Question

Outline the salient features of nitrogen cycling in an ecosystem.

Answer 1

Nitrogen is a gaseous cycle. The atmosphere (about 79% \[\mathrm{N}_{2}\]) is the main reservoir. Nitrogen is essential for proteins, nucleic acids and chlorophyll.

Salient features:

1. Nitrogen fixation: Atmospheric \[\mathrm{N}_{2}\] is converted into nitrogen compounds by:

• Symbiotic bacteria: Rhizobium in legume root nodules, Frankia in non-legumes such as Casuarina
• Free-living bacteria: Azotobacter (aerobic), Clostridium (anaerobic)
• Cyanobacteria: Anabaena, Nostoc, Aulosira
• Industrial fixation and lightning

2. Ammonification: Decomposers convert organic nitrogen in dead matter and wastes into ammonia.

\[\text{Organic nitrogen} \xrightarrow{\text{Ammonifying microbes}} \mathrm{NH}_{3}\]

3. Nitrification: Ammonia is converted to nitrites, then to nitrates.

\[\mathrm{NH}{3} \xrightarrow{\text{Nitrosomonas}} \mathrm{NO}{2}^{-} \xrightarrow{\text{Nitrobacter}} \mathrm{NO}_{3}^{-}\]

4. Uptake: Plants absorb nitrates and form proteins. Animals get nitrogen by eating plants.

5. Denitrification: Denitrifying bacteria such as Pseudomonas convert nitrates back to \[\mathrm{N}{2}\] under poorly aerated conditions.

\[\mathrm{NO}{3}^{-} \xrightarrow{\text{Pseudomonas}} \mathrm{N}_{2}\]

Nitrogen cycles repeatedly between atmosphere, soil, organisms and microbes, and it is maintained mainly by biological fixation.