Global distribution of N2O emissions from aquatic systems: natural emissions and anthropogenic effects
Introduction
Total global N2O emissions include a wide range of sources from terrestrial and aquatic systems, with both natural and anthropogenic components (e.g., Khalil and Rasmussen, 1992, Matthews, 1994, Bouwman et al., 1995, Bange et al., 1996). A clear understanding of the magnitude of the various natural and anthropogenic sources and their geographic distribution is necessary to quantify factors contributing to the current increases in atmospheric N2O concentrations, to predict future potential increases, and to develop realistic and cost-effective strategies to reduce future emissions. There have been a number of analyses of the magnitude and global distribution of terrestrial and deep ocean sources (e.g., Nevison, 1994, Bouwman et al., 1995); however only recently have such analyses been done for freshwater and coastal marine systems (Seitzinger and Kroeze, 1998).
N2O emissions from freshwater and coastal marine ecosystems are estimated to amount to 1.9 Tg N y−1 (Seitzinger and Kroeze, 1998), which is similar in magnitude to estimated N2O emissions from a number of other sources, including arable land, grasslands, and animal excreta (Bouwman et al., 1995). Much of the N2O emission from these aquatic systems is anthropogenic and thus contributes to the current increase in atmospheric N2O. In addition, large increases in future N2O emissions from these aquatic systems are predicted as a result of increases in human activities (Kroeze and Seitzinger, 1998). Bouwman et al. (1995) presented an analysis of the total and latitudinal distribution of most of the known terrestrial sources and Nevison et al. (1995) has done so with oceanic sources. We now extend that analysis by adding freshwater and coastal marine N2O emissions.
In the current paper we compare: (1) N2O emissions from rivers, estuaries and continental shelves with N2O emissions from oceanic regions, (2) anthropogenic and natural emissions from aquatic systems, and (3) aquatic and terrestrial N2O emissions, both natural and anthropogenic. These comparisons are done at a global scale as well as on a latitudinal basis to provide insight into the geographic distribution of the different sources. Gridded (1° × 1°) estimates of N2O emissions from published studies are used in these analyses.
Section snippets
Methods
We calculated N2O emissions for the following latitudinal zones for the northern and southern hemisphere: 0°–20°, 20°–45°, 45°–66° and 66°–90°.
Emissions from oceans: We obtained the gridded (1° × 1°) global N2O emission inventory for ocean emissions from Nevison (1994) through the Emission Database for Global Atmospheric Research (EDGAR) (Olivier et al., 1996). The EDGAR database had been converted from the original 2.8° × 2.8° grid of Nevison (1994) to a 1° × 1° grid, which resulted in a loss of 0.2
Aquatic N2O emissions
N2O emissions from rivers, estuaries and continental shelves: Global watersheds receive inputs of nitrogen (N) from a variety of anthropogenic sources (Fig. 1). The three anthropogenic sources of new fixed N are synthetic fertilizer, combustion of fossil fuels and biological N2-fixation associated with cultivated crops. Together, these sources contribute an amount of N (about 90–130 Tg N y−1) similar to the natural biological fixation of N2 in non-agricultural soils (∼140 Tg N y−1) (Galloway et
Concluding remarks
It should be kept in mind that the above analyses are based on N2O emissions inventories currently available in gridded format. As noted above, there are a number of missing sources (Mosier et al., 1998), as well as more recent estimates of abiotic sources (Prather et al., 1995). Emissions from all sources have considerable uncertainty associated with them. As we improve our understanding of factors controlling N2O emissions in ecosystems worldwide, most certainly our estimates will need to be
Acknowledgements
We would like to thank L. Bouwman and J. Olivier for assistance with obtaining the terrestrial and ocean data bases from EDGAR. This work was supported in part with funding from NOAA NJ Sea Grant (SPS) (NJSG-99-423), NOAA Coastal Ocean Program (SPS) and Wageningen University (CK).
Sybil Seitzinger’s current research includes studies of: denitrification in rivers, estuaries and continental shelves; dissolved organic nitrogen inputs and bioavailability in aquatic ecosystems; and global modeling of N transport by world rivers and associated nitrous oxide production. Anthropogenic impacts are key components of her studies. She also enjoys windsurfing, ice climbing, snow boarding, white water kayaking, and traveling.
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Sybil Seitzinger’s current research includes studies of: denitrification in rivers, estuaries and continental shelves; dissolved organic nitrogen inputs and bioavailability in aquatic ecosystems; and global modeling of N transport by world rivers and associated nitrous oxide production. Anthropogenic impacts are key components of her studies. She also enjoys windsurfing, ice climbing, snow boarding, white water kayaking, and traveling.