Wetland plant species and biochar amendments lead to variable salinity reduction in roadway-associated soils

Salinization is an emerging threat in freshwater wetlands, with few techniques available to mitigate anthropogenic inputs such as road salts. Phytoremediation and biochar addition have each been proposed to remediate salt-affected soils generally, but interactive effects in wetland environments to improve soil conditions adjacent to roadways are not well understood. We conducted an 88-day fully factorial greenhouse experiment to quantify the effects of three plant treatments (unvegetated, Typha × glauca and Phragmites australis) and three biochar rates (0.0, 2.5, 5.0 % wt/wt) on the soil and leachate of a simulated wetland system. Both plant species significantly reduced soil Cl^- content relative to unvegetated controls, while Typha also significantly reduced Cl^- content of leachate and soil Na⁺. The difference in effects was likely due to different salt tolerance strategies: the salt-accumulating Typha contained a significantly higher volume of Na⁺, Cl^-, and water in its tissue than Phragmites, whose greater K⁺:Na⁺ ratio and similar soil Na⁺ to controls indicated a salt exclusion strategy. Biochar did not influence the growth of either species but moderately increased tissue Na⁺ concentration in Typha. Furthermore, biochar's effects on soil and leachate salt levels varied by application rate with the medium rate moderately increasing soil Na⁺ and Cl^- and leachate Cl^-, while the highest application did not differ from controls across all metrics. Our results suggest that phytoremediation can be optimized with salt-accumulating species, whose mechanisms of salt tolerance involve the accumulation of salt ions from the surrounding environment. The consistent flooding in our study may have inhibited the influence of biochar. We recommend future studies parse the effects of water levels and redox potential on biochar's ability to influence wetland salinity. Data repository: doi.org/10.17605/OSF.IO/9QFZ7.