Effects of irrigation sources on ammonia-oxidizing bacterial communities in a managed turf-covered aridisol

Ammonia-oxidizing bacteria (AOB) perform the rate-limiting step of nitrification, a key process in the global nitrogen cycle. In this study, chemical factors controlling AOB activity, diversity, and composition in a turfgrass-covered aridisol irrigated with groundwater, Colorado River water, or reclaimed wastewater were examined. Activity of AOB contributed an average of 96% of potential nitrification activity in four soils examined, and this activity correlated positively with ammonium concentration and negatively with salinity of the irrigation water. AOB abundance, as determined by quantitative polymerase chain reaction, also correlated positively with ammonium concentration in the irrigation water but negatively with soil salinity. Characterization of AOB communities by denaturing gradient gel electrophoresis showed the presence in every soil of AOB taxa, most commonly found in high-ammonia environments. The soil with the fewest years of management had the least diverse AOB population, compared to the other three soils, and much lower specific nitrification activity. This soil was irrigated with highly saline Colorado River water, which likely exerted acute negative effects on the activity of AOB. In summary, this study revealed that, although AOB activity and growth responded positively to ammonium availability in irrigation water, the salinity of the water and soil had strong negative effects on these aspects of the AOB community.