Release of natural organic matter sorbed to oxides and a subsoil

Sorption to mineral surfaces is an important process controlling the mobili ty and stability of natural organic matter (NOM) in soil, yet only knowledg e of the reversibility of this process enables the prediction:of element cy cling and NOM-induced transport in soils. We have elucidated the desorption of mineral-bound NOM in batch experiments with amorphous Al(OH)(3), goethi te, and a subsoil low in organic C. These sorbents were equilibrated with i ncreasing amounts of water-extractable NOM from the Oa horizon of a mor for est-floor layer and then extracted with solutions of different ionic streng ths, pH,: and concentrations of inorganic anions (Cl-, SO42-, H2PO4-). Sorb ed NOM was extracted after 24, 48, 72, and 120 h. We investigated structura l and functional characteristics of the desorbed NOM by XAD-8 (macroporous resin) fractionation and by C-13-NMR spectroscopy. Desorption of NOM from m inerals and soils was negligible (<3%) under solution conditions similar to those during the sorption (hysteresis).:It was not influenced by increasin g concentrations of noncompeting inorganic anions such as Cl-. Increased co ncentrations (less than or equal to 0.1 M) of-competing anions like SO42- o r H2PO4- increased the NOM desorption. Though H2PO4- was most efficient in desorbing NOM, the extractability was only less than or equal to 60% at the highest H2PO4- concentration. The,most significant desorption occurred whe n solution pH was raised. For goethite, NOM desorption reached a maximum at a pH above the paint of zero charge (PZC) of the mineral. With increasing surface coverage of the sorbent by NOM, the proportion of desorbable NOM de creased for all extractants. Increased sorption hysteresis was also observe d with an increasing time period between sorption and desorption. The desor ption was more pronounced for NOM compounds that exhibit hydrophilic proper ties and have low contents of aromatic structures and carboxyl groups. The irreversible binding of NOM, especially of the lignin-derived portion, to s oil minerals seems to result from its polvelectrolytic nature. This may fav or the formation of multi-site coordinative bonds and effective shielding o f the binding ligands by; other parts of the sorbed molecule.