Which one of the following is not a nitrogen fertilizer?

Nitrogenous fertilizers are classified based on the chemical form in which they provide nitrogen to plants, primarily as ammonium, nitrate, or amide [1]. Ammonium sulfate ((NH4)2SO4) is an ammoniacal fertilizer containing approximately 20.6% nitrogen [1]. Ammonium nitrate (NH4NO3) is a dual-form fertilizer containing both nitrate and ammonium nitrogen [1]. Urea ((NH2)2CO) is an amide fertilizer and is the most widely used nitrogenous fertilizer globally due to its high nitrogen content of 46% [1]. In contrast, molecular nitrogen (N2) is an inert atmospheric gas. While it is the ultimate source of nitrogen, plants cannot assimilate N2 directly from the air; it must first be 'fixed' into reactive forms like ammonia or nitrates by microorganisms or industrial processes to serve as a fertilizer [2]. Therefore, N2 itself is not used as a fertilizer product.

Sources

1. [1] https://agritech.tnau.ac.in/agriculture/agri_nutrientmgt_fertilizers.html
2. [2] Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 25: Agriculture > Bio-fertilizers > p. 364