Solubilization of iron by water-extractable humic substances

The capability of water-extractable humic substances (WEHS) to solubilize Fe from sparingly soluble Fe-hydroxide was studied. Addition of WEHS (1.7 mmol organic C l(-1)) to a dialysis tube containing labeled insoluble Fe-hydroxide caused an increase in the amount of (59)Fe measured in the external solution. The humic fraction was also able to solubilize Fe from soil samples, with levels comparable to those obtained using a solution containing 100 mu M DTPA. By measuring the amount of (59)Fe eluted from soil columns pre-loaded with (59)Fe-WEHS it was possible to evaluate the mobility of Fe complexed to the humic molecules. The recovery of 59Fe varied from 2% to 25% in respect to the soil type used. The ability of Fe-WEHS to serve as an Fe source far the phytosiderophore hydroxy-mugineic acid (HMA) was also analyzed. The removal of (59)Fe from the Fe-WEHS complex by HMA was demonstrated by adding the phytosiderophore to a dialysis tube containing the (59)Fe-WEHS complex. The observations suggested a ligand exchange between the phytosiderophore and the humic fraction. The results indicate that WEHS is able to increase the amount of Fe present in the soil solution, possibly by forming mobile complexes with the micronutrient. These complexes could act as easily available Fe sources in Fe acquisition processes by both monocot and dicot plants, playing an important role particularly in soils with low available Fe.