Industrial development and agricultural intensification are projected to increase in the humid tropics over the next few decades1, increasing the emissions, transport and deposition of nitrogen-containing compounds2. Most studies of the consequences of enhanced nitrogen deposition have been performed in temperate ecosystems in which biological processes are limited by nitrogen supply; they indicate that added nitrogen is retained up to decades before losses as nitrogen oxides or as nitrate (NO3−) begin3,4,5. We measured soil emissions of two gases that are important in the atmosphere, nitrous oxide (N2O) and nitric oxide (NO), after experimental additions of nitrogen in two tropical rainforests of Hawai'i. Growth of one of the forests was limited by nitrogen; in the other, nitrogen was abundant and growth was limited by phosphorus, as is more characteristic of most tropical forests6. Here we show that the phosphorus-limited forest lost more nitrogen oxides than the nitrogen-limited forest, and it lost equally large amounts after first-time additions of nitrogen as after chronic, long-term nitrogen additions. This forest seems to be naturally ‘nitrogen saturated’7; it and perhaps other tropical forests may not retain as much anthropogenic nitrogen as do forests in northern latitudes.