Extrapolation of point measurements and fertilizer-only emission factors cannot capture statewide soil NOxemissions

Maazet al. argue that inconsistencies across scales of observation undermine our working hypothesis that soil NOxemissions have been substantially overlooked in California; however, the core issues they raise are already discussed in our manuscript. We agree that point measurements cannot be reliably used to estimate statewide soil NOxemissions—the principal motivation behind our new modeling/airplane approach. Maazet al.’s presentation of fertilizer-based emission factors (a nonmechanistic scaling of point measures to regions based solely on estimated nitrogen fertilizer application rates) includes no data from California or other semiarid sites, and does not explicitly account for widely known controls of climate, soil, and moisture on soil NOxfluxes. In contrast, our model includes all of these factors. Finally, the fertilizer sales data that Maazet al. highlight are known to suffer from serious errors and do not offer a logically more robust pathway for spatial analysis of NOxemissions from soil.