Changes in microbial biomass C, N, and P and enzyme activities in soil incubated with the earthworms Metaphire guillelmi or Eisenia fetida

Earthworms are ubiquitous in soil and ingest large amounts of soil, organic matter and leaf litter. To assess changes in organic matter fractions after passage through the earthworm gut, we measured microbial biomass C, N and P and the fungal-to-bacterial ratios in worm-worked soil (WWS), obtained by incubating soil for 24 h with large numbers of the anecic earthworm Metaphire guillelmi (1.5 ratio of fresh weight worms: dry weight soil). Microbial biomass C, N and P were estimated by the fumigation-extraction methods, and fungal-to-bacterial ratios by selective inhibition using substrate induced respiration. Enzyme activities in the gut of M. guillelmi were also compared with an epigeic earthworm species Eisenia fetida. Activities of cellulase, protease, chitinase, acid and alkaline phosphatases, in gut, casts and uningested soil were measured. In WWS, microbial biomass decreased (130 mug C g(-1) soil), and there was a concomitant increase of available nutrients (27 and 10 mug g(-1) soil for ninhydrin-reactive N and inorganic P, respectively). There was no difference between the glucose-sensitive microbial biomass (NIB) and the control but the respiratory quotient was greater (2.85 +/- 0.17 and 2.95 +/- 0.07 mug CO2-C g(-1) soil h(-1) for WWS and control, respectively). The fungal-to-bacterial ratio was slightly higher in WWS than in uningested soil (1.61 vs. 1.35). Cellulase activity was greater in the gut of the epigeic earthworm than in that of the anecic one (152.8 +/- 18.7 vs. 18.9 +/- 1.3 mug glucose g(-1) worm fw h(-1)); conversely, protease and phosphatase activities were significantly higher in gut of the anecic specie as opposed to the epigeic species. The activity of cellulolytic enzymes was slightly higher in casts than in soil; while activities of protease, acid (pH 6.5) and alkaline (pH 9.0) phosphatases were lower in earthworm casts than in the uningested soils (protease, acid and alkaline phosphatase activity were 29.5 +/- 1.7 mg tyrosine g(-1) worm fw h(-1), 570 +/- 2.9, 748 +/- 7.3 mug p-nitrophenol g(-1) soil h(-1) in soil and 17.8 +/- 2.0 mg tyrosine g(-1) worm fw h(-1), 327 +/- 26.7, 549 +/- 19.7 mug p-nitrophenol g(-1) soil h(-1) in casts, respectively). We conclude that micro-organisms are used by earthworms as a secondary food resource; and that passage through earthworm gut decreases the total soil MB and increase the active components of MB. (C) 2000 Elsevier Science Ltd. All rights reserved.