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 element in living things. it is a component of photosynthetic pigments in plants, and part of proteins and nucleic acids in all organisms. consumers obtain their nitrogen from the soil or via symbioses, e.g. by eating nitrogen - fixing bacteria. plants obtain their nitrogen from other organisms (about 80%) or directly from the atmosphere. the earths 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 nitrogen cycling, transforming nitrogen between the biotic and abiotic environments through nitrogen fixation and other nitrogen transformations. humans intervene in the nitrogen cycle by manufacture and application of nitrogen fertilizers. manufacture of nitrogen fertilizer is an energy - intensive artificial nitrogen fixation process in which atmospheric nitrogen and hydrogen are combined under high temperature and pressure. this process (the haber process) was responsible for the growth in intensive agriculture and is one of the largest contributors to a buildup of reactive 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)

Explanation:

Step1: Identify nitrogen - fixing bacteria action

Nitrogen - fixing bacteria (e.g., Azotobacter, Rhizobium) convert atmospheric nitrogen ($N_2$) into ammonia ($NH_4^+$). The process is called nitrogen fixation.

Step2: Identify nitrifying bacteria action 1

Nitrosomonas spp. (nitrifying bacteria) convert ammonia ($NH_4^+$) into nitrite ($NO_2^-$). The process is nitrification.

Step3: Identify nitrifying bacteria action 2

Nitrobacter spp. (nitrifying bacteria) convert nitrite ($NO_2^-$) into nitrate ($NO_3^-$). This is also part of nitrification.

Step4: Identify plant assimilation

Plants take up nitrate ($NO_3^-$) from the soil and incorporate it into organic compounds during assimilation.

Step5: Identify denitrifying bacteria action

Denitrifying bacteria (anaerobic) convert nitrate ($NO_3^-$) back into atmospheric nitrogen ($N_2$). The process is denitrification.

Answer:

(a) Nitrogen fixation by nitrogen - fixing bacteria (Azotobacter, Rhizobium): $N_2$ to $NH_4^+$
(b) Nitrification by Nitrosomonas spp.: $NH_4^+$ to $NO_2^-$
(c) Nitrification by Nitrobacter spp.: $NO_2^-$ to $NO_3^-$
(d) Assimilation by plants: Uptake of $NO_3^-$ and incorporation into organic compounds
(e) Denitrification by denitrifying bacteria: $NO_3^-$ to $N_2$