Earthworms, flooding, and sewage sludge

Earthworms are ecosystem engineers. Their burrowing increases soil water holding capacity,
changes microbial populations, and improves soil nutrient content. In the UK, flooding is
predicted to increase with climate change, and it is important to understand how increased
flooding may affect earthworm populations.
Increases in flooding may also impact soils amended with sewage sludge. The use of sewage
sludge as a soil amendment increases soil organic matter content, but the interaction between
sludge amendment and flooding is not understood. Oxygen concentrations of flooded soils rapidly
decrease as the soil microbial community breaks down soil organic matter. This thesis examined
whether sewage sludge amendment of soil led to faster rates of oxygen depletion in flooded soils,
and how this could affect earthworm populations.
Earthworm populations in pasture and crop fields known to regularly flood were surveyed every
three months, and soil environmental factors measured. Populations subject to the stresses of both
crop growth and flooding were lower than in pasture soils, with both fields dominated by the
species Allolobophora chlorotica. Laboratory experiments investigating oxygen concentration
depletion in flooded soil amended with sludge found little effect of sludge application on oxygen
depletion; an unexpected result likely caused by the recalcitrant nature of sewage sludge.
Experiments examining the oxygen requirements of some earthworm species found that the green
morph of A. chlorotica appeared to aestivate in response to low oxygen conditions: a novel finding
suggesting a mechanism for their survival in regularly flooded areas. Experiments examining how
earthworms, flooding, and sewage sludge affected soil emissions of greenhouse gases found high
emissions of N2O from unflooded soils treated with both earthworms and sewage sludge.
The findings provide insight into earthworm responses to flooding at both the individual and
population level, and increase understanding of interactions between earthworms, flooding, and
sewage sludge.