| Landfills are large sources of CH4, but a considerable amount of CH4 can be removed in situ by methanotrophs if their activity can be stimulated through the addition of nitrogen. Nitrogen can, however, lead to increased N2O production. To examine the effects of nitrogen and a selective inhibitor on CH4 oxidation and N2O production in situ, 0.5 M of NH4Cl and 0.25 M of KNO3, with and without 0.01% (w/v) phenylacetylene, were applied to test plots at a landfill in Kalamazoo, MI from 2007 November to 2009 July. The addition of NH4+ and NO3 increased N2O production, but had no effect on CH4 concentrations. The simultaneous addition of phenylacetylene reduced N2O production and enhanced CH4 oxidation. PCR analyses showed that methanotrophs, especially those possessing particulate methane monooxygenase, were more abundant than those possessing soluble methane monooxygenase, and, interestingly, archaeal ammonia-oxidizers were more abundant than their bacterial counterpart. Microarray analyses showed NH4+ and NO3 caused the overall methanotrophic diversity to decrease, with a significant reduction in the presence of Type I methanotrophs. The simultaneous addition of phenylacetylene caused methanotrophic diversity to increase, with greater presence of Type I methanotrophs. Also, archaeal amoA gene clone libraries were constructed to examine the long-term effects of the amendment on the AOA community structure. Clone libraries showed that the addition of NH 4+ and NO3 increased the presence of Group 1.1b archaeal ammonia-oxidizers, while their presence decreased with the simultaneous addition of phenylacetylene. Several methanotrophs were investigated to examine the relative importance of methanotrophic mediated N2O production. Five out of six Type II methanotrophic strains produced 32--342 ppmv of N20, while two Type I strains did not produce detectable amount of N 2O. Collectively, these results suggest that the addition of phenylacetylene with NH4+ and NO3 reduces N2O production by selectively inhibiting archaeal ammonia-oxidizers and/or Type II methanotrophs, but it is currently unknown what the magnitude of N 2O production might be from archaeal ammonia-oxidizers. Once the major contributors on N2O production are identified, we may be able to come up with a better strategy to mitigate in situ GHG emissions from a landfill. |
