Lumbricus terrestris in a soil core experiment: Nutrient-enrichment processes (NEP) and gut-associated processes (GAP) and their effect on microbial biomass and microbial activity

For 5 weeks, lettuce was supplied to the surface of three types of soil cores. In the first treatment, Lumbricus terrestris (L.) was added before the lettuce supply. The average specific growth rate of L. terrestris was 0.01 d−1 with an average feeding rate of 47 mg lettuce dw g−1 earthworm dw d−1 (2.1 mg lettuce Kjeldahl(Kj)-N g−1 earthworm dw d−1. In the second treatment, no worms were added, but the lettuce was mixed with the soil. Relative to this “Mixed control”, microbial biomass, nitrifying activity and respiration rate were significantly reduced in the presence of L. terrestris by 20, 25 and 49%, respectively. Proteolytic bacteria were significantly stimulated, but the inverse was noticed for fluorescent pseudomonads. Total plate counts, siderophore-producing bacteria and fungi were similar in both treatments. In the third treatment, no worms were added but the lettuce was left unmixed on the soil surface. The “Earthworm treatment” scored significantly higher for most values relative to this “Unmixed control”. Nutrient-enrichment processes (NEP) associated with the organic matter incorporation were distinguished from gut-associated processes (GAP) associated with the transit of soil and organic matter through the earthworm gut. It was concluded that the NEP rather than the GAP are responsible for the increased numbers of microorganisms reported in the presence of earthworms. On the other hand, the specific metabolic activity (qCO2) of the microbial biomass was decreased by 35% due to the GAP. The lower qCO2 indicated a better energy to biomass conversion and an alteration in the microbial community in favour of a higher proportion of K-strategists.