Fires and Nitrogen Cycle
Country: United States
Date: January 29, 2005
My group and I are in participating in a group 4 project
required for IB Science. Our question that was given to us asked: Does
forest fires effect the nitrate levels in the soil?
To answer this
question we took several samples, some being of burnt soil, and others
being of a control group of unburnt soil in the same environment. Our
hypothesis was that nitrate levels would be higher in burnt soil than in
unburnt soil. However we found the opposite.
Why would this be?
At high temperature, like might be encountered in a forest fire or prairie
fire, nitrate that is formed by the reaction of ammonia with oxygen, could
further react by oxidizing organic matter, to produce carbon dioxide and
nitrogen gas (among many other possible reactions). Possibly the critical
parameter is the temperature of the ground. It is a pretty complex system,
so it is hard to predict all the possible outcomes that might occur.
Most forest soils have only 10% to 20% organic material, and
forest fires usually stay in the forest canopy. Sometimes the
litter on the ground will burn also, but the soil usually
does not burn much unless it is very dry.
At any rate, if you burn the organic material in the soil
you will release nitrogen from the soil either as nitrate
aerosol or nitrogen oxides gas. So, your finding of less nitrate
may not be a surprise if soil did burn and the forest material
was burned off without depositing much ash on the soil.
David R. Cook
Atmospheric Physics and Chemistry Section
Environmental Research Division
Argonne National Laboratory
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