Priming effects of Aporrectodea caliginosa on young rhizodeposits and old soil organic matter following wheat straw addition

• Aporrectodea caliginosa caused the strongest true positive priming effect on old SOM.
• N-rich rhizodeposits have a higher palatability than old SOM.
• Rhizodeposits were preferentially anabolized to biomass and not catabolized to CO2.
• Straw addition increased the assimilation of rhizodeposits by A. caliginosa.
• Straw addition caused a small negative true priming effect on young rhizodeposits.

Previous work has shown that endogeic earthworms cause different, i.e. apparent as well as true positive priming effects, presumably due to unknown interactions of substrate-colonising fungi and labile SOM, e.g. rhizodeposits. To explore these interactions, a soil that had previously been enriched with 13C- and 15N-labelled rhizodeposits of pea (Pisum sativum L.) plants was used in an incubation experiment. The objective was to determine whether Aporrectodea caliginosa causes a priming effect on the decomposition of young rhizodeposits and old soil organic matter (SOM) following wheat straw addition. After 56 days of incubation at 12 °C, earthworm effects on autochthonous SOM-derived CO2 (+88%) were higher than on rhizodeposit-derived CO2 (+16%), indicating a stronger true positive priming effect on old SOM than on young rhizodeposits. Feeding of A. caliginosa significantly reduced microbial biomass C (−12%) and N (−30%) derived from rhizodeposits. In contrast, SOM-derived microbial biomass C and N remained unaffected, indicating a higher palatability of rhizodeposits. However, they were not catabolized to CO2, but preferentially anabolized, i.e. transferred to the biomass of microorganisms and earthworms. Not only straw but also A. caliginosa generally caused a shift in the microbial community towards saprotrophic fungi, as indicated by increased ergosterol contents and ergosterol to microbial biomass C ratios. A. caliginosa decreased δ15N, total N, and N derived from rhizodeposits in the non-decomposed straw recovered as particulate organic matter, indicating the importance of rhizodeposits as an N source.