Title Spatially explicit estimates of N 2 O emissions from croplands suggest climate mitigation opportunities from improved fertilizer management Running head Spatially explicit N 2 O emission estimates List of authors

With increasing nitrogen (N) application to croplands required to support growing food demand, mitigating N2O emissions from agricultural soils is a global challenge. National greenhouse gas emissions accounting typically estimates N2O emissions at the country scale by aggregating all crops, under the assumption that N2O emissions are linearly related to N application. However, field studies and meta-analyses indicate a nonlinear relationship, in which N2O emissions are relatively greater at higher N application rates. Here we apply a super-linear emissions response model to crop-specific, spatially-explicit synthetic N fertilizer and manure N inputs to provide subnational accounting of global N2O emissions from croplands. We estimate 0.66 Tg of N2O-N direct global emissions circa 2000, with 50% of emissions concentrated in 13% of harvested area. Compared to estimates from the IPCC Tier 1 linear model, our updated N2O emissions range from 20-40% lower throughout Sub-Saharan Africa and Eastern Europe, to >120% greater in some Western European countries. At low N application rates, the weak non-linear response of N2O emissions suggests that relatively large increases in N fertilizer application would generate relatively small increases in N2O emissions. Since aggregated fertilizer data generate A cc ep te d A rt ic le This article is protected by copyright. All rights reserved. underestimation bias in nonlinear models, high-resolution N application data are critical to support accurate N2O emissions estimates. Introduction Agriculture accounts for ~20-30% of global greenhouse gas emissions (Vermeulen et al., 2012) and produces the majority (~59%) of anthropogenic N2O emissions (Ciais et al., 2013). Nitrous oxide is a potent greenhouse gas, and is the most important contributor to stratospheric ozone depletion, with associated negative health impacts (Wolfe & Patz, 2002), and decreased plant productivity (Sitch et al., 2007). The largest source of N2O emissions from agriculture is synthetic N fertilizer and manure application to croplands (Syakila & Kroeze, 2011), which is projected to increase by ~50% from 2000 to 2050 (FAO, 2012). Between 2001-2011, annual N2O emissions from synthetic and manure fertilizers increased by 37% and 12%, respectively (FAO 2014b). Consequently, reducing N2O emissions from croplands is critical for addressing climate change and ozone depletion concerns. N2O is produced from microbially-mediated nitrification and de-nitrification processes in soils, leading to emission rates that are modified by diverse climate, soil, and vegetative conditions, and are highly variable over time and space (Stehfest & Bouwman, 2006; Philibert et al., 2012). These ‘direct’ emissions are distinct from ‘indirect’ emissions in which N2O is formed from N volatilized or leached from managed soils (De Klein et al., 2006), and N2O emissions associated with land use change (Flynn et al., 2012).