What initiates the nitrogen cycle?

The initiation of the nitrogen cycle is primarily marked by the fixation of atmospheric nitrogen. This process involves converting nitrogen gas (N2), which makes up about 78% of the Earth's atmosphere and is inert or unusable by most living organisms, into forms that can be absorbed and utilized by plants.

Nitrogen fixation can occur through several mechanisms, such as biological fixation by certain bacteria (e.g., Rhizobium in legumes) that convert nitrogen gas into ammonia (NH3), or through abiotic processes like lightning. Once nitrogen is fixed into a usable form, it allows for the creation of amino acids and nucleic acids, which are essential for plant growth and are subsequently passed up the food chain to herbivores and carnivores.

The other options, while related to processes occurring in the nitrogen cycle, do not initiate it. Decomposition of organic matter plays a vital role in recycling nitrogen back into the soil, digestion by herbivores allows for the assimilation of nitrogen into animal protein, and evaporation pertains to water cycles rather than nitrogen cycles. Therefore, fixation of atmospheric nitrogen is the crucial first step that sets in motion the complex interactions in the nitrogen cycle.