Review of the IPCC methodology for estimating nitrous oxide emissions associated with agricultural leaching and runoff

Context abstract: The constraint imposed by the observed atmospheric N₂O increase suggests that the IPCC may overestimate the anthropogenic N₂O source. The 1996 Revised IPCC methodology, which will be used by Parties to the Kyoto Protocol, predicts that N₂O accounts for 10% or more of national aggregate greenhouse gas emissions in many countries. This percentage contribution is comparable to or greater than the overall emissions reductions required by the Protocol. N₂O emissions associated with agricultural leaching and runoff contribute a significant share of the IPCC N₂O source. The current methodology may significantly overestimate these emissions, with implications for the total IPCC anthropogenic N₂O source and for national greenhouse gas inventories.

Main abstract: N₂O emissions associated with leaching and runoff play an important role in determining both the magnitude and the uncertainty of the agricultural N₂O source, as estimated by the 1996 revised IPCC methodology. According to the methodology, leaching/runoff emissions account for over 1/4 of the total agricultural N₂O source and nearly 1/2 of the range of uncertainty in the total source. Notably, the observed atmospheric N₂O increase of 3.9 Tg N/yr, which provides an important and well documented constraint on the anthropogenic N₂O source, is significantly lower than the IPCC total agricultural source of 6.3 Tg N/yr.

Several areas of uncertainty in the IPCC estimate of leaching and runoff-related N₂O emissions are identified in this review. First, in the current methodology, a default-leaching fraction for fertilizer and animal waste of 30% is recommended for all countries, despite large variations within individual watersheds and agricultural systems. Second, the N₂O emission factor associated with groundwater may be overestimated by an order of magnitude. Currently, groundwater accounts for 60% of leaching-related N₂O emissions, with the remainder assumed to occur from rivers and estuaries. Finally, leaching fractions and associated N₂O emission factors may not be defined in a conceptually consistent manner.