Uncertainties in the global source distribution of nitrous oxide

Inventories with 1°×1° resolution were compiled of nitrous oxide (N2O) emissions from fertilized arable land, animal excreta, postclearing effects on soil emissions, fossil fuel and fuelwood combustion, and industrial N2O sources. For other sources of N2O, including soils under natural vegetation, oceans, and biomass burning, published inventories were used. From these inventories the annual N2O emission was calculated for four broad latitudinal zones covering the globe. Uncertainties were assessed by comparing variants of inventories with source estimates inferred from inverse modeling techniques. Major uncertainties occur in the tropics, where small errors in both soil and oceanic emission estimates may have large repercussions for the zonal distributions. Although there may still be many poorly known and unidentified N2O sources, the analysis has resulted in improved understanding of some sources, i.e., (1) the oceanic N2O emission may be more important than assumed in recent global N2O budgets, with a major portion stemming from the 30°-90°S zone; (2) the N2O emission from animal excreta forms a significant global source; (3) most of the N2O from arable lands and grasslands, including effects of synthetic fertilizers and animal excreta, comes from the northern hemisphere; accounting for only the synthetic-fertilizer effect on N2O emission leads to an underestimation of the emission from arable lands; (4) fossil fuel combustion and industrial N2O sources are dominant in the 30°-90°N zone, while N2O from fuelwood combustion is mainly produced in the 0°-30°N zone; (5) the estimation of enhanced N2O soil emission following tropical forest clearing that has accounted for gradually declining N2O fluxes, along with aging of the clearing leads to a global emission that is significant but lower than previous estimates; (6) most of the N2O from coastal marine and freshwater systems and soil N2O emission resulting from N deposition probably comes from the northern hemisphere.