Methods of producing a treated gas by removing nitrogenous compounds are disclosed. Methods of recovering ammonia from a gas stream having nitrogenous compounds are disclosed. Methods of producing a fertilizer product from organic waste are disclosed. The methods may include introducing aqueous sulfurous acid into a gas stream having nitrogenous compounds to absorb the nitrogenous compounds in a liquid and produce a treated gas. The methods may also include maintaining the pH of certain solutions above 5 or introducing an oxidant into certain solutions to produce sulfate ions. Systems for removing nitrogenous compounds including a reaction subsystem, a solids-liquid separator, a temperature control subsystem, an oxidation control subsystem, and a recirculation line are also disclosed. The systems may be employed to remove nitrogenous compounds from a gas stream, recover the ammonia from the gas stream, or produce a fertilizer product from the recovered ammonia.