UNCERTAINTIES IN THE GLOBAL SOURCE DISTRIBUTION OF NITROUS-OXIDE

Inventories with 1 degrees x1 degrees resolution were compiled of nitr ous oxide (N2O) emissions from fertilized arable land, animal excreta, postclearing effects on soil emissions, fossil fuel and fuelwood comb ustion, and industrial N2O sources. For other sources of N2O, includin g soils under natural vegetation, oceans, and biomass burning, publish ed inventories were used. From these inventories the annual N2O emissi on 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 unc ertainties occur in the tropics, where small errors in both soil and o ceanic emission estimates may have large repercussions for the zonal d istributions. Although there may still be many poorly known and uniden tified N2O sources, the analysis has resulted in improved understandin g of some sources, i,e., (1) the oceanic N2O emission may be more impo rtant than assumed in recent. global N2O budgets, with a major portion stemming from the 30 degrees-90 degrees 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; ac counting for only the synthetic-fertilizer effect on N2O emission lead s to an underestimation of the emission from arable lands; (4) fossil fuel combustion and industrial N2O sources are dominant in the 30 degr ees-90 degrees N zone, while N2O from fuelwood combustion is mainly pr oduced in the 0 degrees-30 degrees N zone; (5) the estimation of enhan ced N2O soil emission following tropical forest clearing that has acco unted for gradually declining N2O fluxes, along with aging of the clea ring leads to a global emission that is significant but lower than pre vious estimates; (6) most of the N2O from coastal marine and freshwate r systems and soil N2O emission resulting from N deposition probably c omes from the northern hemisphere.