Question

which depiction of changing chemical compounds best describes the flow of nitrogen from the atmosphere, through nitrogen cycle, and back into the atmosphere?
n2 → no3- → nh4+ → n2
n2 → nh3 → no4- → n2
n2 → nh4+ → no3- → n2
n2 → nh3 → no2- → n2

Explanation:

In the nitrogen - cycle, nitrogen in the atmosphere ($N_2$) is first fixed into ammonia ($NH_3$) or ammonium ($NH_4^+$) by nitrogen - fixing bacteria. Then, through nitrification, ammonium is converted to nitrite ($NO_2^-$) and then to nitrate ($NO_3^-$). Finally, through denitrification, nitrate is converted back to atmospheric nitrogen ($N_2$). The correct sequence is $N_2
ightarrow NH_4^+
ightarrow NO_3^-
ightarrow N_2$.

Answer:

$N_2
ightarrow NH_4^+
ightarrow NO_3^-
ightarrow N_2$ (the third option)