Which forms of nitrogen are usable by living organisms?

Living organisms primarily utilize nitrogen in the form of nitrates and ammonia. Nitrates are a major source of nitrogen for plants, which absorb them from the soil. Once absorbed, plants can assimilate nitrates to synthesize essential amino acids and proteins, forming the foundational building blocks of their cells.

Ammonia also serves as a usable form of nitrogen for many organisms, particularly in aquatic environments. Certain bacteria can directly utilize ammonia, and it can also be converted into nitrate through a process called nitrification, making it accessible for plant uptake as well.

While there are other forms of nitrogen such as nitrites and urea, they are either less commonly used or require specific processing by the organism to become usable. Nitrites, while part of the nitrogen cycle, are typically intermediate products and not a direct source of nitrogen for most organisms. Phosphates, on the other hand, are essential for life but do not contribute to nitrogen, which is specifically what the question is asking about. Thus, nitrates and ammonia together represent the primary forms of nitrogen that living organisms can readily use.