Nitrous oxide is a greenhouse gas and also contributes to ozone loss. It seems that fertilizer run-off into coastal waters stimulates its production — at least to the west of India.
The gas nitrous oxide (N2O) influences the Earth's climate both directly and indirectly. Occurring at trace levels in the atmosphere, most of the gas comes from only two sources: soils and oceans. But changing environmental conditions, stemming from both natural variability and human-induced perturbations, are increasing nitrous oxide emissions to the atmosphere. On page 346 of this issue, Naqvi et al.1 present the first evidence for a phenomenon that might have far-reaching consequences for the Earth's climate. They show that large amounts of nitrous oxide can temporarily build up in the waters of the Indian continental shelf, and that this is caused by changes in the coastal environment that favour formation of the gas.
In the lower atmosphere (the troposphere) nitrous oxide is present at about only 0.00003%. Nonetheless, it punches well above its weight in terms of its effect on climate and atmospheric chemistry2. It is a strong greenhouse gas: a molecule of nitrous oxide is about 200–300 times more powerful in warming potential than a molecule of carbon dioxide. In addition, nitrous oxide is not removed from the troposphere by chemical reactions and so can reach the middle atmosphere (the stratosphere). Here it is photochemically destroyed, forming nitric oxide radicals which are involved in one of the cycles that destroy ozone. So, indirectly, nitrous oxide also affects the stratospheric ozone layer which protects us from harmful ultraviolet radiation.
For the past 100 years, the atmospheric concentrations of nitrous oxide have been increasing steadily3. Besides the soils and oceans4, there are anthropogenic sources of nitrous oxide such as biomass burning and emissions from industrial processes and automobiles. But the increase of the past century is probably mainly due to the intensification of agriculture5: nitrogen-containing fertilizers are known to increase nitrous oxide release from soils6. But how the coastal zone responds to increasing inputs of nutrients, in the form of fertilizer run-off from land, remains largely a mystery.
Both the terrestrial and the oceanic nitrogen cycles are driven by the activities of microorganisms. Some of them can obtain their energy by transforming various forms of nutrients such as nitrate (NO3−), nitrite (NO2−) and ammonium (NH4+). Nitrous oxide is mainly produced during two processes: nitrification and denitrification.
In nitrification, ammonium is transformed into nitrate. Nitrous oxide is formed as a by-product during the step from ammonium to nitrite:
Denitrification occurs when nitrate is transformed into dinitrogen (N2):
But the formation of dinitrogen can be incomplete, generating nitrous oxide — estimates vary, but it seems that a yield of 0.1–6% of nitrous oxide can be produced.
But things are more complex than this because the amount of nitrous oxide produced by either nitrification or denitrification depends on the prevailing oxygen conditions — maximum yields of the gas occur only in a narrow range of low oxygen concentrations. This means that significant nitrous oxide accumulations in the ocean will probably mainly be found under oxygen-depleted (suboxic) conditions7. In turn, this implies that small variations in the levels of dissolved oxygen might lead to large changes in the rate at which nitrous oxide is formed.
Naqvi et al.1 provide evidence for a dramatic decrease in dissolved oxygen in the coastal waters off India during the monsoon season, which in turn is associated with the large-scale formation of nitrous oxide. It seems that fertilizer run-off intensifies oxygen depletion, by increasing the productivity of marine algae and the subsequent recycling of the freshly produced organic matter. We should be concerned about these observations because seasonal oxygen depletion seems to have developed only over the past few years, and it is not locally restricted — it spreads along the whole Indian continental shelf.
A rough calculation underlines this point. Naqvi et al. estimate that the nitrous oxide emissions from the area of ocean that they investigated over a period of only six months range from 0.06×1012 g to 0.39×1012 g. Comparing those figures with estimates of the global annual output of nitrous oxide from the oceans8,9 (between 1.9×1012 g and 17×1012 g) reveals a contribution of 0.4–21% from an area which is only about 0.05% of that of the world's oceans. From Naqvi and colleagues' data, it seems likely that increasing nutrient inputs into the coastal zone from nitrogen fertilizers cause the seasonal shift in oxygen regime. But natural variability stemming from mechanisms such as the monsoon-induced upwelling of coastal waters, which delivers nutrients to the surface, cannot yet be excluded.
Several questions remain. Are there comparable accumulations of nitrous oxide in other oxygen-depleted coastal environments (for instance the Gulf of Mexico)? Are such accumulations a short- or a long-term phenomenon, and do they affect the atmospheric concentrations of the gas? Answers might come from intensive time-series measurements in selected coastal areas. Meantime, plenty of readers will have recognized nitrous oxide in a more familiar guise as laughing gas. But the results of Naqvi et al. show that, in environmental terms, this may be no laughing matter.
Naqvi, S. W. A. et al. Nature 408, 346– 349 (2000).
Prather, M. et al. in Climate Change 1995: The Science of Climate Change, Contribution of Working Group I to the Second Assessment of the Intergovernmental Panel on Climate Change (eds Houghton, J. T. et al.) 86– 103 (Cambridge Univ. Press, 1996).
Battle, M. et al. Nature 383, 231–235 (1996).
Bouwman, A. F., Van der Hoek, K. W. & Olivier, J. G. J. J. Geophys. Res. 100, 2785 –2800 (1995).
Kroeze, C., Mosier, A. & Bouwman, L. Glob. Biogeochem. Cycles 13, 1–8 (1999).
Robertson, G. P., Paul, E. A. & Harwood, R. R. Science 289, 1922– 1925 (2000).
Codispoti, L. A. et al. in Oceanography of the Indian Ocean (ed. Desai, B. N.) 271–284 (Balkema, Rotterdam, 1992).
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