Long-term effects of amendment with municipal solid waste compost on the elemental and acidic functional group composition and pH-buffer capacity of soil humic acids

The main objective of this research was the evaluation of cumulative and residual, long-term (9 years) effects of amendment with municipal solid waste compost (MWC) used at rates of 20 and 80 t ha(-1) on the chemical, structural and functional properties of soil humic acids (HAs). For this propose, HAs were isolated from the MWC sample, the unamended control soil and MWC-amended soils by conventional procedures, and analysed for elemental and acidic functional group composition, and by differential scanning potentiometry to determine their pH-buffer capacity and average apparent dissociation constants. With respect to the control soil HA, the MWC-HAs had a greater content of N and smaller contents of O and acidic groups, smaller molecular size and a limited humification degree. In the residual-effect experiment, the application of MWC affected only partially the elemental composition of soil HA, whereas an increase was measured for the acidic functional group contents, especially at the larger amendment rate. In the cumulative-effect experiment, the repeated application of MWC affected markedly both the elemental and acidic functional composition of soil HA, thus suggesting the occurrence of a major incorporation of structural units of MWC-HA into amended soil HAs. Up to six apparent average dissociation constants were calculated for the HAs studied, which corresponded to the various acidic functional groups in HAs. The MWC-amended soil HAs from residual treatments exhibited a buffer capacity and a number of apparent dissociation constants (K-app) greater than those of HAs from the control soil and the cumulative experiment soils. The buffer capacity of HAs from the cumulative treatments increased below pH 7 and decreased above this pH, with respect to that of the control soil HA. No significant differences were observed as a function of the amendment rate used. (C) 2003 Elsevier B.V. All rights reserved.