Selective sorption and precipitation of dissolved organic matter in Podzols - evidence from a column experiment

Complexation of dissolved organic matter (DOM) with Fe and Al plays an important role in the stabilization of soil carbon in Podzols. However, the influence of DOM source on chelation processes remains poorly understood. In this study, a Brazilian tropical coastal Podzol was used as a model environment to isolate the effects of DOM composition on sorption and desorption processes associated with podzolization. Columns containing B-horizon material (B-columns), and E-horizon material (E-columns), amended with Al-nitrate, kaolinite, and gibbsite were percolated with DOM derived from different sources: a stream, peat, litter, and charred litter. The results showed large differences in dissolved organic carbon (DOC) retention, with retention values varying widely among DOM source, which remained consistent across all E-column treatments. DOC retention in the columns decreased in the order stream > peat > litter > char. Spectroscopic (FTIR) and molecular (Py-GC/MS) analyses revealed selective retention in E-columns of compounds associated with macromolecular materials from biopolymers (aliphatics from roots, lignin phenols from lignin, and carbohydrates from cellulose), except for the DOM from char. In contrast, phenolic, carboxylic, and low-molecular-weight aromatic compounds were associated with desorption processes in the B-columns. Overall, the results indicate that sorption and desorption of organic matter in Podzol horizons are strongly dependent on the source of DOM, which implies that changes in environmental conditions may highly influence the fate of soil carbon in Podzols.