2. Cycles Carbon and Nitrogen Cycles

3. Nitrogen cycle THE NITROGEN CYCLE Nitrogen (N) is an element like carbon. All creatures need nitrogen to survive. There are huge amounts of nitrogen gas in the atmosphere, but most animals and plants have no way of using it. It needs to be fixed (put into a biologically useful compound). After it is fixed, it can then start to move through the cycles and organisms in an ecosystem.

6. Nitrogen is essential to all living systems, which makes the nitrogen cycle one of Earth's most important nutrient cycles. Eighty percent of Earth's atmosphere is made up of nitrogen in its gas phase. Atmospheric nitrogen becomes part of living organisms in two ways. The first is through bacteria in the soil that form nitrates out of nitrogen in the air. The second is through lightning. During electrical storms, large amounts of nitrogen are oxidized and united with water to produce an acid that falls to Earth in rainfall and deposits nitrates in the soil. Plants take up the nitrates and convert them to proteins that then travel up the food chain through herbivores and carnivores. When organisms excrete waste, the nitrogen is released back into the environment. When they die and decompose, the nitrogen is broken down and converted to ammonia. Plants absorb some of this ammonia; the remainder stays in the soil, where bacteria convert it back to nitrates. The nitrates may be stored in humus or leached from the soil and carried into lakes and streams. Nitrates may also be converted to gaseous nitrogen through a process called denitrification and returned to the atmosphere, continuing the cycle.

7. Nitrogen is essential to all living systems, which makes the nitrogen cycle one of Earth's most important nutrient cycles. Eighty percent of Earth's atmosphere is made up of nitrogen in its gas phase. Atmospheric nitrogen becomes part of living organisms in two ways. The first is through bacteria in the soil that form nitrates out of nitrogen in the air. The second is through lightning. During electrical storms, large amounts of nitrogen are oxidized and united with water to produce an acid that falls to Earth in rainfall and deposits nitrates in the soil. Plants take up the nitrates and convert them to proteins that then travel up the food chain through herbivores and carnivores. When organisms excrete waste, the nitrogen is released back into the environment. When they die and decompose, the nitrogen is broken down and converted to ammonia. Plants absorb some of this ammonia; the remainder stays in the soil, where bacteria convert it back to nitrates. The nitrates may be stored in humus or leached from the soil and carried into lakes and streams. Nitrates may also be converted to gaseous nitrogen through a process called denitrification and returned to the atmosphere, continuing the cycle.

8. Human activities and the nitrogen cycle Human activities cause increased nitrogen deposition in a variety of ways, including * burning of both fossil fuels and forests, which releases nitrogen into the atmosphere * fertilizing crops with nitrogen-based fertilizers, which then enter the soil and water * ranching, during which livestock waste releases ammonia into the soil and water * allowing sewage and septic tanks to leach into streams, rivers, and groundwater