Release of natural organic matter sorbed to oxides and a subsoil

Sorption to mineral surfaces is an important process controlling the mobility and stability of natural organic matter (NOM) in soil, yet only knowledge of the reversibility of this process enables the prediction of element cycling and NOM-induced transport in soils. We have elucidated the desorption of mineral-bound NOM in batch experiments with amorphous Al(OH)3, goethite, and a subsoil low in organic C. These sorbents were equilibrated with increasing amounts of water-extractable NOM from the Oa horizon of a mor forest-floor layer and then extracted with solutions of different ionic strengths, pH, and concentrations of inorganic anions (Cl-, SO42-, H2PO4-). Sorbed NOM was extracted after 24, 48, 72, and 120 h. We investigated structural and functional characteristics of the desorbed NOM by XAD-8 (macroporous resin) fractionation and by 13C-NMR spectroscopy. Desorption of NOM from minerals and soils was negligible (-. Increased concentrations (≤0.1 M) of competing anions like SO42- or H2PO4- increased the NOM desorption. Though H2PO4- was most efficient in desorbing NOM, the extractability was only ≤60% at the highest H2PO4- concentration. The most significant desorption occurred when solution pH was raised. For goethite, NOM desorption reached a maximum at a pH above the point of zero charge (PZC) of the mineral. With increasing surface coverage of the sorbent by NOM, the proportion of desorbable NOM decreased for all extractants. Increased sorption hysteresis was also observed with an increasing time period between sorption and desorption. The desorption was more pronounced for NOM compounds that exhibit hydrophilic properties and have low contents of aromatic structures and carboxyl groups. The irreversible binding of NOM, especially of the lignin-derived portion, to soil minerals seems to result from its polyelectrolytic nature. This may favor the formation of multi-site coordinative bonds and effective shielding of the binding ligands by other parts of the sorbed molecule.