat, smoke, and loss of habitat all combine to either
kill these organisms (as well as their eggs or larva) or drive
them from the system. In some natural ecosystems, fire is
probably as much a factor in the natural fluctuations of
arthropod populations as climatic factors or trophic inter-
actions. Fire suppression in forests may actually be upset-
ting the natural equilibrium, allowing outbreaks of such
common pests as bark beetles, leaf miners, and lepidopter-
ous leaf eaters such as tent caterpillars. In some ecosystems,
however, fire may not impact arthropod populations. Joern
(2005), for example, found that different burn frequencies
had no effect on grasshopper species diversity or density
in North American tall grass prairies. In general, studies

Weed Management

Fire is used for weed management most effectively and
practically when the weeds are either in the litter or soil
as seed, or shortly after the seeds have germinated. Seeds
or seedlings in the litter are most likely to be killed by
fire, since litter at the surface burns at high temperatures
and down to the soil surface. For this reason, it is necessary
to have some kind of mulch cover or crop residue to carry
the fire. Slash and burn systems are very effective at
destroying seed in the litter and on the immediate soil
surface.
A more recently developed practice for weed control
has been used in Europe for many years. A propane tank
is connected to a hose and a nozzle so that a flame can