The transport of mineral and organic matter into the soil profile by Lumbricus rubellus Hoffmeister

The biology and ecology of the earthworm Lumbricus rubellus Hoffmeister, 1843, and its effects on the turn-over of organic matter and soil are not well known. To gather this information, the ingestion and egestion rates were measured using a litterbag technique and the transport of organic matter was quantified with a newly developed method, using soil columns to which ¹⁴C labelled plant material was added. The feeding habits of the worm were positively influenced by temperature in wet soils (> -15m of water) and were negatively influenced in dry soil (< -15 m of water). The total egestion rate changed from 0.3 g.g⁻¹.day⁻¹ at 5 °C to 1.0 g.g⁻¹. day⁻¹ at 20° C in moist soil (- 5 m of water). The egestion rate at medium range temperatures, 10 and 15° C, was less affected by drought stress than at 5 and 20 °C. The egestion rate of carbon was a more stable parameter than the total egestion rate, and ranged from approximately 20 mg.g⁻¹.day⁻¹ at 5 °C, to 50 mg.g⁻¹.day⁻¹ at 20 °C. The moisture and temperature effects were apparent in the Q₁₀ of the total egestion rate and of the egestion rate of carbon. The Q₁₀ ranged from 1.66 in wet soils to 3.27 in dry soils in the 5-15 °C interval and from 1.98 to 0.32 in the 10-20 °C range. For the egestion rate of carbon, the Q₁₀ ranged from 1.92 to 3.21 and from 1.28 to 0.47, respectively. The body water content of the worm varied considerably with the soil water potential, and reached a maximum level of 5.5 kg.kg⁻¹ (dwt) between -15 metres of water and -30 metres of water. When under drought stress, worms stopped ingesting large quantities of soil, switched to a diet high in organic matter and lowered their activity. In the ¹⁴C column experiment, the total cast production was significantly related to depth. L. rubellus produced 15 % of the cast on the surface of the soil, 46 % in the 0-5 cm layer, 22 % in the 5-10 cm layer and 16 % in the 10-15 cm layer. Independent calculations from a) the uptake of ¹⁴C labelled carbon in earthworms, b) removal of litter from the surface and c) ¹⁴C label recovered from cast, showed that the worms ingested 78-82 % of the offered organic matter as shoot litter and 18-22 % as root litter. ¹⁴C originating from shoot and root litter was recovered in casts throughout the profile, indicating that the worms mixed food from all layers. The total egestion rate found in the column experiment was 5.2 times higher than was found in the litterbag technique under comparable conditions (2.34 vs 0.45 g.g⁻¹.day⁻¹). The egestion rate of carbon was similar in both techniques (37.1 vs. 46.1 mg.g⁻¹.day⁻¹, 10 °C). In preliminary litterbag trials, it was found that L. rubellus egested 15.5 mg.g⁻¹.day⁻¹ of carbon (5 °C) for each of four food types offered. The 5 °C temperature trial of the litterbag technique, showed a similar amount of carbon egested. It was concluded that the worm needed a constant amount of carbon to provide nutrients and energy, of which a part or all may originate from ingested microorganisms. Based on the distribution of cast in the profile and the feeding strategies of L. rubellus, it was concluded that this earthworm cannot be classified as an epigeic worm. A new strategy class was proposed: eurygeic worms, earthworms living in the litter-soil interface, mixing organic matter into the profile and mineral soil into the litter layer. Based on the literature and results from the present study, a computer model was developed to simulate the longterm effects of earthworms on an agricultural soil system. Simulations of the mixing of soil and organic matter in a limited-till agricultural system, showed that earthworms negatively affected the accumulation rate of surface litter and positively affected the organic matter content of the mineral soil. The model can be used to predict the trends in organic matter in soils, important in soil conservation, mine reclamation and reforestation.Land and Food Systems, Faculty ofGraduat