Effects of mercury on the activity and community composition of soil ammonia oxidizers

Experiments were conducted to examine the effects of mercury (Hg) on soil nitrification activities and the microbial communities of ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA). The soil samples spiked with different Hg concentrations were incubated for a period of 1, 2, 4, and 8 weeks in triplicate and the potential nitrification rate (PNR) of the samples was determined. The abundance of AOB and AOA was measured after an 8-week incubation by real-time polymerase chain reaction (PCR) assay of the amoA genes, while the community compositions by cloning and sequencing approaches. The soil PNR differed with different incubation periods. It tended to decrease with increasing soil Hg concentrations at week 1, basing on which the half-maximal effective concentration (EC50) was 1.59 mg kg(-1). There was no significant difference in the abundance of AOB or AOA among the treatments. The AOB community was dominated by Nitrosospira-like sequences and more than 70% of the obtained clones were affiliated with the cluster 3a.2. The percentage of cluster 3a.1 in AOB community appeared to a consistent trend of decreasing with ascending soil Hg concentrations. While all the AOA sequences in the clone libraries were grouped into cluster S (soil and sediment origin). This study revealed that Hg could inhibit soil potential nitrification and the extent varied with incubation periods. Soil Hg pollution changed the composition of soil AOB to some extent. These findings will be helpful to recognize the effects of Hg on the activity and community composition of soil ammonia oxidizers.