When NAD+ becomes NADH, it is being

NAD+ accepts a hydrogen ion (H+) and two electrons (2e−), as it becomes reduced to NADH + H+. The NADH moves to the electron transport chain and donates a pair of electrons (becomes oxidized) to the first compound in the chain.

Explanation:

step 1: nitrogen fixation- conversion of atmospheric no₂ into forms useful to plants by lightning, bacteria, and cyanobacteria.

step 2: nitrification-process of converting ammonia to nitrite or nitrate.

step 3: assimilation-plant roots absorb new form of nitrogen.

step 4: ammonification-conversion of organic nitrogen to ammonia by bacteria involved in decomposition.

step 5: denitrification-bacteria turns the decomposed matter back into n₂o, which is released into atmosphere; repeat cycle.