Steady-State Nitrogen Isotope Effects of N 2 and N 2 O Production in Paracoccus denitrificans

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Abstract
Nitrogen stable-isotope compositions (δ 15 N) can help track denitrification and N 2 O production in the environment, as can knowledge of the isotopic discrimination, or isotope effect, inherent to denitrification. However, the isotope effects associated with denitrification as a function of dissolved-oxygen concentration and their influence on the isotopic composition of N 2 O are not known. We developed a simple steady-state reactor to allow the measurement of denitrification isotope effects in Paracoccus denitrificans . With [dO 2 ] between 0 and 1.2 μM, the N stable-isotope effects of NO 3 − and N 2 O reduction were constant at 28.6‰ ± 1.9‰ and 12.9‰ ± 2.6‰, respectively (mean ± standard error, n = 5). This estimate of the isotope effect of N 2 O reduction is the first in an axenic denitrifying culture and places the δ 15 N of denitrification-produced N 2 O midway between those of the nitrogenous oxide substrates and the product N 2 in steady-state systems. Application of both isotope effects to N 2 O cycling studies is discussed.