Question

15 the nitrogen cycle
key question what processes are involved in the cycling of nitrogen between the atmosphere and the biosphere?
nitrogen is an essential plant nutrient. it is a component of photosynthetic pigments in plants and part of proteins and nucleic acids in all organisms. plants obtain their nitrogen from the soil or via symbiotic relationships, whereas consumers obtain their nitrogen from other organisms (e.g., by eating or via symbiosis). the earth atmosphere is about 80% nitrogen gas (n₂), but molecular nitrogen is so stable that it is directly available only to those few organisms (all of them bacteria) that can fix it (capture and combine it into another molecule).
bacteria play a crucial role in moving and transforming nitrogen between the biotic and abiotic components of the nitrogen cycle by nitrogen fixation and other nitrogen transformations.
industrial processes also play a role in the nitrogen cycle by manufacture of nitrogen fertilizers. manufacture of nitrogen - containing fertilizers requires a large amount of energy - intensive artificial nitrogen fixation processes where atmospheric nitrogen and hydrogen are combined at high temperature and pressure. this process (the haber - bosch process) was responsible for the growth in nitrogen - containing fertilizers and is one of the largest contributors to a boost in the amount of nitrogen in the biosphere.
1 use the diagram above to describe five instances in the nitrogen cycle where bacterial action is important. include the name of each of the processes and the changes to the form of nitrogen involved.
(a)
(b)
(c)
(d)
(e)

Explanation:

1. Nitrogen - fixing: Nitrogen - fixing bacteria (e.g., Azotobacter, Rhizobium) convert atmospheric nitrogen ($N_2$) into ammonia ($NH_4^+$). This makes nitrogen available for plants to use in building proteins and other nitrogen - containing compounds.
2. Ammonification: Decomposer bacteria break down dead organisms and organic matter, releasing ammonia ($NH_4^+$) as a by - product. This returns nitrogen from organic compounds back to the soil.
3. Nitrification (first step): Nitrifying bacteria like Nitrosomonas convert ammonia ($NH_4^+$) into nitrite ($NO_2^-$). This is an important step in making nitrogen more accessible to plants in a different chemical form.
4. Nitrification (second step): Another group of nitrifying bacteria, such as Nitrobacter, convert nitrite ($NO_2^-$) into nitrate ($NO_3^-$). Nitrate is a form of nitrogen that plants can readily absorb and use for growth.
5. Denitrification: Denitrifying bacteria convert nitrate ($NO_3^-$) back into atmospheric nitrogen ($N_2$) under anaerobic conditions. This process helps to complete the nitrogen cycle by returning nitrogen to the atmosphere.

Answer:

(a) Nitrogen - fixing; $N_2$ to $NH_4^+$
(b) Ammonification; Organic nitrogen to $NH_4^+$
(c) Nitrification (first step); $NH_4^+$ to $NO_2^-$
(d) Nitrification (second step); $NO_2^-$ to $NO_3^-$
(e) Denitrification; $NO_3^-$ to $N_2$