Ammonia oxidizing bacteria community structure and richness under coniferous/deciduous tree species at three temperate forest sites

Ammonia Oxidizing Bacteria community structure and richness under coniferous/deciduous tree species at three temperate forest sites
Malchair S. and Carnol M.
Laboratory of Plant and Microbial Ecology
Department Biology, Ecology and Evolution
University of Liege, Belgium
Introduction:
Despite the crucial roles of soil microorganisms in ecosystem processes, numerous uncertainties subsist on the relationship between soil microbial diversity and function. Furthermore, the link between aboveground and belowground diversity remains also unclear. In European forests, at the beginning of the 19th century, plantations of Norway spruce and Scots pine for timber production were substantially increased. However, concerns were expressed with regard to the ecological risks posed by coniferous monocultures. The conversion of these monocultures into broadleaved or mixed stands has been suggested as a solution. Nevertheless, belowground effects of such a change in the dominant tree species is largely unknown, although bacteria regulate many soil processes and some groups, like ammonia oxidizing bacteria (AOB) are highly sensitive to environmental stresses.
Objectives:
The aims of this study were to investigate (i) AOB community structure and richness under several tree species, (ii) microbial/environmental factors influencing AOB diversity, (iii) the relationship between AOB diversity and the nitrification process.
Materials and methods:
Forest floor (Of, Oh) was sampled under Norway spruce, Douglas fir, European beech and sessile oak at three temperate forest sites. AOB diversity (community structure and richness) was assessed by PCR-DGGE and sequencing. Samples were analysed for microbial (net N mineralization, potential nitrification, basal respiration, microbial biomass, microbial or metabolic quotient) and environmental parameters (pH, total nitrogen, extractable ammonium, organic matter content and exchangeable cations).
Results:
AOB sequences retrieved in this study were related to different uncultured strains from soils, related to both Nitrosospira-like and Nitrosomonas-like sequences. AOB community structure and tree species effects on AOB diversity were site-specific. AOB community structure was influenced by environmental/microbial parameters, i.e. net N mineralization or organic matter content, regulating ammonium availability. AOB richness was not related to nitrification but a very weak correlation between potential nitrification and AOB community structure was observed.
Conclusions:
At larger spatial scales, site specific characteristics may be more important that tree species in determining AOB richness and community structure. However, within sites, tree species influence AOB diversity. The absence of a clear relationship between AOB diversity and nitrification points to a possibly role of AOB abundance, phenotypic plasticity or the implication of ammonia oxidizing archaea in this process.