Ammonia oxidizers in a grazing land with a history of poultry litter application
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Poultry litter (PL) is widely applied on grazing lands in Georgia, but it is not clear how it affects the function and community structure of ammonia-oxidizing bacteria (AOB) and archaea (AOA). In 2009, soil samples were collected from plots that received PL for 15 years at 8 Mg ha-1 annually and those plots were resampled in 2013 after 2 years of PL stoppage, with the objective of examining the long-term impacts of PL and its legacy effects on AOB and AOA. In 2009, the abundance of AOB and AOA were significantly higher in PL treated soils (6.98 and 7.06 log copies g soil-1 for AOB and AOA, respectively) than controls plots that received equivalent amount of nitrogen (N) in the form of urea ammonium nitrate (6.39 and 6.53 log copies g soil-1 for AOB and AOA, respectively). In 2013, AOB abundance decreased significantly in response to the discontinuation of PL application. We used dicyandiamide to separate the roles of AOB and AOA in nitrification in PL and CL (no N added) soils. At early stage of incubation, AOA dominated nitrification in both PL and unamended CL, accounting for ≥70% of nitrification potential (NP). With time, AOB contribution to NP grew in PL, accounting for ≥50% at high ammonium level. We designed a laboratory study to examine how AOB and AOA respond to elevated levels of Zinc (Zn) and Copper (Cu). Zn application resulted in greatest decrease in abundance of AOB (78%) and AOA (85%) at the high ammonium level over 28 days. Likewise, Cu application significantly reduced AOB (92%) and AOA (63%) abundance at the high ammonium level over 28 days. The relative contribution of AOB to NP was significantly higher in both Zn (~60%) and Cu (~70%) treated soils despite the numerical dominance of AOA over AOB. Overall, AOB were the dominant players in PL soil. As such, understanding the effects of land application of PL on AOA and AOB communities and their respective roles in nitrification is essential to target the right group to achieve efficient N use.