Nitrogen Cycle

Question 1

five steps in the nitrogen cycle

Answer 1

1) Nitrogen fixation
2) Nitrification
3) Assimilation
4) Ammonification
5) Denitrification

Question 2

1) Nitrogen fixation

Answer 2

the conversion of N2 to NH3 (O2 free environment required)

Certain cyanobacteria and soil bacteria can fix nitrogen with the enzyme nitrogenase (some free living, others in symbiotic relationships with plants)

Wild plants with symbiotic nitrogen fixers are often colonizers of poor soils (e.g., alders). Cultivated legumes have nitrogen fixers

Root nodules on a pea plant contain nitrogen fixing bacteria (Rhizobium sp.)

Aquatic filamentous nitrogen fixing Cyanobacteria
(algae)

Question 3

2) Nitrification

Answer 3

the conversion of ammonia (NH3) or ammonium
(NH4+) to nitrate (NO3-)

As in the N fixation step, nitrification is also accomplished by a series of bacteria (e.g., Nitrosomonas sp., and Nitrobacter sp.)

Question 4

3) Assimilation

Answer 4

Plants (via roots), bacteria & algae can absorb
NH3, NH4+ NO3- and NO2- and incorporate the nitrogen into organic compounds. These organic N compounds may then move up the food web through consumers.

Question 5

4) Ammonification

Answer 5

the breakdown of organic nitrogen compounds into ammonia (NH3) or ammonium (NH4+)

Decomposers (e.g. bacteria and fungi) are important in the breaking down of nitrogenous waste to ammonium

This ammonium can be recycled back to plants and algae for re-assimilation

This recycling within the food web is a major process in
most ecosystems

Question 6

5) Denitrification

Answer 6

the reduction of nitrate (NO3-) to gaseous
nitrogen (N2)

Denitrifying bacteria complete the cycle by converting NO3- to gaseous N2

Requires a low O2 environment

Question 7

Summary

Answer 7

• Bacteria perform the majority of chemical transformations

* Recycling of reduced nitrogen (NH4, NOx) compounds is a significant process in the N cycle in most ecosystems