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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). This process is recognized to be large enough to be taken into account into the ecosystem carbon balance. Yet, the exact mechanisms of PE are still unknown. Compounds differ with respect to the amount of energy that can be used for microbial growth. Here, we hypothesized that addition of energy-rich compounds to soils will stimulate microbial growth and consequently PE. We also tested the competing hypothesis that the addition of energy-poor, complex compounds will stimulate certain groups of microorganisms that are specialized in degrading recalcitrant compounds, thereby co-metabolizing SOM fractions and stimulating PE. Our last hypothesis was that addition of mineral N will negatively affect PE by preventing microorganism to mine for N in SOM. Therefore, we compared the effect of adding energy-rich, simple compounds to adding energy-poor, complex compounds to a grassland soil on PE, in combination with or without addition of ammonium nitrate. Three isotope-labelled carbon substrates (glucose, cellobiose and vanillic acid), which differ in substrate complexity and the energy they provide for microbial growth were added to soil using a mesocosm approach. To test the effect of energy content versus the effect of complexity of added compounds on PE, the amount of carbon and the amount of energy content of added compounds was kept constant among treatments, respectively. We observed that the effect of substrate complexity was much larger than the effect of the energy-level of added compounds on PE. The most complex compound, vanilic acid, induced the highest CO2 respiration and PE. Overall, stable isotope probing (DNA-SIP) revealed that the microbial community responded quickly in soils amended with glucose and cellobiose, with an increase in DNA copies numbers in both heavy and light fractions after 5 hours and 3 days of incubation and a subsequent drop after 13 days of incubation. In treatments where the energy-level of added compounds was kept constant, vanillic acid caused an increase in DNA copies number in the light fraction for both bacteria and fungi soils after 13 days of incubation. The contribution of fungi to cause PE was quite low compared to bacteria, reflecting the low F:B ratio of the grassland soil used for the experiment. β-proteobacteria, Sphingobacteriia, and γ-proteobacteria responded quickly (5 hours) after the addition of the substrates and Acidobacteria, α-proteobacteria were dominantly present in both heavy and light fractions. After 3 and 13 days these differences disappeared and the bacterial community (in the heavy as well as in the light fractions) was dominated by Acidobacteria, Actinobacteria and α-proteobacteria. Different substrates did not stimulate the growth of different bacterial groups, although we found differences in PE among the treatments. Discrepancy in PE can thus not be explained by the stimulation of different microbial groups, but it appears to be due to stimulation of different enzyme systems within similar phylogenetic bacterial communities, thereby possibly co-metabolizing SOM fractions. Overall, addition of nitrogen caused a slight increase in PE. This may indicate that addition N and C to soils in our study better matched the N demands for microbial growth and PE, confirming the stoichiometric decomposition theory and rejecting the mining for N theory. |