Global estimates of indirect emissions

Based on IPCC default EFs of 1996 (Mosier et al, 1998), indirect emissions from atmospheric deposition had been estimated to be 0.3 Tg N yr-1 with an uncertainty range of 0.06 to 0.6 Tg N. The respective indirect flux from leaching and runoff had been 1.6 Tg N yr-1 (0.13-7.7 Tg N yr-1). Indirect emissions had thus been close to the direct emission from agricultural soils of 2.1 Tg N yr-1.

Using 2006 IPCC data, del Grosso et al (2008) calculated much lower indirect emissions of 0.76Tg N yr-1, which are also much lower than the current estimates of direct soil emission of 3.77Tg N yr-1. Indirect emissions from leaching and runoff are now much lower than before, with a total of 0.4Tg yr-1 or 0.13Tg yr-1 for emissions from groundwater, rivers and estuaries, respectively (S. del Grosso, personal communication, 2009). The downward revision of the EF for N2O from groundwater (IPCC-EF5g) is supported by recent groundwater data (Weymann et al, 2008, Deurer et al, 2008). Indirect emissions from atmospheric deposition slightly increased to 0.36Tg N yr-1. However, the flux from atmospheric N deposition is probably higher than this value because IPCC-EF4 is likely to be underestimated by at least a factor of two (see previous section).

Based on river N loads, global fluxes from rivers and estuaries have recently been modelled, giving 0.3 to 2.1Tg N yr-1 (Kroeze et al, 2009), which is clearly higher than the current IPCC-based estimate for rivers and estuaries (0.13 + 0.13 = 0.26Tg N yr-1). This inconsistency needs further attention. However, it must be noted that the extent of uncertainty for all estimates of indirect N2O fluxes is still huge and much larger than estimates of direct fluxes.

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