Organic matter dynamics in a compost-amended anthropogenic landfill capping-soil

The application of municipal waste compost and other organic materials may serve to improve fertility and organic carbon (C) stocks of soils used in land reclamation activities, particularly in the recovery of degraded areas at exhausted quarries, mines and landfill sites amongst others. We investigated long-term organic matter dynamics in such anthropogenic soils by collecting samples at different depths over a 10-year chronosequence subsequent to compost application to the top layer of a landfill capping-soil. Variations in the stable isotope composition (¹³C and ¹⁵N) of the soil samples showed that, even after 10 years, amended topsoils were enriched in compost-derived organic matter. The addition of compost to the superficial layer also resulted in an input of soluble organic compounds that was subject to leaching along the soil profile. Sorption isotherms for compost-derived water-extractable organic matter onto mineral materials used for landfill covering suggest that sorptive preservation was primarily responsible for the increase in C content and the shift in the C isotopic signature to values similar to that of the applied compost, in the deeper soil horizons over the 10-year period. This was also confirmed by the accumulation of lignin-derived phenolic compounds in the deeper horizons. Nevertheless, analysis for non-cellulosic carbohydrates in soil samples and their respective water-extractable fractions suggest that a proportion of the compost-derived, labile organic matter fraction is leached through the profile and potentially lost from the soil system.