Seasonal effect and anthropogenic impact on the composition of the active bacterial community in Mediterranean orchard soil.

Several anthropogenic interventions, common in agriculture, may influence active bacterial communities in soil without affecting their total composition. Therefore, the composition of an active bacterial community in soil may reflect its relation to biogeochemical processes. This issue was addressed during two consecutive years in olive-orchard soil, irrigated with treated wastewater (TWW) in a Mediterranean climate, by following the active (rRNA) and total (rRNA gene) bacterial community in the soil. Although TWW irrigation did not affect the composition of the total soil bacterial community, it had an effect on the active fraction of the community. These results, based on 16S rRNA data, indicate that the organic matter and minerals in TWW were not directly utilized for the rapid proliferation of specific taxonomic groups. Activity levels, manifested by variance in the relative abundance of the active and total communities of selected operational taxonomic units, revealed annual and seasonal fluctuations and fluctuations dependent on the type of irrigation. The potential activity (nitrification rates) and community composition of ammonia-oxidizing bacteria were affected by TWW irrigation, and this group of bacteria was therefore further explored. It was concluded that irrigation with TWW had little effect on "who is there", i.e. which bacteria were present, but influenced "who is active", with a distinct effect on bacteria associated with the biochemical cycling of nitrogen.