Lab 7 Identifying Limiting Nutrient - Sayre School

Lab 7: The Ecosystem Drawing
Nitrogen is used to produce proteins and DNA. Despite its abundance in the atmosphere,
nitrogen is often the limiting nutrient for plant growth. This occurs because most plants can
only use nitrogen in two solid forms: ammonium and nitrate. Animals receive the nitrogen they
need for metabolism, growth, and reproduction by eating living or dead organic material.
In the process of nitrification, ammonium is chemically altered by bacteria to form nitrate.
Nitrate is very soluble and it is easily lost from the soil system by leaching. Some of this leached
nitrate flows through the water system where it is returned to the atmosphere by denitrification.
Denitrification is also common in soils and is carried out by heterotrophic bacteria.
Nitrifying Bacteria
Almost all of the nitrogen found in any terrestrial ecosystem originally came from the
atmosphere. Significant amounts enter the soil in rainfall or through the effects of lightning.
The majority, however, is fixed within the soil by bacteria. Members of the bean family form
symbiotic relationships with nitrogen fixing bacteria. In exchange for some nitrogen, the
bacteria receive carbohydrates and a moist environment from the plants.
Humans have altered the nitrogen cycle by:
• The over-use of nitrogen fertilizers has caused nitrate to leach into water. The additional
nitrogen flows into streams, rivers, lakes, and estuaries which can lead to eutrophication.
• The farming of livestock which release large amounts of ammonia into the environment
from their wastes.
• Sewage waste and septic tank leaching.
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glossary 7.8