Abstract

Earthworms were sampled after two and eleven years following experimental addition of organic matter to a semi-restored colliery spoil site at Chisnall Hall in Lancashire. Organic matter (OM) in the form of anaerobic digestate (AD) and/or organic growth medium (OGM) was incorporated into 200 m2 plots on site using a mechanical digger. Six treatments, including a control with no amendment were replicated 3 times. A quarter of each plot was planted with ash, cherry, willow or reed canary grass. After 2 years, results showed that addition of OM gave rise to significantly higher community densities of earthworms with the greatest (638 m-2) in a high (1875 t ha-1) digestate application treatment, compared with 192 m-2 in the unamended control (p<0.05). Of the nine species found, Allolobophora chlorotica and Aporrectodea caliginosa, both shallow-working, and Aporrectodea longa, a deep burrowing earthworm, contributed to greatest numbers. Eleven years after OM addition, the density of earthworms had fallen significantly in most of the OM-amended plots (maximum of 428 m-2 in high anaerobic digestate) with no change in the control. Overall, earthworm community biomasses had also fallen but not significantly. Species composition on site had changed during the intervening period with less epigeic species present (e.g., Bimastos rubidus no longer present after eleven years) and a change in dominance of endogeic species to A. rosea. Tree-related species, e.g., Dendrobaena octaedra were also found more recently. Experimental plant species used did not affect earthworm abundance or biomass. Addition of organic matter to a colliery spoil site initially enhanced earthworm community density, most likely through a combination of immigration from surrounding areas and through increased reproduction associated with the abundance of food. After eleven years, the reduction in earthworm numbers was most likely a function of nutrient depletion.