Methods of producing a treated gas by removing nitrogenous compounds are disclosed. Methods of recovering nitrogenous compounds from a gas stream are disclosed. Methods of producing a fertilizer product from organic waste are disclosed. The methods may include introducing a gas stream having nitrogenous compounds into a nitrogenous liquid containing a salt of ammonia to absorb the nitrogenous compounds in the liquid and produce a treated gas. The methods may also include controlling the pH of certain solutions or introducing an oxidant into certain solutions to produce nitrogen ions. Systems for removing nitrogenous compounds including a reaction subsystem, an oxidation control subsystem, a dissolved solids concentrator, and a recirculation line are also disclosed. The systems may be employed to remove nitrogenous compounds from a gas stream, recover the nitrogenous compounds from the gas stream, or produce a fertilizer product from the recovered nitrogenous compounds.