Sorption of dissolved organic carbon in soils: effects of soil sample storage, soil-to-solution ratio, and temperature

Experiments on the sorption of dissolved organic carbon (DOC) in soils were mainly conducted in batch approaches. Because varying setups were used in these studies, comparison of the results requires knowledge on the effects that different experimental conditions may have on the sorption of DOC. This investigation evaluated the DOC sorption of soils using differently pretreated soil samples (field-fresh (two sampling dates), air-dried, stored at 3°C and −18°C), at different soil-to-solution ratios (1:40, 1:20, 1:10 and 1:5 w/v) and different temperatures (5°C, 15°C, 25°C and 35°C). The sorption of DOC was analyzed using the initial mass (IM) approach, which regressed the initial amount of sorbate (normalized to soil mass) against the sorbed amount (normalized to soil mass). The DOC release — when a solution without DOC was added — strongly increased with temperature and soil-to-solution ratio. Among the different types of sample storage and preparation, air-drying resulted in the largest DOC release. The smallest release was from the field-fresh samples. Freezing and storage at 3°C resulted in intermediate DOC release with freezing having the greater effect. The release from air-dried samples exceeded that of field-fresh samples by a factor of four at maximum. In contrast, none of the experimental setups influenced the slope of the IM isotherms. Thus, it seems possible to compare directly the binding affinity of DOC to different soils as determined at varying experimental conditions.