Fractionation of ammonia nitrogen isotopes by ruminal bacteria in vitro

Fractionation of ammonia N isotopes was tested in vitro as ruminal ammonia N was labeled by addition of <F>15N</F>-ammonium sulfate enriched at 10.7 at.% (15N10) or 20.5 at.% (15N20) excess. Ruminal inoculum was obtained from a lactating dairy cow fed a 45% forage:55% concentrate diet (dry matter (DM) basis). Duplicate incubations were carried out for 6 h and vessels were sampled at 0, 1, 2, 4, and 6 h. Total and individual VFA, acetate to propionate ratio, and ammonia concentration did not differ (<F>P>0.05</F>) between treatments. The <F>15N</F>-enrichment of bacterial N increased (<F>P<0.001</F>) with incubation time, but was not affected (<F>P=0.36</F>) by the degree of enrichment of the ammonium sulfate. However, at the latter incubation hours, <F>15N</F>-enrichment of bacterial N was higher (<F>P<0.05</F>) for 15N20 than 15N10. The proportion of bacterial N originating from ammonia N was not affected (<F>P=0.81</F>) by the <F>15N</F> source. At <F>15N</F>-enrichment levels considerably higher than natural abundance, fractionation of N isotopes by ruminal bacteria is negligible and does not significantly affect the reliability of <F>15N</F> as an N tracer in nutritional studies with ruminants. [Copyright &y& Elsevier]