Abundance of Ammonia Oxidizing Bacteria and Archaea Under Long-Term Maize Cropping Systems.

Nitrification involves the oxidation of ammonium and is an important component of the overall N cycle. Nitrification occurs in two steps; first by oxidizing ammonium to nitrite, and then to nitrate. The first step is often the rate limiting step. Until recently ammonia-oxidizing bacteria were thought to be the sole contributors to this process; however, the discovery of crenarchaeota, ammonia-oxidizing archaea, in marine environments has led to further study of their role in nitrification. Current literature supports the dominance of archael over bacterial ammonia oxidizers in terrestrial ecosystems; however, little is known about what drives their abundance. To investigate the role of cropping system management on nitrifier abundance we sampled long-term continuous maize (25+ years) under two tillage treatments (tillage and no tillage) and five N fertilizer rates (0, 40, 80, 120, 160 kg ha^-1 yr^-1). Samples were collected four times during 2012; before fertilization, 2 weeks after fertilization and pre- and post- harvest. Quantitative PCR was used to determine the abundance of ammonia-oxidizing bacteria and archaea. In agreement with current literature, it was found that ammonia oxidizing archaea greatly outnumber bacteria. The low abundance of ammonia oxidizing bacteria in this long-term fertilized plot may be attributed to niche differentiation between archaea and bacteria, as it was found that bacterial abundance rather than archaeal abundance respond to fertilizer rates and tillage treatments in a monoculture system.This may have repercussions for N transformation important to plant N acquisition, carbon storage and greenhouse gas emissions.