Soil Properties Controlling the Adsorption of Dissolved Organic Carbon to Mineral Soils

Dissolved organic C (DOC) is the most reactive and mobile component of soil C and can be retained within mineral soils by adsorption. We determined the adsorption characteristics. for 52 mineral soil samples from 17 temperate and boreal soil profiles, using a modified Langmuir equation. The DOC solution used for batch experiments was derived from the organic horizons of a Spodosol. We analyzed the extent to which soil properties, such as the sum of poorly crystalline Fe and Al (Fe(pc) + Al(pc)), texture, and soil C, are related to DOC adsorption to mineral soils. Sorption characteristics including the maximum adsorption capacity (Q(max)) and the null point (np), where adsorption equals desorption, were best explained by Fc(pc) + Al(pc) (R(2) = 0.55 and 0.28, respectively). The Al(pc) exerted a stronger influence than Fe(pc) on Q(max). A simple method for estimating Q(max) was developed whereby the change in pOH after treatment with NaF is well correlated to Fe(pc) + Al(pc) (R(2) = 0.71, P < 0.0001) and Q(max) (R(2) = 0.50, P < 0.0001). The influence of clay content on Q(max) was of secondary importance and was largely masked by the dominant influence of Fe(pc) + Al(pc). Soil C did not have any influence on Q(max), but a slight negative influence on np. The B horizons of Spodosols and volcanic soils had the greatest Q(max), while large levels of soil C in Spodosols produced a high desorption potential. Results from this study emphasize the importance of considering the adsorption potential in conjunction with the desorption potential for better prediction of changes to the size of mineral soil C pools and DOC export to aquatic systems.