Lumbricus terrestris in a soil core experiment: Effects of nutrient-enrichment processes (NEP) and gut-associated processes (GAP) on the availability of plant nutrients and heavy metals

Over 5 weeks, 2.4 g lettuce dw kg−1 dry soil was applied to a soil core containing Lumbricus terrestris (“Earthworm treatment”, ET). Earthworms grew well (μ = 0.01 d−1) in response to lettuce application. In a “Mixed control” (MC) without earthworms, lettuce was mixed with the soil, while it was left unmixed at the soil surface in an “Unmixed control” (UC). Relative to the MC, L. terrestris decreased soil NO3−N and microbial biomass-N by 25 and 20% respectively. It was estimated that N-immobilization in earthworm biomass balanced these decreases. Dry matter yield of maize and spinach in the worm-worked soil were decreased by 13% and 6%, with a 15 and 19% lower Kjeldahl (Kj)-N content respectively. Gut-associated processes (GAP) were considered responsible for these decreases in soil NO3−N content and plant growth. When comparing the cases directly related to practice, i.e. ET and UC, NO3−N, maize dry weight and spinach Kj-N content were increased by L. terrestris by 18, 9 and 15% respectively. In this case the reported effects are the result of both (physical) nutrient-enrichment processes (NEP) and (biological) GAP. The NEP as well as the GAP appeared to increase the availability of most plant nutrients (inorganic P (Pi), Mg, Ca, Fe and Mn) and heavy metals (Cr and Co). This yielded an overall increase of 8 to 23% and 20 to 39% for plant nutrient and heavy metal availability respectively. Cu availability (+6%) seemed to be increased only by the GAP. Smaller or no differences were found for K, Zn, Cd and Pb. However, the NEP as well as the GAP seemed to decrease Ni-availability.