Millar, N., G. P. Robertson, A. Diamant, R. J. Gehl, P. R. Grace, and J. P. Hoben. 2013. Quantifying N2O emissions reductions in US agricultural crops through N fertilizer rate reduction. Verified Carbon Standard, Washington, DC, USA.
Summary Description of the Methodology
This methodology quantifies emissions reductions of nitrous oxide (N2O) from agriculture in the United States (US), as brought about by reductions in the rate of nitrogen (N) fertilizer (synthetic and organic) applied to cropland. The methodology encourages the application of economically optimum N fertilizer rates that do not harm productivity, and requires the use of verifiable best management practices for N timing, placement, and type. Depending on the US state where a project is implemented, the methodology uses either a generally accepted IPCC Tier 1 emission factor or an empirically derived Tier 2 regional emission factor (applicable in the 12 state North Central Region) to aid in calculating N2O emissions reductions. The approach is straightforward and transparent and is a practical solution to help reduce N2O emissions and other reactive N pollutants from agriculture. The field research that underpins the methodology is publicly available in the peer-reviewed literature.
Nitrous oxide production in agricultural soil occurs predominantly through the microbial transformations of inorganic N (Robertson and Groffman 2007). The potential to produce and emit N2O increases with the increasing availability of N (Bouwman et al. 1993). Due to the strong influence of available soil N on N2O emissions, some emissions of N2O are an unavoidable consequence of maintaining highly productive cropland (Mosier 2002). However, any activity or process that acts to keep available soil N low will lead to lower N2O emissions. Anthropogenic activities that lower the input of N into cropland agriculture help to reduce emissions of N2O (Robertson and Vitousek 2009).
To date the vast majority of evidence supports N input as the most robust and reliable default proxy for calculating N2O emissions. It is consistent and straightforward to quantify as a metric and its use is substantiated by the IPCC, which uses annual N rate as the default method for calculating N2O emissions from managed land in national greenhouse gas inventories. Moreover, its alteration is readily accessible to management intervention.
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