The hydrologic sciences must play a major role in improving our understanding of the transport and fate of the vast amount of reactive nitrogen that is being added to the environment by human activities. Detailed understanding of the function of different landscape units will help predict watershed losses of nitrogen. A better understanding of the processes that control denitrification in surface and groundwaters is essential to ascertain total gaseous N loss and the percentage that is nitrous oxide, a greenhouse gas that is accumulating in Earth's atmosphere.