ATMOSPHERIC INPUTS OF DISSOLVED ORGANIC NITROGEN TO THE OCEANS

THE input of fixed nitrogen to the oceans by in situ fixation, river/g roundwater supply and atmospheric deposition represents an important c ontrol on marine productivity on long timescales, and hence on ocean-a tmosphere CO2 exchange and its effects on climate(1-3). Any assessment of human perturbation of the global nitrogen cycle also requires an a ccurate estimate of these inputs. The current best estimates suggest t hat the natural fluvial and atmospheric inputs are of similar magnitud e(3,4), and that globally both have been increased by a factor of two above natural levels as a result of human activity(3-5). Dissolved org anic nitrogen represents more than half of the fluvial input of dissol ved fixed nitrogen, but current estimates of atmospheric inputs are us ually based on only the inorganic (NO3- + NH4+) component, although so me authors have recognized the potential importance of organic nitroge n(6-9). Here we present analyses of dissolved organic nitrogen in rain and snow which show that it is a ubiquitous and significant component of precipitation, even in remote marine areas. Our results require an approximate doubling of present estimates of the atmospheric input of fixed nitrogen to the oceans, and an increase in estimates of the tot al fixed-nitrogen input by a factor of about 1.5. These results indica te that the human impact on the global nitrogen cycle may be larger th an has been thought.