Question

Why plants don't absorb nitrogen from the air directly like carbon dioxide and oxygen

Answer 1

Plants can’t absob nitrogen direcly from atmosphere (although is is abundant around here) because of the nature of molecular nitogen. in molecular nitrogen, two atoms of nitrogen are bonded with each other by three covalent bonds (one is sigma, the other two are pi bonds); and this bonding is too strong to be broken by any of the biochemical processes happening in the plant cells as it requires a good amount of energy which if plant cells provide would cost them their survival.

Nitrogen is the most essential element for living organisms. Nitrogen is present in the form of proteins, vitamins, cytochromes, nucleic acid and hormones. Hence, nitrogen is the fundamental constituent of nucleic acid which play vital role in regulating metabolism, growth reproduction and heredity.

therefore plants require nitrogen in larger amount and acts as principal macronutrients. Majority of plants can not utilize nitrogen in elemental form (N2) even though it constituent 78% of earth atmosphere.thus plants unable to uptake atmospheric nitrogen (N2) as such.