What plays multiple roles in the cycling of nitrogen, including converting nitrogen gas into a form of nitrogen usable by plants?

Animals do not directly convert nitrogen gas into usable forms for plants; instead, they rely on the nitrogen compounds found in plants and other organisms. While animals play a role in the nitrogen cycle through excretion and decomposition, they do not perform the initial nitrogen fixation necessary for plant nutrient uptake.

Lightning can convert nitrogen gas in the atmosphere into nitrogen oxides, which can then fall to the ground as precipitation. While this process contributes to the nitrogen cycle, it is less significant compared to the role of bacteria in directly fixing nitrogen into forms that plants can use.

Fungi play a role in decomposing organic matter and recycling nutrients, including nitrogen, but they do not fix atmospheric nitrogen into usable forms for plants. Their primary function is associated with nutrient absorption and symbiotic relationships with plants rather than direct nitrogen conversion.