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 ¹⁵N-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 (P > 0.05) between treatments. The ¹⁵N-enrichment of bacterial N increased (P < 0.001) with incubation time, but was not affected (P = 0.36) by the degree of enrichment of the ammonium sulfate. However, at the latter incubation hours, ¹⁵N-enrichment of bacterial N was higher (P < 0.05) for 15N20 than 15N10. The proportion of bacterial N originating from ammonia N was not affected (P = 0.81) by the ¹⁵N source. At ¹⁵N-enrichment levels considerably higher than natural abundance, fractionation of N isotopes by ruminal bacteria is negligible and does not significantly affect the reliability of ¹⁵N as an N tracer in nutritional studies with ruminants.