Isotopic fractionation of dissolved organic carbon in shallow forest soils as affected by sorption

Isotopic fractionation of dissolved organic carbon percolating through the soil is often interpreted as due to microbial transformation. We investigated the potential effects of sorption on the δ¹³C of dissolved organic C in field and laboratory experiments. We sampled the organic C in soil water at two forested sites and measured sorption with intact mineral soil and individual minerals (dolomite, ferrihydrite, goethite, and quartz). The dissolved organic C was separated into hydrophilic and hydrophobic fractions using a resin approach. The δ¹³C values of bulk soils, alkaline‐extractable organic C, and dissolved organic C and its fractions were measured. Hydrophilic and hydrophobic fractions in forest floor seepage water were characterized by ¹³C‐NMR spectroscopy. At both sites, δ¹³C of dissolved organic C increased with increasing depth, suggesting that decomposition contributes to the loss of the dissolved organic C. However, there was an enrichment of hydrophilic organic C in the soil solution as the water moved down the soil. The δ¹³C values of hydrophilic fractions were less negative than those of hydrophobic fractions. The smaller δ¹³C in the hydrophobic fraction was due to the large contribution of compounds derived from lignin that are depleted in ¹³C. As the isotope composition of both fractions of dissolved organic C did not change throughout the profile, changes in δ¹³C of total organic C reflected changes in the relative proportions of its hydrophilic and hydrophobic fractions. The sorption experiments with minerals and soil cores gave similar results. When dissolved organic C came into contact with mineral material, the δ¹³C of that remaining in solution increased due to preferential sorption of the ¹³C‐depleted hydrophobic fractions. Moreover, the soils released hydrophilic organic C with large δ¹³C values, increasing the δ¹³C of organic C in effluents from soil compared with that in the inflow. Thus, selective sorption of organic C fractions changes δ¹³C in a way that mimics metabolic transformation and decomposition.