Nitrogen management challenges in major watersheds of South America - Bustamante, Mercedes Maria da Cunha; Martinelli, Luiz Antonio; Pérez, Tibisay; Rasse, Rafael; Ometto, Jean Pierre Henry Balbaud; Pacheco, Felipe Siqueira; Lins, Silvia Rafaela Machado; Marquina, Sorena (Environmental Research Letters, 2015)