NOVELTY - Fermenting nitrogenous biomass (1), comprises (i) supplying the biomass into a mixing device (10), mixing a recirculate into the biomass to produce a mixture, supplying back the mixture into a fermenter (15), (ii) removing the fermentation residue (21), supplying the residue to a separator and separating the residue into a solid and a liquid phase, supplying only the liquid phase into a nitrogen removal device (26), reducing nitrogen in the liquid phase, producing a nitrogen-reduced fermentation residue liquid phase, and (iii) supplying nitrogen reduced liquid phase into the mixing device.    USE - The method is useful for fermenting nitrogenous biomass, preferably poultry manure in a biogas plant.    ADVANTAGE - The method is carried out in an easy manner.    DETAILED DESCRIPTION - Fermenting nitrogenous biomass (1), preferably poultry manure in a biogas plant, comprises (i) supplying the nitrogenous biomass, which is to be fermented, into a mixing device (10), mixing a defined amount of a recirculate into the biomass so that the nitrogen concentration of the produced biomass/recirculate mixture lies below a defined nitrogen threshold value, supplying back the biomass/recirculate mixture into at least one fermenter (15) of the biogas plant and generating biogas, (ii) removing the fermentation residue (21) from at least one fermenter, supplying the fermentation residue to a separator and/or a separator device and separating the fermentation residue into a fermentation residue solid phase and a fermentation residue liquid phase, supplying only the fermentation residue liquid phase into a nitrogen removal device (26), reducing or removing nitrogen in the form of ammonium and/or ammonia dissolved or bound in the liquid phase, producing a nitrogen-reduced, preferably a nitrogen-free, fermentation residue liquid phase, and (iii) supplying nitrogen reduced fermentation residue liquid phase into the mixing device in a defined time and amount as a recirculate. An INDEPENDENT CLAIM is also included for a device for carrying out the fermentation of nitrogenous biomass, preferably poultry manure in the biogas plant by the above mentioned method, comprising the mixing device, the fermenter, which is connected downstream to the mixing device, the separator and/or separator device, which is connected downstream to the fermenter, the nitrogen removal device, which is connected downstream to the separator and/or separator device, and at least one recirculation device comprising a return line and/or an intermediate storage or a buffer reservoir, by means of which the nitrogen-reduced fermentation residue liquid phase is supplied as the recirculate to the mixing device.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic process flow of the method carried out in the device.    Nitrogenous biomass (1)    Mixing device (10)    Fermenter (15)    Fermentation residue (21)    Nitrogen removal device (26)