Nitrogen is returned to the soil when animals urinate. During urine excretion, animals release nitrogenous waste, which can be converted into forms of nitrogen that plants can use. Additionally, decaying organic matter from animals and plants also contributes to the nitrogen content in the soil, but in the context of the choices you provided, the most direct answer regarding the return of nitrogen would be "urinate."
So, the most fitting answer would be Urinate.