Significance Acid rain has stripped forests of soil calcium, with consequences for forest health and downstream ecosystems. In 1999, researchers initiated a whole-watershed experiment, with the goal of replacing all the calcium lost. This experiment increased the pH and acid-neutralizing capacity of soils and streamwater, and forest growth increased. In 2010, nitrogen export from the treated watershed began to increase, and by 2013, annual inorganic N losses from the experimental watershed were 30-times higher than from the adjacent reference watershed, a proportional increase only seen in whole-watershed clear-cutting experiments. The discovery that calcium enrichment can convert a watershed from a sink to a source of N suggests unforeseen consequences of acid rain mitigation and provides new insights into watershed dynamics.