Does the addition of leaf litter affect soil respiration in the same way as addition of macrofauna excrements (of Bibio marci Diptera larvae) produced from the same litter?

•Addition of Bibio excrement reduced respiration in soil while litter addition increased it.•Priming effect of litter corresponded with bacterial increase while excrements addition increased fungi.•Comparing grounded litter and excrements indicated that fragmentation is not a major reason for litter excrement differences. The aim of this study was to compare how litter and excrements of Bibio marci larvae affect respiration. We measured changes in microbial respiration over a two-week period to ascertain the effects of added leaf litter and excrements produced from the same litter by larvae of St Mark's fly (B. marci). At the same time, we measured the effect of litter and excrement introduction into soil on soil respiration. We used litter pieces and ground litter to evaluate the effect of fragmentation. Two tree species (alder and oak) and two soil types (with high and low organic carbon content) were used to investigate the interaction between litter quality and soil C content. Bacterial counts were obtained and ergosterol content was measured at the end of the cultivation period. The soil with high organic matter content exhibited higher respiration than the soil with low organic matter content. In the absence of soil, both alder litter and excrements induced higher respiration than oak litter and excrements. For both tree species, litter pieces elicited higher respiration than excrements. Concerning the effect of substrate addition, ground litter of alder induced higher respiration than litter pieces. The opposite applied to oak litter, most likely because of a high content of phenolics in oak litter that were made more available in the ground litter. When the substrates were introduced into soil, litter (both in pieces and ground) increased microbial respiration more than excrements. The increase due to respiration was much higher in the soil with low organic matter content. Bacterial counts increased more in the soil with low organic matter content, while the opposite was the case for ergosterol content. Organic matter mineralization of macrofauna excrements was lower compared to intact litter. This was particularly true when litter or excrements were introduced into soil. A high-pH substrate and biomass with few microbes probably induces a large priming effect in low-carbon soils.