Root exudate input stimulates peatland recalcitrant DOC decomposition by r-strategic taxa of Gammaproteobacteria and Bacteroidetes

Background In peatlands, decomposition of organic matter is limited by harsh environmental conditions and low decomposability of the plant material. Increased microbial decomposition of organic matter in peatland ecosystems may become an important phenomenon in the near future after the expected shift in plant community composition from Sphagnum to vascular plants due to climate change. Such a change in plant community composition will lead to increased root exudates flux to the soil and stimulation of microbial growth and activity. The aim of our study was to evaluate the effect of root exudates on the decomposition of recalcitrant dissolved organic carbon (DOC) and identify the microorganisms responsible for this process. Results Decomposition of recalcitrant DOC was stimulated by a high levels of 13 C labelled root exudates addition whereas it was suppressed by a low levels of root exudates addition. Recalcitrant DOC decomposition was positively related to the exudate C/N ratio as a result of enhanced “microbial nutrient mining” due to a deepening of microbial nutrient limitation. The early stage of incubation immediately following the exudate addition was characterized by the preferential use of the added compounds at the expense of recalcitrant DOC. At the same time, r-strategic bacteria (identification based on average 16SrRNA gene copy number) belonging to mainly to Gammaproteobacteria and Bacteriodete s phyla relatively increased within the microbial community. At the later stage, this more abundant bacterial community was replaced by a less abundant community composed of bacteria mostly belonging to Alphaproteobacteria and Acidobacteria . The most important taxa with the potential to decompose complex compounds were indentified: Mucilaginibacter ( Bacteriodete s), Burkholderia and Pseudomonas ( Gammaproteobacteria ) among r-strategists and Bryocella and Candidatus Solibacter ( Acidobacteria ) among K-strategists. Conclusions Increased inputs of root exudates, with a higher C/N ratio, may stimulate decomposition of peatland recalcitrant DOC. In this, bacteria were found to be more important than fungi. Our experiment indicates that r-strategic bacteria as well as K-strategists can be important in stimulated decomposing of recalcitrant peatland DOC.