Earthworm activity affecting organic matter, aggregation and microbial activity in soils restored after opencast mining for coal

Earthworms were introduced into physically degraded soils restored after opencast coal mining. Their effects on soil organic matter and associated soil aggregation were then measured after a period of 9 yr. Earthworm inputs increased stable aggregation and resulted in a higher proportion of the soil organic matter as carbohydrates. Although the total amount of organic matter in the top 15 cm was unaffected by the presence of earthworms, there was some redistribution to depth. Earthworms also caused an increase in the carbon content of the clay-sized fraction. Aggregation was closely correlated with organic indices, particularly total carbohydrates. There was also evidence of organic matter having greater efficiency in stabilising aggregates where earthworms were abundant. Earthworm activity increased soil microbial biomass near the surface but caused a decrease at depth. Metabolic quotient was lower near the surface but higher at depth in soils with earthworms; mineralisation constants were lower in soils with earthworms at both sampling depths. Overall, the results emphasise the important influence of earthworm activity on aggregate and organic matter stabilisation, processes which are closely linked.