Porosity and hydrological changes in surface mine soils

Soils replaced after mining are subject to various treatments prior to revegetation and in the years following planting to promote their rehabilitation. Despite these treatments, compaction (bulk densities > 1.5 g cm-3) and a complete loss of macro-pore systems are typical features of recently replaced substrates, whether these are topsoils or soil forming materials. The highly unstable nature of these substrates limits the effectiveness of mechanical treatments aimed at recreating these macro-pore systems. As a consequence, replaced soils shed rainfall with important implications for their ability to supply plants with water, for land management and for local hydrology. Over time, soil porosity and hydrologic characteristics change and the management of this process will largely determine whether there is a sustainable outcome to the land restoration process. Results from studies into long-term changes in soil pore and moisture characteristics on land restored after surface mining in the UK will be presented. Recolonisation of soils by earthworms is a key factor in developing a new macro-pore system but in its early stages this can lead to waterlogging of surface soils and reduced surface bearing strength. During this transition phase the soils are particularly susceptible to damage by treading or wheel pressures and careless management can lead to a cycle of improvement and degradation. The importance of ensuring a progressive transition from surface runoff to infiltration and through soil drainage, and the problems of achieving this objective, are emphasised. Implications for land management (both agricultural and woodland) during this transition period, and the need for informed long-term management, are discussed.