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

Abstract

Maaz et al. argue that inconsistencies across scales of observation undermine our working hypothesis that soil NOx emissions 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 NOx emissions—the principal motivation behind our new modeling/airplane approach. Maaz et 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 NOx fluxes. In contrast, our model includes all of these factors. Finally, the fertilizer sales data that Maaz et al. highlight are known to suffer from serious errors and do not offer a logically more robust pathway for spatial analysis of NOx emissions from soil.