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 Fe 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.