Soluble organics from forest litter and their role in metal dissolution

The effect of soluble organics from O1 and O2 litter of seven overstory and understory forest species on dissolution of Al, Fe, Mg, and Mn from two forest soils was investigated. Litter was leached in absence and presence of soil under controlled laboratory conditions, and the leachates were analyzed. In absence of soil, different litters varied widely in their ability to release dissolved organic carbon (DOC) and metals. No significant correlations (p greater than 0.1) between protons consumed and DOC, Al, Mg, and Mn released from litter were found. In the presence of soils there was a direct relationship between organics released from litters and Al and Fe dissolved from soils but not for the other two metals. The predominant class of organics found in all litter leachates were acids and neutrals, and their contents were directly proportional to total organics dissolved. Thus hydrophobic plus hydrophilic acid, base, and neutral content of litter leachates corresponded to mean values of 49.6, 8.8, and 41.1%, respectively, for all litters. Total acid content was composed of 30.6% hydrophobics and 18.5% hydrophilics. A good correlation was obtained between soil dissolved Al and hydrophobic acids (r = 0.91, p less than 0.001), Al and Fe, and hydrophilic acids (r =0.93, r = 0.85, p less than 0.001, respectively). Polyphenolics constituted from 10 to 23% of the DOC content of both O1 and O2 litter extracts. Polyphenolic content was highly correlated with dissolved Al (r = 0.94, p less than 0.001) and with dissolved Fe (r =0.80, p less than 0.001). Five aliphatic acids and six aromatic acids were detected and quantified. Free-organic acids with high chelating abilities such as oxalic, malic, gallic, and protocatechuic acids made up 60 to 80% of the identified free-acid content of six litter leachates. Significant correlation was obtained between total free-organic acid content and dissolved Al (r =0.87, p less than 0.001) and Fe (r = 0.82, p less than 0.001) from soils.