Composition changes of dissolved organic matter in a soil amended with municipal waste compost

A spectroscopic study was carried out to determine the changes in composition of dissolved organic matter (DOM) in a soil repeatedly amended with a municipal waste compost. In addition to humic-like substances, composted products contain dissolved organic matter composed of a complex mixture of relatively low molecular weight polymeric materials. The FT-IR and H-1-NMR spectra of DOM extracted from a clay-loam soil (SDOM), municipal waste compost (CDOM), and soil-compost mixtures incubated for varying time periods are reported. One of the incubated samples was taken from a freshly prepared laboratory mixture (LASDOM); the rest were taken from a 6-year field experiment on the soil collected at four months (6YASDOM) after the last amendment. The analyses were carried out also on a CDOM sample after being kept in contact with the soil for 24 h (BEER). The main findings of the IR and H-1-NMR spectra concern polysaccharides and olefinic groups. The former increased in concentration in both 6YASDOM (vs. SDOM and LASDOM), and BEER (vs. CDOM). The 6YASDOM finding proves that the increase in polysaccharides was caused by the evolution of the added municipal waste compost rather than by a direct addition of the compost. The polysaccharide increase in BEER testifies to a selective adsorption by the soil of humus-like organic macromolecules containing COOH groups, thereby increasing the concentration of the less adsorbed hydrophilic polysaccharide molecules, which would also explain the increase in polysaccharides in the 6YASDOM. Conversely, the concentration of olefinic groups was high in SDOM and low in CDOM, LASDOM, and BEER. This finding supports the hypothesis that the highly reactive olefinic groups present in SDOM react rapidly with CDOM molecules, thus lowering their concentration. In conclusion, compared with soil DOM, the increase of polysaccharides and the decrease of olefinic groups seem to be the most important characteristic of DOM in soils amended with municipal waste compost.