The abundance of microbial nitrogen assimilation genes within metagenome.

Genes from families associated with nitrogen fixation (nifD, nif H, nifK), urea assimilation (ureA–G), nitrate reduction (nar, nir) and ammonium utilization (glnA) were identified and enumerated within each metagenome using the COG database and phylogenetically classified using RefSeq. Results represent the total number of hits to functional categories (COG database) within each library and are normalized for library size.