Retrieval of aboveground crop nitrogen content with a hybrid machine learning method

•PROSPECT-PRO model accounting for proteins is coupled with the 1-D SAIL model.•Gaussian processes (GP) identifies optimal spectral bands for nitrogen (N) sensing.•GP and PROSAIL-PRO modelling are combined for N retrieval at agricultural sites.•Heteroscedastic GP provides accurate estimations of crop N from leaves plus stalks.