| Abstract
| - The IHSS soil humic acid (HA) standard and two HAsfrom soils of very different origin (Chernozem and Ranker)were fractionated by tandem size-exclusion chromatography−polyacrylamide gel electrophoresis. From each HA,three fractions with different molecular sizes (MSs) andelectrophoretic mobilities were obtained and investigatedfor their fluorescence properties and ability to photoinducethe transformation of 2,4,6-trimethylphenol and herbicidefenuron. Regardless of the source of the HA, the two highMS fractions were found to be very weakly fluorescent.They photoinduced the degradations of fenuron and 2,4,6-trimethylphenol less efficiently than the bulk HA (10−50-fold and 1.4−5.3-fold, respectively). In contrast, the low MSfraction was proved to be fluorescent and to photoinducethe transformation of probes as least as efficiently thanthe bulk HA. These results show that (i) most of fluorophoresand a great part of photoinductive chromophores arelocated in the low MS fractions of soil HAs and (ii) thisdistribution of photochemically active constituents may becharacteristic across broad soil types.
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