Priming of soil organic matter: Chemical structure of added compounds is more important than the energy content

The addition of easily degradable compounds to soil (e.g. root exudates, plant residues) can result in priming effects (PE), a short-term change in the turnover of soil organic matter (SOM). Although PE are recognized to be large enough to be taken into account into the ecosystem carbon balance, the exact mechanisms are still unknown. Here, we examined the effect of two characteristics of added compounds on PE, namely metabolic usable energy for microbes and resemblance to recalcitrant fractions of soil organic matter (SOM). For this purpose, glucose, cellobiose (energy rich compounds, low resemblance with recalcitrant SOM) and vanillic acid (energy-poor compound, higher resemblance with recalcitrant SOM) were selected. In addition the effect of mineral nitrogen (N) on PE was tested. 13C labelled compounds were mixed with sandy soil from an ex-arable site. To separate the effect of energy content from that of resemblance to SOM, the amount of carbon and the amount of energy content of added compounds was kept constant in treatments, respectively. The community structure of microbes that were able to use added compounds was evaluated using stable isotope probing (DNA-SIP) combined with qPCR and Illumina sequencing. When corrected for energy content, vanilic acid induced the highest CO2 respiration and PE. DNA-SIP revealed that bacterial classes like β- and γ-Proteobacteria, that are known to harbour many opportunistic bacteria, responded quickly (5 h) with incorporation of 13C from added substrates, whereas classes like Acidobacteria and Actinobacteria responded over a longer incubation time. In treatments where the energy-level of added compounds was kept constant, vanillic acid caused an increase in DNA copy numbers of bacteria and fungi using native SOM after prolonged incubation. The contribution of fungi to PE was minor, reflecting the low F:B ratio of the soil used for the experiment. Different substrates resulted in different PE but appeared to stimulate the growth of similar bacterial groups. This suggests that the added compounds stimulate different enzyme systems within similar bacterial taxa. Although combined addition of mineral nitrogen (ammonium nitrate) and organic compounds caused a slightly extra increase in PE in most treatments, this might be an artefact as addition of mineral N only decreased respiration. Overall our results indicate that the effect of chemical structure of added compounds on PE is much larger than the effect of energy-content.