Table 43 Changing assessment since 1990 of agricultural contribution to global N2O emissions

Assessment Estimated global N2O Description of source emission from agriculture (Tg N2O-N yr1)

Mosier et al (1998) and 4.2(0.6-14.8) IPCC (2001) 2.1 (0.6-3.1)

Fertilizer, including emission from groundwater Cultivated soils

Mineral N (fertilizer) + animal waste + N-fixation

N added to soils + indirect emissions Manure management Biomass burning

By contrast, the trend in the estimates of N2O emissions from the largest natural terrestrial source, wet tropical forests, has gone in the opposite direction. McElroy and Wofsy (1986) estimated an annual global emission from this biome of 7.4 ± 4Tg N2O—N, nearly ten times their estimate for fertilized agricultural land (Table 4.1). The forest estimate was based on measurements at only five sites, and not long afterwards Matson and Vitousek (1990), using data from about 30 sites, reduced the estimate to 2.4Tg N2O-N

yr-1, and the assessments of Mosier et al (1998) and Kroeze et al (1999) were similar. The scaling of observed fluxes in wet tropical forests to annual emissions was aided by the relatively small diurnal and month-to-month variations in N2O emissions (Matson and Vitousek, 1990) because of the rapid and continuous N mineralization that takes place (Keller and Reiners, 1994), providing a steady supply of mineral N (NH4+ and NO3-) as substrates for the N2O-producing microorganisms (see Chapter 2). Nonetheless, the global upscaling has large associated uncertainties.

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