Ammonia Oxidizers as Biological Health Indicators of Elevated Zn and Cu in Poultry Litter Amended Soil

Ammonia-oxidixing bacteria (AOB) and archaea (AOA) mediate the first and rate-limiting step of nitrification and are responsive to agricultural management practices. These two attributes make them ideal indicators of biological soil health. We conducted a laboratory incubation study to determine their response to elevated levels of zinc (Zn) and copper (Cu) in poultry litter treated soil at three substrate concentrations: 0 (low), 50 (medium) and 100 (high) mg ammonium (NH4+-N) kg(-1) soil. Nitrification potential (NP) was measured to characterize changes in their function in which 1-octyne was used to separate their contributions. Quantitative polymerase chain reaction was used to measure their abundance by targeting amoA. Increasing Zn from 21 to 250 mg kg-1 resulted in large reductions in AOB (78%) and AOA (85%) abundance at the high NH4+ level over 28 days. Likewise, increasing Cu from 20 to 120 mg kg(-1) significantly reduced AOB (92%) and AOA (63%) abundance at the high NHthorn 4 level over 28 days. The relative contribution of AOB to NP was significantly higher than that of AOA in both Zn (similar to 60%) and Cu (similar to 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 NH4+. The results also indicated that AOB are functionally more important than AOA under elevated Zn and Cu concentrations and that management practices to improve N use efficiency should focus on AOB under this condition.