Nitrification is a minor source of nitrous oxide (N₂O) in an agricultural landscape and declines with increasing management intensity

Abstract

The long-term contribution of nitrification to nitrous oxide (N₂ 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 N₂ O flux measurements with site-specific Michaelis-Menten kinetics of nitrification-derived N₂ O, we test the relative importance of nitrification-derived N₂ 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 N₂ 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 N₂ O yields in cultured nitrifiers (typically 0.04%-8% of NH₃ oxidized) and competing sinks for available <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mtext>NH</mml:mtext><mml:mn>4</mml:mn><mml:mo>+</mml:mo></mml:msubsup></mml:math> in situ. Most nitrification-derived N₂ O was produced by ammonia-oxidizing bacteria rather than archaea, who appeared responsible for no more than 30% of nitrification-derived N₂ O production in all but one ecosystem. Although the proportion of nitrification-derived N₂ O production was lowest in annual cropping systems, these ecosystems nevertheless produced more nitrification-derived N₂ O (higher V_max ) than perennial and successional ecosystems. We conclude that nitrification is minor relative to other sources of N₂ O in all ecosystems examined.