Bulk and water-extractable organic matter from compost: evaluation of the selective dissolution in water using infrared absorbance ratios

Land application of composts affects concentration and composition of dissolved organic matter (OM) which plays important roles in soil functioning and may have effects on spreading of environmental pollution. Linking between the composition of bulk compost OM and its water-soluble fraction may, therefore, allow better understanding and prediction of the environmental impact of compost added to soil. The objectives of this study were to (i) examine composition-based links between bulk compost OM and water-extractable OM (WEOM), and (ii) evaluate and quantify selectivity of bulk compost OM dissolution, based on infrared (IR) absorbing functional groups. For that, 8 different composts and their freeze-dried WEOMs were characterized by mid-IR transmission spectroscopy. Compositions of compost OM and of WEOM were characterized in terms of ratios (R) defined on the basis of both areas and heights of specific IR absorbance bands in relation to absorbance by aliphatic CH groups. A simple novel approach is suggested, whereby selective dissolution of compost OM components is quantified by relating the R values determined for WEOM to those associated with compost OM. Significant similarities of IR spectra found in a series of WEOMs (and, to a lesser extent, in a series of compost OMs) suggest significant contributions of OM carboxylic groups to various bands. IR absorbance of compost OM contributed by hydrophilic and, specifically, carboxyl and carboxylate groups, when related to absorbance by aliphatic CH groups, can be used for predicting the indices characterizing WEOM composition, such as IR absorbance-based R values and aromaticity estimated from specific UV absorbance.