1. Humans and the Carbon Cycle – Respiration releases __________ into the air. – Burning fossil fuels releases CO 2 into the atmosphere. – Burning of wood and natural forest fires also release CO 2 into the atmosphere. – Half of this CO 2 remains in the atmosphere, which may contribute to global warming. – The rest dissolves into the ocean or is absorbed by plants.

2. The Nitrogen Cycle

3. Nitrogen: – is needed to build proteins. – makes up 78% of the atmospheric gases. – cannot be used straight out of the atmosphere by most organisms; it must be “fixed” first. Nitrogen-fixing bacteria: – can “fix” atmospheric nitrogen into chemical compounds. – live in soil and in nodules on some plant roots. – release “fixed” nitrogen into the soil, where it is used by other plants.

4. The Nitrogen Cycle (cont.) How is nitrogen in the atmosphere changed into a form that can be used by organisms? 1.Nitrogen-fixing bacteria: a)Take atmospheric nitrogen and “fix” it into useable compounds (ammonia). b)Release “fixed” nitrogen into the soil. 2.Lightning converts some nitrogen into useable nitrates.

6. The Nitrogen Cycle (cont.) 4. Some soil bacteria convert ammonia into nitrates. 5. Bacteria (in soil AND water) change a small amount of the nitrogen back into atmospheric nitrogen. 6. Decomposers release nitrogen from organisms back into the soil. So where does MOST of the nitrogen stay in the cycle?