Livestock slurry is an important source of methane (CH4). Depending on dry matter content, a floating crust may form where methane-oxidizing bacteria (MOB) and CH4 oxidation activity have been found, suggesting that surface crusts may reduce CH4 emissions from slurry. However, it is not known how MOB in this environment interact with inorganic nitrogen (N). We studied inhibitory effects of ammonium (NH4+), nitrate (NO3–) and nitrite (NO2–) on potential CH4 oxidation in a cattle slurry surface crust. Methane oxidation was assayed at salt concentrations up to 500 mM at 100 and 10,000 ppmv headspace CH4. First-order rate constants were used to evaluate the strength of inhibition. Nitrite was the most potent inhibitor, reducing methanotrophic activity by up to 70% at only 1 mM NO2–. MOB were least sensitive to NO3–, tolerating up to 30 mM NO3– at 100 ppmv CH4 and 50 mM NO3– at 10,000 ppmv CH4 without any decline in activity. The inhibition by NH4+ increased progressively, and no range of tolerance was observed. Methane concentrations of 10,000 ppmv resulted in 50- to 100-fold higher specific CH4 uptake rates than 100 ppmv CH4, but did not change the inhibition patterns of N salts. MOB in slurry surface crusts maintained activity at higher concentrations of NH4+ and NO3– than reported for MOB in soils and sediments, possibly showing adaptation to high N concentrations in the slurry environment. Yet, it appears that the effectiveness of surface crusts as CH4 sinks will depend on inorganic N concentrations.
Journal of Environmental Quality, 2013, Vol 42, Issue 2, p. 507-515