Bulk and water-extractable organic matter from compost: evaluation of the selective dissolution in water using infra-red 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 compositions of bulk compost OM and its water-soluble fraction may, therefore, allow better understanding and prediction of environmental impact of compost land use. The objectives of this study were to (i) examine compositional links between bulk compost OM and its water-extractable OM (WEOM), and (ii) evaluate and quantify selectivity of bulk compost OM dissolution, based on infra-red (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 WEOM were characterized in terms of ratios (R) defined on the basis of both areas and heights of specific IR 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. WEOM composition (characterized by the R values) is associated, to a certain extent, with compost OM composition expressed by similar type indices. The WEOM aromaticity estimated by specific UV absorbance correlated strongly with some R values determined for bulk OM.

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