August 5, 2024
Plants At Work
Using native halophytes to remediate salt-impacted soils
BY DR. LYNDSAY CARTWRIGHT, LAUREN NAWROTH, DR. BARB ZEEB
Have you ever wondered what happens to the salt we apply to roads, parking lots and sidewalks once winter is over?
Salt, often in the form of sodium chloride (NaCl), is used in the winter to improve safety for winter travel. While some of the salt stays on surfaces to melt snow and ice, it also sprays (if liquid form) or bounces (if rock form) to nearby areas, including ditches, lawns or gardens. If it rains or the snow melts, some of the salt moves rapidly into streams and lakes, while the rest ends up being stored in soils, stormwater management ponds and groundwater. This stored salt is called legacy salt because it will still make its way to freshwater streams and lakes for years to come.
A negative legacy
More salt in freshwater is not good. We need freshwater to drink and irrigate our crops and many aquatic species rely on freshwater as their home. Freshwater is getting saltier across Canada. This makes it vitally important to put less salt into the environment and to find ways to remove legacy salt.
Phytoremediation involves using plants to clean up contaminated environments and has been used effectively to remediate sites contaminated with industrial chemicals, petroleum hydrocarbons, metals and salt. Halophytes are plants that can tolerate and, in some cases, remove salt from soils. They make up a very small percentage of all plants, accounting for only about two per cent of terrestrial species. Halophytes tolerate salty soils in three main ways.
First, some plants stop salt from entering roots (i.e., excluder halophytes). Second, some plants take up salt from soil and store it in shoots and leaves (i.e., accumulator halophytes). Finally, some plants take up salt from the soil and excrete it through salt glands on their leaf surfaces (i.e., excretor halophytes). Both accumulator and excretor halophytes provide an innovative solution for removing legacy salt from soil.
Promising new research
In the spring of 2022, we set up experimental plots in Mississauga, Ont., to compare the ability of four native perennial halophytic grasses to remediate soils containing salt from nearby roads. Two accumulator species, switchgrass (Panicum virgatum) and sand dropseed (Sporobolus cryptandrus), and two excretor species, prairie cordgrass (Sporobolus michauxianus) and sideoats grama (Bouteloua curtipendula) were planted.
Plant samples were collected in the fall of 2022 and 2023 and results indicate that all four plant species can take up high levels of chloride. Timelines for remediation were calculated based on plant biomass production, the amount of chloride taken up by each plant and the soil salt concentration. Switchgrass, prairie cordgrass and sideoats grama are projected to remediate this location in less than three years.
But what happens to the salt after it moves from the soil to the plants?
Phytoremediation using halophytes is a preferable and less invasive method of soil remediation compared to conventional methods (such as excavation or chemical amendments) with many co-benefits (such as aesthetics, thermal mitigation and creation of wildlife habitat). Using accumulator halophytes requires harvesting the above ground parts of the plants after each season of growth and repurposing the plant biomass. Repurposing could include composting to reduce the volume of biomass, using it as feed and fodder for livestock, pyrolyzing it to produce biochar or even using it to produce biofuels.
Using excretor halophytes may be a less intensive intervention as the need to harvest plants is eliminated. Once the salt is excreted to the leaf surfaces of excretor halophytes, the wind can disperse the salt to other areas diluting the amount of salt in problem areas, like roadsides. Dilution is not the solution to pollution for most contaminants; however, sodium and chloride in low amounts are important nutrients for plants and are often not readily available in the environment.
The results of this study are relatively new, and we look forward to testing implementation of this biotechnology with landowners and landscapers in the coming months. This testing will contribute to developing guidelines or checklists to help identify what to plant, where to plant it, maintenance requirements and potential limitations and challenges.
Nature can often provide us with the best solutions to environmental problems. Let’s do our part by reducing the amount of salt we add to the environment and removing what we can from salt-contaminated soils.
Several nurseries supply native halophytes including NVK Nurseries, Sassafras Farms, Grow Wild! Native Plants Nursery and Native Plant Nurseries.
If you are interested in participating as a landowner or landscaper to help develop guidelines for implementation, would like to discuss using halophytes on your lands, or have any questions, please contact Lyndsay Cartwright lyndsay.cartwright@trca.ca.