Decreasing soil water Ca2+ reduces DOC adsorption in mineral soils: Implications for long-term DOC trends in an upland forested catchment in southern Ontario, Canada Original Research Article

Positive trends in dissolved organic carbon (DOC) concentration have been observed in surface waters throughout North America and northern Europe. Although adsorption in mineral soils is an important driver of DOC in upland streams, little is known about the potential for changes in DOC adsorption to contribute to these trends. We hypothesized that long-term declines in soil water Ca2+ levels, in response to declining acid deposition, might influence DOC adsorption and that this could contribute to long-term DOC trends in an upland forested catchment in south-central Ontario, Canada. Between 1987 and 2009, DOC concentrations increased significantly (p < 0.05) in stream water and B horizon soil water (2 out of 3 sites). The null point DOC concentration (DOCnp), which is a measure of the soil water DOC concentration at equilibrium with the soil, ranged from 1.27 to 3.75 mg L− 1 in B horizon soils. This was similar to the mean DOC concentrations of B horizon soil water (2.04–6.30 mg L− 1) and stream water (2.20 mg L− 1), indicating that soil and stream water DOC concentrations are controlled by equilibrium processes at the soil–water interface. Adsorption experiments using variable Ca2+ concentrations demonstrated that as Ca2+ decreased the DOCnp increased (1.96 to 4.74 mg L− 1), which was consistent with the observed negative correlation between DOC and Ca2+ in B horizon soil water (p < 0.05; r2 = 0.21). Additional adsorption experiments showed that Na+ had no effect on DOC adsorption (p > 0.05), indicating that changes in DOC adsorption might be related to cation bridging. We conclude that declines in soil water Ca2+ concentration can contribute to increasing DOC trends in upland streams by reducing DOC adsorption in mineral soils.