Soil amendment with organic materials is a common practice to increase the quality of soils, particularly in those areas where soils present a low content of organic carbon. Despite the overall beneficial effects of surrogate organic inputs, further insight in the composition of organic material added to soil is necessary to evaluate the potential consequences for metal bioavailability and the ultimate fate of the added organic carbon.

Two soils with low organic matter content (<0.1%) and slightly alkaline pH (7.25) were amended with 0.1% hay, peat or maize straw and incubated for one month at field capacity or under saturated conditions. Soil solutions were then analyzed for total metal and dissolved organic carbon contents and UV-Absorbance at 254 nm. Metal speciation analysis was performed with WHAM-VI.

For soils incubated at field capacity, 10-fold increase in concentration of lead (Pb) in solution was determined for soils amended with hay compared to control soils, while peat or maize straw addition resulted in maximum a 3-fold increase. Soil saturation increased Pb solubility circa 3-fold, but no significant effect was observed for the addition of organic materials under anaerobic conditions. Lead solubility was strongly correlated (R=0.7) with dissolved organic matter. Results indicate that organic inputs easily oxidizable may increase the concentration of soluble complexes of Pb, which increases the risk of Pb uptake by plants or groundwater contamination.

Besides, addition of hay promoted the reduction of Mn under saturated conditions, while peat or maize straw slowed down Mn reduction. Hay provides a substrate easily oxidizable by bacteria while peat or maize straw may constitute a more recalcitrant material that can be used as electron acceptor under anaerobic conditions.