Dynamics of Trace Metals and De-icing Salt in Compost and Amended Soils
by Maryam Kargar
In urban environments, street trees are often stressed due to heavy traffic and chemical injury from deicing salt, commonly used to melt snow and ice on the pavement and sidewalks. The accumulation of potentially toxic metals, mainly from traffic, and also deicing salt in the tree pit soils, can be toxic to trees and shorten their life span. As a collaborative study between McGill University and the City of Montreal, Quebec, Canada, this research attempts to design a new soil mixture in order to promote tree health and improve urban runoff water quality. A preliminary chemical analysis was done on soils sampled from tree pits in Montreal in order to determine the trace metals with highest concentrations in the tree pit soils. The addition of organic amendments was tested, such as compost and biochar, to verify if it can improve the physicochemical conditions of soil; through increase in adsorption of potentially toxic metals from runoff water and/or providing more nutrients for tree roots.
Adsorption, desorption and phytotoxicity tests were conducted to evaluate the mobility and bioavailability of potentially toxic metals in the amended soil mixtures, as compared with un-amended city soil. The results will indicate how the addition of biochar or compost affect the fertility of the soil mixtures and the uptake of metals by plants grown in the soil mixtures. The soil mixture with the highest fertility and the least bioavailability of potentially toxic metals will be recommended as the optimum soil mixture for use in sidewalk tree pits.
As the next step, a two-year study will involve monitoring the recommended soil mixture in 40 new tree pits in the Hochelaga-Maisonneuve neighbourhood of Montreal. Water quality in the tree pits will be monitored by sampling from zero-tension lysimeters and other environmental data will also be logged. The results of the monitoring study will be used to plan large-scale deployment of the optimum soil mixture in tree pits in Montreal, in order to reduce stress on street trees, improve their vigor, and lengthen their lives.
