The effect of Lumbricus terrestris on soil in relation to plant growth: effects of nutrient-enrichment processes (NEP) and gut-associated processes (GAP)

In an experiment with lettuce supplied as a C and N source, three types of treatments were carried out. In the first treatment, Lumbricus terrestris was added prior to the lettuce which was supplied on the surface. The average specific growth rate of L. terrestris was 0.01 d-1 for an average feeding rate of 47 mg lettuce dry wt g-1 earthworm dry wt d-1. No worms were added in the second treatment, but the lettuce was manually 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. Similarly, in the worm-worked soil, the NO3(-) concentration was decreased by 25%, while dry matter production and N content of spinach plants were reduced by 6 and 19%, respectively. Analysis of N uptake in plants and initial mineral N concentration in soils revealed that the lower plant growth was caused by the lower availability of N. In contrast with the effect on the NO3(-) concentration, the availability of PO4(3-) (6%) and the elements Mg (5%), Ca (4%), Fe (6%), Mn (12%) and Cu (6%) was increased in the soil in the presence of earthworms. These differences were significant only for Mn and Cu. No differences were found for the elements K and Zn. In the third treatment, no worms were added but the lettuce was left unmixed on the soil surface. Relative to this "unmixed control", the "earthworm treatment" always scored higher values, except for microbial biomass and nitrifying activity, which were unchanged. In the presence of L. terrestris, two processes occur at the same time: the nutrient enrichment processes (NEP) due to the incorporation of organic matter in the soil and the gut-associated processes (GAP) due to the transit of soil and organic matter through the gut of the earthworm. Based on this experimental set-up, it was concluded that the GAP decreased microbial biomass and microbial activity as well as NO3(-) concentration in soil and N content of the crop. On the other hand when the NEP was also taken into account, all of these effects were counteracted, mostly yielding an increase in the presence of L. terrestris.