Impact of incorporated fresh 13C potato tissues on the bacterial and fungal community composition of soil

Soil microbial communities play a major role in organic matter decomposition, however the importance of the individual species involved is still unclear. To identify the dynamics and identity of bacterial species involved in decomposition of potato tissue as well as the assimilation of carbon from fresh plant material, 13C-labeled green potato tissues (13C 99.2%) were incorporated in soil microcosm for 39 days at the level of 2.5% (w/dry weight soil). The DNA was extracted from the soil after 1, 6, 15, 25 and 39 days. The heavy (13C) and light (12C) fractions of DNA were separated by ultracentrifugation and the structures of the bacterial and fungal communities were characterized by DGGE. Primary and secondary 13C-sequestrators were identified by sequencing DGGE bands that had appeared only in the heavy DNA fractions. Over the course of the experiment, the most dominant 13C-labeled phylogenetic group (class or phylum) was γ-Proteobacteria (51.4%), followed by Actinobacteria (27%), β-Proteobacteria (8.1%) and α-Proteobacteria (5.4%). Two taxa, namely Firmicutes and Verrucomicrobia, were represented by just one sequence type. These bacterial taxa were differentiated into primary (Arthrobacter, Pseudomonas) and secondary sequestrators (Actinobacteria, Dyella, Mesorhizobium and Sphingomonas). The latter were possibly involved in either the redistribution of previously consumed carbon or in a possible degradation of the more complex plant compounds. On the basis of this analysis, only 5 to 8 bacterial taxa were involved in carbon sequestration at any one measured time point. Our results show the importance of specific microbial taxa in the decomposition and mineralization of plant residues in soil, which will allow us to better understand the role of such communities in carbon cycling.