Nitrification gene ratio and free ammonia explain nitrite and nitrous oxide production in urea-amended soils

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Abstract

The atmospheric concentration of nitrous oxide (N2O), a potent greenhouse gas and ozone-depleting chemical, continues to increase, due largely to the application of nitrogen (N) fertilizers. While nitrite (NO2) is a central regulator of N2O production in soil, NO2 and N2O responses to fertilizer addition rates cannot be readily predicted. Our objective was to determine if quantification of multiple chemical variables and structural genes associated with ammonia (NH3)- (AOB, encoded by amoA) and NO2-oxidizing bacteria (NOB, encoded by nxrA and nxrB) could explain the contrasting responses of eight agricultural soils to five rates of urea addition in aerobic microcosms. Significant differences in NO2 accumulation and N2O production by soil type could not be explained by initial soil properties. Biologically-coherent statistical models, however, accounted for 70–89% of the total variance in NO2 and N2O. Free NH3 concentration accounted for 50–85% of the variance in NO2 which, in turn, explained 62–82% of the variance in N2O. By itself, the time-integrated nxrA:amoA gene ratio explained 78 and 79% of the variance in cumulative NO2 and N2O, respectively. In all soils, nxrA abundances declined above critical urea addition rates, indicating a consistent pattern of suppression of Nitrobacter-associated NOB due to NH3 toxicity. In contrast, Nitrospira-associated nxrB abundances exhibited a broader range of responses, and showed that long-term management practices (e.g., tillage) can induce a shift in dominant NOB populations which subsequently impacts NO2 accumulation and N2O production. These results highlight the challenges of predicting NO2 and N2O responses based solely on static soil properties, and suggest that models that account for dynamic processes following N addition are ultimately needed. The relationships found here provide a basis for incorporating the relevant biological and chemical processes into N cycling and N2O emissions models.

Original languageEnglish (US)
Pages (from-to)143-153
Number of pages11
JournalSoil Biology and Biochemistry
Volume111
DOIs
StatePublished - Aug 1 2017

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