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BGIs are a driving force for the establishment of microbial communities: Influence of plant litter and soil composition on the establishment and structure of bacterial communities

Due to its heterogeneity soil is one of the most complex ecosystems where microorganisms can live. It consists of a multitude of microhabitats and biogeochemical interfaces (BGIs) that strongly influence the microbial ecology. In this experiment, we investigated the effect of plant litter addition and soil composition on the colonization and development of bacterial communities in different artificial soils and a natural soil. The four artificial soils consisted of quartz sand, silt and (i) montmorillonite, (ii) illite, (iii) montmorillonite and charcoal or (iv) illite and ferrihydrite. All artificial soils were inoculated with microorganisms extracted from a Luvisol and sterile manure and matured for two years to allow BGI formation (Pronk et al., 2012). The aged artificial soils and one natural Luvisol were mixed with equal amounts of 15N and 13C labelled maize and potato litter and incubated in the dark up to 63 days at constant temperature of 14°C and soil moisture of 60 % mWHC. Controls without litter were treated in the same way. Soil samples were taken 0, 7, 21 and 63 days after the plant litter addition. Bacterial community abundance and structure in soils with different composition were analysed on DNA and mRNA level using quantitative PCR (qPCR) and terminal restriction fragment length polymorphism analysis (T-RFLP). The 16S rRNA gene copy numbers were similar in natural and artificial soils. Despite the litter addition an increase in 16S rRNA gene copy numbers in natural soil was not detected. In comparison to 16S rRNA the 16S cDNA copy number values were significantly higher in all soils. In all artificial soils incubated with plant litter transcript values were higher than those in control soils. The increase of transcript values followed immediately after the litter addition, whereas in the natural soil the increase was delayed for 7 days. This implies that the easily available nutrients have a stronger influence on the activity of bacterial communities than the BGIs. T-RFLP analysis of 16S rRNA gene suggests that the bacterial community structure is dependent on soil composition as well as on time and litter addition. In contrast, T-RFLP of 16S cDNA revealed a different active community in the Luvisol compared to the artificial soils, while no significant difference was observed among the artificial soils. Taken together the data indicate that the mineral composition of artificial soils has an impact on the overall bacterial community composition but not on the active community. Moreover, the positive effect of released nutrients is delayed in the Luvisol where already complex BGIs exist, which might have reacted with nutrient particles.

Soil composition ; plant litter ; artificial soils ; bacterial diversity ; qPCR ; T-RFLP

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