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

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.