FIGURE 1. Forms of nitrogen storage in, or export from, two ecosystems in North and South America.

a, Data for a largely forested region in the northeastern United States³. Here, nitrogen is found to be exported primarily in inorganic form. b, The pristine forest area in Argentina and Chile studied by Perakis and Hedin¹. Most notably, most of the nitrogen exported in streams and rivers here is in organic, rather than inorganic, form. The two regions have about the same annual precipitation of 1,100 mm. But the total 'sinks' in a are almost ten times those in b (about 36.4 kg ha ^-1 yr ^-1 compared with 0.4 kg ha ^-1 yr ^-1). The difference is due to a much greater input of nitrogen through human agency in the United States. As well as nitrogen export in water, and denitrification, sinks in the forested northeastern United States include export in agricultural and forestry products (food and wood); volatilization of ammonia from manure and fertilizers; storage in growing forests (biomass increase); and accumulation in soils, mainly in forests and in suburban land. The budget in b was estimated from the new data¹, with the assumptions that the nitrogen in soils and biomass is in steady state, and that the ratio of denitrification to nitrate export is the same in both regions.