Describe the nitrogen cycle's key processes and why nitrogen is often a limiting nutrient in ecosystems.

• A. Fixation, nitrification, assimilation, mineralization, denitrification; nitrogen is often limiting because most available forms are N2 or locked in rocks.
• B. Photosynthesis and respiration drive nitrogen cycling.
• C. Nitrogen is abundant and never limiting.
• D. Only nitrate fixation occurs.

Answer: (A)

Nitrogen moves through ecosystems by transforming through a sequence of steps that convert inert atmospheric N2 into forms living things can use, and then back to the atmosphere. The key steps begin with fixation, where certain microbes and, at times, lightning convert N2 into ammonium (NH4+). Nitrification follows, as ammonium is oxidized to nitrite and then to nitrate (NO3−), making nitrogen available in a form plants can take up. Assimilation is the process by which plants (and microbes) incorporate nitrate or ammonium into amino acids and other organic molecules. Mineralization, or ammonification, is carried out by decomposers when they break down organic nitrogen from dead matter and waste back into ammonium. Denitrification returns nitrogen to the atmosphere when nitrate is reduced and released as N2 (or N2O) under low-oxygen conditions. Although nitrogen is abundant as N2 in the air, most organisms cannot use it directly, so usable forms of nitrogen are often scarce in ecosystems. When available nitrogen is limited, plant growth and productivity are constrained, making nitrogen a common limiting nutrient.