CLOSING THE GLOBAL N2O BUDGET - NITROUS-OXIDE EMISSIONS THROUGH THE AGRICULTURAL NITROGEN-CYCLE - OECD IPCC/IEA PHASE-II DEVELOPMENT OF IPCC GUIDELINES FOR NATIONAL GREENHOUSE-GAS INVENTORY METHODOLOGY/

In 1995 a working group was assembled at the request of OECD/IPCC/IEA to revise the methodology for N2O from agriculture for the National Gr eenhouse Gas Inventories Methodology. The basics of the methodology de veloped to calculate annual country level nitrous oxide (N2O) emission s from agricultural soils is presented herein. Three sources of N2O ar e distinguished in the new methodology: (i) direct emissions from agri cultural soils, (ii) emissions from animal production, and (iii) N2O e missions indirectly induced by agricultural activities. The methodolog y is a simple approach which requires only input data that are availab le from FAO databases. The methodology attempts to relate N2O emission s to the agricultural nitrogen (N) cycle and to systems into which N i s transported once it leaves agricultural systems. These estimates are made with the realization that increased utilization of crop nutrient s, including N, will be required to meet rapidly growing needs for foo d and fiber production in our immediate future. Anthropogenic N input into agricultural systems include N from synthetic fertilizer, animal wastes, increased biological N-fixation, cultivation of mineral and or ganic soils through enhanced organic matter mineralization, and minera lization of crop residue returned to the field. Nitrous oxide may be e mitted directly to the atmosphere in agricultural fields, animal confi nements or pastoral systems or be transported from agricultural system s into ground and surface waters through surface runoff. Nitrate leach ing and runoff and food consumption by humans and introduction into se wage systems transport the N ultimately into surface water (rivers and oceans) where additional N2O is produced. Ammonia and oxides of N (NO ,) are also emitted from agricultural systems and may be transported o ff-site and serve to fertilize other systems which leads to enhanced p roduction of N2O. Eventually, all N that moves through the soil system will be either terminally sequestered in buried sediments or denitrif ied in aquatic systems. We estimated global N2O-N emissions for the ye ar 1989, using midpoint emission factors from our methodology and the FAO data for 1989. Direct emissions from agricultural soils totaled 2. 1 Tg N, direct emissions from animal production totaled 2.1 Tg N and i ndirect emissions resulting from agricultural N input into the atmosph ere and aquatic systems totaled 2.1 Tg N2O-N for an annual total of 6. 3 Tg N2O-N. The N2O input to the atmosphere from agricultural producti on as a whole has apparently been previously underestimated. These new estimates suggest that the missing N2O sources discussed in earlier I PCC reports is likely a biogenic (agricultural) one.