Preliminary study on the distribution of ammonia oxidizers and their contribution to potential ammonia oxidation in the plant-bed/ditch system of a constructed wetland

Ammonia oxidation-as the rate-limiting step of nitrification-has been found to be performed by both ammonia-oxidizing archaea (AOA) and bacteria (AOB). However, how ammonium content and oxidation-reduction status regulate the distribution of ammonia oxidizers in constructed wetlands and their contribution to potential ammonia oxidation rate are still in dispute. This study aimed to explore the effects of ammonium content and oxidation-reduction status on the abundances of AOA/AOB and examine the contributions of AOA and AOB populations to ammonia oxidation rates in the plant-bed/ditch system of a constructed wetland. Sampling was carried out in the plant-bed/ditch system of the Shijiuyang Constructed Wetland, China. Three plant-bed soil cores were collected using a soil auger and sampled at depths of 0, 20, and 50 cm in 5-cm increments. Five ditch surface sediments (0-5 cm) were collected along the water flow direction. The abundances of AOA and AOB were investigated by quantitative polymerase chain reaction based on amoA genes. The potential ammonia oxidation rate was determined using the chlorate inhibition method. The results showed that AOA outnumbered AOB in the plant-bed surface soil which had lower ammonium content (4.67-7.63 mg kg(-1)), but that AOB outnumbered AOA in the ditch surface sediment which had higher ammonium content (14.0-22.9 mg kg(-1)). Ammonium content was found to be the crucial factor influencing the relative abundances of AOA and AOB in the surface samples of the plant-bed/ditch system. In the deep layers of the plant bed, AOA abundance outnumbered AOB, though much lower oxidation-reduction potential occurred along the water flow direction. Thus, the oxidation-reduction potential may be another factor influencing the distributions of AOA and AOB in the deep layers of the plant bed without significant difference in ammonium content (p < 0.05). Moreover, the potential ammonia oxidation rate was significantly dominated by AOB rather than AOA in the plant-bed/ditch system. The high ammonium content in the ditch sediment likely favored AOB. AOA seemed to persist more readily even under low oxidation-reduction potential in the deep layers of the plant bed. Ammonium content and the oxidation-reduction potential were important parameters influencing the distribution of AOA and AOB in the plant-bed/ditch system of Shijiuyang Constructed Wetland. AOB contributed more to ammonia oxidation than AOA, both in the plant-bed soils (r = 0.592, p = 0.0096) and in the ditch sediments (r = 0.873, p = 0.0002).