Labile organic carbon fractions after amendment of sandy soil with municipal sewage sludge and compost

The paper deals with the effect of organic amendments three years after their application on the transformation of organic matter in sandy soil. The labile part of organic carbon has been suggested as a sensitive indicator of changes in soil organic matter. Our study was based on quantification of the hot water-extractable carbon fraction and its optical properties, as well as the easily mineralizable fraction of organic carbon and fulvic acids. The hot water-extractable carbon content was lower in plots amended with sewage sludge than with compost, and it was correlated with easily mineralizable carbon as well as the light fraction of fulvic acids. The analyzed amended soil contained amounts of hot water-extractable carbon that are typical of sandy soils. The optical properties of the carbon fraction soluble in hot water seem to implicate that this dissolved organic carbon in amended plots contained fewer polar functional groups which were hydrophilic. This may suggest the formation of more complex organic compounds with lower affinity to water. The Carbon Management Index values increased after the application of organic amendments as compared to the untreated soil, and were higher for the plots amended with compost. The measurable dynamics of carbon expressed by Lability Index, Carbon Pool Index and Carbon Management Index showed that both compost and sewage sludge increased the lability of organic carbon and, irrespective of some decline in the organic carbon content, an overall balance of carbon is favorable. However, changes in these indices should be monitored over time.