A New Nitrogen Index That Can be Used to Improve the Methodology for Assessing National N2O Emissions from Cropping Systems of Mexico.

Recent reports such as the IPCC's Fifth Assessment Report and the 2014 U.S. National Climate Assessment stress the importance of climate change mitigation and/or adaptation. In order to assess the effects of practices on cropping systems and how to mitigate and/or adapt to climate change, there is a need for national trace gas inventories. One of the top twelve largest emitters of trace gases and users of nitrogen fertilizer in the world is Mexico. The agency in charge of Mexico's national trace gas inventory is INECC-SEMARNAT, which estimates that the agricultural sector contributes to about 12% of Mexico’s national emissions in CO₂ equivalents (INECC-SEMARNAT, 2013). Half of this percentage is from nitrous oxide-N (N₂O-N) emissions generated from the use of nitrogen fertilizers. Mexico’s national trace gas inventory is currently calculated using a 1% emission factor to assess the contribution of nitrogen fertilizers to N₂O emissions. We found that when this methodology was compared to measured N₂O emissions values, it significantly underestimated N₂O emissions. Additionally, we tested the Nitrogen Index across different cropping systems from Mexico and found that N₂O-N emissions estimated by the Nitrogen Index were significantly correlated with measured N₂O emissions. Based on these results, we suggest that the current methodology used to assess Mexico's trace gas inventory, which is submitted to the United Nations Framework Convention on Climate Change (UNFCCC), underestimates N₂O emissions. We propose that the use of a Tier 2 tool such as the Nitrogen Index to assess Mexico’s N₂O emissions from agricultural systems, could improve assessment of N₂O emissions by more closely approximating N₂O-N emissions values measured in the field. The results suggest that Mexico’s national emissions (from all sources) reported to UNFCCC may be underestimated by 6%.