HUMIFICATION PROCESSES IN RECLAIMED OPEN-CAST LIGNITE MINE SOILS

To identify the principal humification pathways which occur during the initial stages of pedogenesis, organic matter from mine soils of diff erent ages from the Meirama lignite mine in NW Spain was subjected to a series of physicochemical and chemical fractionations. Although humi c molecules of molecular weight lower than 10,000 Da were always predo minant, the percentage of C associated with molecules of between 50,00 0 and 200,000 Da increased with soil age, as did the percentage of che mically stabilized humus and the percentage of C associated with immob ile complexes. In general, our results suggest that the principal humi fication pathway in these soils involves abiotic condensation reaction s, as in the case of natural soils of this region. The low complexed m etal content of the mine soils in comparison with natural soils indica tes the low degree of weathering of inorganic fraction which has occur red in the former.