Sorption of organic carbon compounds to the fine fraction of surface and subsurface soils

Dissolved organic carbon (DOC) transported from the soil surface is stabilized in deeper soil profiles by physicochemical sorption processes. However, it is unclear how different forms of organic carbon (OC) compounds common in soil organic matter interact with soil minerals in the surface (A) and subsurface (B) horizons. We added four compounds (glucose, starch, cinnamic acid and stearic acid) to the silt- and clay-sized fraction (fine fraction) of A and B horizons of eight soils from varying climates (3 temperate, 3 tropical, 1 arctic and 1 sub-arctic). Equilibrium batch experiments were conducted using 0 to 100 mg C L-1 of C-14-labeled compounds for 8 h. Sorption parameters (maximum sorption capacity, Q(max) and binding coefficient, k) calculated by fitting sorption data to the Langmuir equation showed that Q(max) of A and B horizons was very similar for all compounds. Both Q(max) and k values were related to sorbate properties, with Q(max) being lowest for glucose (20-500 mg kg(-1)), highest for stearic acid (20,000-200,000 mg kg(-1)), and intermediate for both cinnamic acid (200-4000 mg kg(-1)) and starch (400-6000 mg kg(-1)). Simple linear regression analysis revealed that physicochemical properties of the sorbents influenced the Q(max), of cinnamic acid and stearic acid, but not glucose and starch. The sorbent properties did not show predictive ability for binding coefficient k. By using the fine fraction as sorbent, we found that the mineral fractions of A horizons are equally reactive as the B horizons irrespective of soil organic carbon content. (C) 2013 Elsevier B.V. All rights reserved.