How do animals get nitrogen necessary for protein synthesis?

Animals obtain nitrogen necessary for protein synthesis primarily by consuming either plants or other animals. This is because nitrogen in its gaseous form (as found in the atmosphere) is not usable by most animals directly. Instead, organisms such as plants absorb nitrogen from the soil, primarily in the form of nitrates and ammonium, which they utilize to synthesize amino acids and proteins.

When animals eat plants (a process known as herbivory), they effectively transfer the nitrogen that the plants have assimilated into a bioavailable form, allowing the animals to incorporate it into their own proteins. In the case of carnivorous animals, they acquire nitrogen by consuming other animals who, in turn, have gotten their nitrogen from either plants or other animals through the same process. Thus, both paths of nutrient acquisition ultimately link back to the initial uptake of nitrogen by plants, making the consumption of either plants or other animals crucial for an animal's nitrogen needs.