Dynamics of spatial and temporal outflow from a soil column influenced by earthworm activity

Earthworms are known as ecosystem engineers, which influence the chemical and physical properties in their own environment and thereby strongly modify soil processes. Soil structure (soil aggregates and macropores) formed by earthworms during burrowing activity may influence the soil moisture retention and water flow, enhancing infiltration into deep soil layers.

We studied the influence of anecic earthworms (Lumbricus terrestris fed on poplar leaves) on the spatial and temporal variability in water outflow and storage through a soil column. Therefore, we established a cylinder (30cm diameter, 50cm high) with silty loamy soil. At the bottom boundary, 15 fiberglass wicks drain the water from the soil column. With these wicks the water outflow is measured with a spatial and temporal resolution.  After an initial wetting of the soil, irrigation of the soil cylinder was done twice per week with a full cone nozzle, with an intensity of 36 mm/h and a duration of 20 minutes After 17 weeks 10 adult earthworms were added to the column. The research design consists of three phases (i) soil-filled column ( 14  weeks, with a gap of 4 weeks in the middle due to the corona lockdown) (ii) transition phase: initial earthworm activity (3 days) (iii) soil column with earthworm created structure (7 weeks).

After the experiment, the column was excavated carefully by layers of 4cm at a time. All of the earthworms were found back alive in the column. There was evidence of earthworm burrows down to 26 cm depth in the soil column, earthworm created aggregates were seen only in the top few centimeters.

We expected the outflow of water from the soil column to change due to the earthworm activity: on the one hand, the creation of macroaggregates was expected to increase the water retention in the soil, and on the other hand, the macropores were expected to create a stronger spatial variability in outflow and a more rapid reaction of outflow to the irrigation events. 

We observed mainly an earlier and slightly higher peak in the total outflow of the column coinciding with an earlier and higher peak in the spatial variability in the outflow of the wicks.