Abstract
<jats:p>Soil salinization refers to the accumulation of water-soluble salts within the soil profile. Two major sources of soil salinization are seawater intrusion and the application of de-icing road salts. These two sources converge in densely populated coastal landscapes, where both road salt and seawater can simultaneously affect soil nitrogen (N) cycling. Here, we assess this “salt squeeze” of the soil N cycle through a comparative synthesis of many, but typically independent, studies for the two sources. Given the differences in compositions for the two salt sources, the effects on soil N dynamics differ across salt concentrations, with both positive and negative feedbacks that cascade through soil physical, chemical, and microbial environments. For example, salinization can suppress denitrification, a microbial process that removes nitrate from soils, with more negative effects from seawater because of sulfide toxicity. On the other hand, salinization may not have a similar negative effect on dissimilatory nitrate reduction to ammonium (DNRA), a competing reductive pathway that retains nitrate within soils as ammonium. Similarly, positively charged sodium and negatively charged chloride ions can displace ammonium and nitrate ions from soils, although the magnitude and consequences of these processes may differ between seawater and road salt inputs. Thus, understanding the net effects of these two salinization sources on soil N concentrations and transformation processes is critical, as management and mitigation strategies could differ considerably. Reducing road salt impacts may require alternatives such as liquid brine applications and optimized winter road management practices, whereas mitigating seawater intrusion may depend on restoring coastal wetlands and constructing living shorelines that function as barriers and buffers against saltwater encroachment.</jats:p>