Provided are a method and a system for treatment of an exhaust stream, from a combustion engine comprising nitrogen oxides. An estimate of at least one future operating condition for the exhaust treatment system is determined based on a road section ahead of the vehicle. A first additive is input into the exhaust stream to cause a first reduction of nitrogen oxides in a first reduction catalyst device. A second additive is input into the exhaust stream to cause a second reduction of a second amount of nitrogen oxides in a second reduction catalyst device, arranged downstream of the first reduction catalyst device. The first supply of additive is controlled based on the estimated at least one future operating condition, such that the first reduction catalyst device is exposed, over time, to a substoichiometric condition, with respect to the first additive and the first amount of nitrogen oxides.