Nitrification is a minor source of nitrous oxide (N O) in an agricultural landscape and declines with increasing management intensity.

The long-term contribution of nitrification to nitrous oxide (N O) emissions from terrestrial ecosystems is poorly known and thus poorly constrained in biogeochemical models. Here, using Bayesian inference to couple 25 years of in situ N O flux measurements with site-specific Michaelis-Menten kinetics of nitrification-derived N O, we test the relative importance of nitrification-derived N O across six cropped and unmanaged ecosystems along a management intensity gradient in the U.S. Midwest. We found that the maximum potential contribution from nitrification to in situ N O fluxes was 13%-17% in a conventionally fertilized annual cropping system, 27%-42% in a low-input cover-cropped annual cropping system, and 52%-63% in perennial systems including a late successional deciduous forest. Actual values are likely to be <10% of these values because of low N O yields in cultured nitrifiers (typically 0.04%-8% of NH oxidized) and competing sinks for available in situ. Most nitrification-derived N O was produced by ammonia-oxidizing bacteria rather than archaea, who appeared responsible for no more than 30% of nitrification-derived N O production in all but one ecosystem. Although the proportion of nitrification-derived N O production was lowest in annual cropping systems, these ecosystems nevertheless produced more nitrification-derived N O (higher V ) than perennial and successional ecosystems. We conclude that nitrification is minor relative to other sources of N O in all ecosystems examined.