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Nitrogen Losses Due to Flooding
Following an anhydrous ammonia or urea application, soil bacteria gradually convert the ammonium-N to nitrate-N. By May 25 fall applied ammonium-N has been exposed to above freezing soil temperatures for about 4 weeks in most of the state, so most of the ammonium-N has probably converted to nitrate-N. Under saturated soil moisture conditions, another type of bacteria (anaerobic) converts nitrate-N to various nitrogen gases. Unlike nitrate or ammonium-N, nitrous gases are not used by crops and escape from the soil to the atmosphere. The conversion from nitrate-N to nitrous gases is called denitrification. The amount of total available soil N denitrified depends on the amount of nitrate-N present, the soil temperature and the length of time the field is saturated.
A good rule for N losses due to denitrification is about 5% per day when soil temperatures are between 50-55 degrees F. If a field was under water for a week, then 30-40% of the nitrate-N would be expected to be lost. When soil temperatures are warmer than this, losses per day increase.
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