Nitrogen oxide emissions after nitrogen additions in tropical forests

Industrial development and agricultural intensification are projected to; i ncrease in the humid tropics over the next few decades(1), increasing the e missions, transport and deposition of nitrogen-containing compounds(2). Mos t studies of the consequences of enhanced nitrogen deposition have been per formed in temperate ecosystems in which biological processes are limited by nitrogen supply; they indicate that added nitrogen is retained up to decad es before losses as nitrogen oxides or as nitrate (NO3-) begin(3-5). We mea sured soil emissions of two gases that are important in the atmosphere, nit rous oxide (N2O) and nitric oxide (NO), after experimental additions of nit rogen 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 forests( 6). Here we show that the phosphorus-limited forest lost more nitrogen oxid es than the nitrogen-limited forest, and it lost equally large amounts afte r first-time additions of nitrogen as after chronic, long-term nitrogen add itions. This forest seems to be naturally 'nitrogen saturated('7); it and p erhaps other tropical forests may not retain as much anthropogenic nitrogen as do forests in northern latitudes.