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Nitrification involves the oxidation of ammonium (NH4+) 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 (AOB) were thought to be the sole contributors to this process; however, the discovery of crenarchaeota, ammonia-oxidizing archaea (AOA), in marine environments has led to further study of their role in nitrification. Current literature supports the dominance of AOA over AOB in terrestrial ecosystems; however, little is known about what drives their abundance. To investigate the role of cropping system management on soil 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 three times during 2012; Pre-plant (5/1/12), after planting (5/14/12) and mid-season (7/6/12). Results show that AOA greatly outnumber AOB. The low AOB abundance may be attributed to niche differentiation between archaea and bacteria, as it was found that AOB were less resilient to N rate and tillage than AOA in monoculture maize. Little, if any, literature has examined soil structure as a possible niche. In previous studies, AOA have been shown to be more resilient to environmental conditions, than AOB. AOB abundance and community structure have been shown to be influenced by tillage, N-rate, and possibly plant growth; they also exhibited spatial heterogeneity. Given the different microenvironments in aggregate size fractions and AOA resistance to environmental conditions and habitat modification a second experiment examined the role of aggregate size fractions on AOB and AOA abundance. AOB were found to respond to soil depth and differed in concentration among aggregate size fractions although N rate still did not influence their abundance. AOA on the other hand was overall unresponsive to soil depth, N rate or aggregate size fraction; however, an N rate by depth interaction affected AOA abundance. Understanding the factors affecting AOB and AOA abundance are important to determine better soil management practices.
Advisor: Rhae Drijber