Response of induced perturbation on replicating β-proteobacterial ammonia-oxidizing populations in soil

The short-term response of induced perturbation by 4-ethylphenol on β-proteobacterial ammonia oxidizers (β-AOB) was investigated in two soils with initial differences in community structure. The hypotheses were that short-term effects of a disturbance of the AOB community is best monitored by specifically looking at the active populations and that soils with dissimilar active AOB populations would display different degree of resistance or resilience. Two soils from a previously characterized long-term field study fertilized with manure or sewage sludge was used. Soil microcosms were incubated in the laboratory over 15 days. The substrate-induced ammonia oxidation was measured, and the composition of β-AOB communities was determined by PCR-DGGE of specific β-AOB 16S rRNA gene fragments. Actively replicating members of the β-AOB were distinguished by the use of bromodeoxyuridine (BrdU) immunocapture. This approach demonstrated that only a minor fraction of the total AOB community was active. Exposure to 4-ethylphenol resulted in approximately 90% lowered substrate-induced ammonia oxidation rates in both soils. This activity inhibition was not accompanied by shifts in β-AOB community structure when total β-AOB DNA was studied. By contrast, changes were seen in the DGGE banding pattern of the BrdU-labeled community DNA after 4-ethylphenol addition in the manure-fertilized soil. In the sewage sludge fertilized soil, the banding pattern of the BrdU-labeled β-AOB remained unchanged, but bands were weaker after the disturbance. In conclusion, it was shown that BrdU immunocapture was applicable to detect shifts in community composition among replicating β-AOB populations in soil. However, this was not reflected by the soils' ammonia oxidation capacity to resist to or recover from the induced perturbation suggesting that rapid population shifts may not influence soil functioning in a short-term perspective.