A method and an exhaust treatment system are provided for treatment of an exhaust stream that comprises nitrogen oxides NOx. The method comprises using a first slip-catalyst SC1, arranged at a first device to create a first impact on a first amount of nitrogen oxides NOx_1 An NOx-storing catalyst may store nitrogen oxides NOx. The first slip-catalyst SC1 may reduce nitrogen oxides NOx, and/or oxidize potential additive in the exhaust stream. This first impact is actively controlled, based on the first amount of nitrogen oxides NOx_1 reaching the first device. The method also comprises a second impact on a second amount of nitrogen oxides NOx_2 reaching a second device. The active control of the first impact may be by active control of the dosage of additive at the first device, and/or through an active control of an exhaust environment, for example, a temperature for the exhaust stream.