Response of Ammonia Oxidizers to Elevated Zinc and Copper Levels in Poultry Litter Amended Soils.

It is not clear how trace metals introduced to soil via poultry liter (PL) affect the ammonia oxidizing bacteria (AOB) and archaea (AOA), which play a crucial role in N cycling. We conducted a laboratory incubation study to determine the response of AOB and AOA to elevated levels of zinc (Zn) and copper (Cu). Nitrification potential (NP) and net nitrification were measured to characterize changes in their function in which 1-Octyne was used in the NP assay to separate the contributions of AOA and AOB. Quantitative polymerase chain reaction was used to measure AOB and AOA abundance by targeting their respective amoA genes. The abundance oftotal bacteria and total fungi was also measured to evaluate the effect on broader groups of microorganisms. Increasing Zn levels resulted in large reductions in AOB (78%) and AOA (85%) at the high ammonium level (100 mg kg soil-1) over 28 days. Likewise, increasing Cu significantly reduced AOB (92%) and AOA (63%) abundance at the high ammonium level over 28 days. Similar reductions in abundance of total bacteria and total fungi were seen under these conditions, indicating the broader effects of the metals. Increasing Zn from 21 ppm to 250 ppm and Cu from 20 ppm to 120 ppm decreased net nitrification by 46% and 27% at high ammonium level, respectively. 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, results indicate that elevated levels of Zn and Cu depressed AOB and AOA abundance and function and that their effect was dependent on availability of ammonium. It also indicates that AOB are functionally more important than AOA under elevated Zn and Cu concentrations.