Effects of nitrogen addition on soil microbial diversity and methane cycling capacity depend on drainage conditions in a pine forest soil

Two forested study sites, one well and one poorly drained, were used for investigation of the effects of variation in drainage, microclimate, and addition of inorganic nitrogen (N) on the whole soil microbial community and its methane cycling capacity. Both sites were capable of consuming and releasing large quantities of methane. The composition of the soil microbial community was investigated using the 3rd generation PhyloChip, a bacterial and archaeal 16S rRNA gene microarray. The PhyloChip was also used to target the composition of methane- and some N-cycling microorganisms. Relative abundance of functional genes involved in methane production and consumption was evaluated with qPCR. rainage condition determined the microbial community structure within and between sites. Greater community structure variation, richness of methanotrophs, and higher abundances of both methanotrophs and methanogens were all found in the poorly drained site, as was higher soil moisture and C content and methane release. In the poorly drained site, high N (67 kg NH4NO3 ha−1 yr−1) increased methanotroph and methanogen abundance, overall taxonomic richness of Bacteria and Archaea, and richness of nitrifiers and methanotrophs. In the well drained site, high N decreased taxonomic richness. Results may indicate that high N concentrations stimulated oxidative reactions, including ammonia and methane oxidation and nitrification in the short term. The resultant increase in release of methane from the high N plots of the poorly-drained site may have been due to indirect inhibition of methane oxidation by the increase in other oxidative reactions. Alternatively, both methanogens and methanotrophs may have been stimulated by high N. Well-drained site high N decreased the taxonomic richness of the soil, but did not impact methane-cycling microbes. These findings begin to bridge the gap between microbial-scale community dynamics and ecosystem-scale ecological functions.