Hotspots of microbial activity induced by earthworm burrows, old root channels, and their combination in subsoil

Abstract

Biopores are pores or voids in soil produced by roots, by earthworms, or by the occupation of earthworms in root pores, which are considered important microbial hotspots, especially in subsoil. We hypothesized that earthworms (Lumbricus terrestris L.) exert stronger effects on microbial activities than decaying plant roots (of Cichorium intybus L.) in the subsoil because of the addition of pre-digested organic material. We tested this hypothesis by analyzing microbial biomass (C-mic), total organic C (C-org), and activities of eight enzymes (cellobiohydrolase, beta-glucosidase, xylanase, acid phosphomonoesterase, leucine aminopeptidase, tyrosine aminopeptidase, chitotriosidase, and n-acetylglucosaminidase) down to 105-cm depth. The C-mic increase was associated with a two- to threefold increase of C-org content in biopores as compared to bulk soil. The highest percentage of C-mic-to-C-org (3.7 to 7.3 %) in the drilosphere demonstrated the enhancement of microbial efficiency for organic matter decomposition by earthworms. The availability of organic matter in biopores increased the activities of C- and N-targeting enzymes by 1.2-11.3 times, but reduced acid phosphomonoesterase activity by 10-40 % in biopores versus bulk soil. Introducing earthworms in root biopores caused 1.5-1.8 times higher microbial biomass and 1.2-1.9 times increased enzyme activities compared to the sole effect of earthworms. Soil depth showed a strong effect on the drilosphere, but only slight effects on the biochemical properties of root biopores and bulk soil. In conclusion, biopores are important microbial hotspots of C, N, and P transformations in subsoil. Earthworms exerted stronger effects on biochemical properties of biopores than decaying roots.