Long-term road salting effects on dispersion of organic matter from roadside soils into drainage water

Sodium chloride has been utilised for decades to maintain road safety in winter and some of its detrimental impacts have been well-documented. However, research on the organic fraction of roadside soils has concentrated upon short-term salt-effects. We hypothesise that decades of past leaching and enhanced mineralisation of organic matter have reduced the concentrations of dissolved organic carbon (DOC) flushes currently occurring. We have examined the effects of salt concentration on organic matter mobilisation in soils that have already experienced varying degrees of exposure to road salting in the field over decades. Applications of salt at concentrations experienced in the field have been simulated to quantify the extent that DOC and dissolved organic nitrogen (DON) are still being mobilised for three prior salt-impact scenarios. A balance occurs between the effects on organic matter of long-term soil pH increase (due to continued cation exchange during salt exposure) which enhances its solubility and organic matter mineralisation, short-term pH suppression (due to the mobile anion effect in soil solution) which reduces its solubility, and short- and long-term sodium-induced dispersion. This now determines the influence of road salt on organic matter leaching from roadside soils and into associated drainage waters.