M Anne Naeth, Sarah R Wilkinson, Allen M Jobson, Alison P Murata, Marian G Fluker, Department of Renewable Resources, University of Alberta
September 30, 2016
Executive Summary
Salt affected soils are those in which salts interfere with normal plant growth. Excessive salt concentrations in soil are one of the major causes of decline in agricultural productivity around the world. Salt affected soils occur naturally on the landscape. In the agriculturally rich prairie regions of Canada, 30 % of the land area is estimated to have salinity or be at risk of salinization and 3.3 % of the land area is considered to be severely saline. Salt affected soils are classified as saline, saline-sodic and sodic, based on amount of salts, type of salts, amount of sodium and soil alkalinity.
Salt affected soils are can be caused by anthropogenic activities such as excessive irrigation, application of organic amendments, road salt runoff, mining and oil and gas exploration and development. Salt affected soils in Alberta must be remediated to meet government criteria for topsoil and subsoil horizons. The criteria are based on soil electrical conductivity and sodium adsorption ratio. Applicability of these salinity measures and values for all salt affected soils in the province and their relationship to successful remediation and reclamation is questioned. Observations in the field suggest successful reclamation can occur on salt affected soils that do not meet current criteria. The Petroleum Technology Alliance Canada (PTAC) contracted the University of Alberta to review and assess the available scientific literature on salt affected soils to evaluate against current regulatory requirements, to identify knowledge gaps in the science, to provide recommendations on the scientific validity of current remediation guidelines in Alberta, and to make recommendations for an alternative approach to reclamation of these salt affected soils. This report was written to provide a scientific basis for potential development of a new risk based assessment system for salt affected soil in Alberta.
Salts, whether natural or a result of anthropogenic activity, can change soil physical, chemical and microbiological properties. How they change soil properties and the magnitude of the change depends on type and concentration of salts. Sodium chloride is one of the most common salts, however, other chloride salts and sulfate salts can be present. Saline soils are those that have high electrical conductivities while sodic soils are those that have high sodium content; both can have detrimental effects on soil, water and plants. High sodium concentrations can cause clay swelling and lead to poor soil structure and reduced permeability.
Phytotoxicity of salt affected soils on species of agronomic importance has been well researched. In general, most common cereal and grain crops in Alberta are not considered salt tolerant. Little research has been conducted on effect of salts on native plants, particularly in prairie and mixedwood regions. Current salinity tolerance indices have been developed for crops based on predetermined and subjective acceptable losses in grain or biomass yields which may not be appropriate for native plant communities. Plant response to salts in soil is physiologically complex and at cellular, organismal and whole plant levels. Germination is one of the plant development stages most sensitive to salinity. Uptake of sodium and transport to shoots is the primary cause of decline in plant health. Plants may not have visual signs of salt stress immediately, even though physiological effects are occurring. Long term, potentially over multiple years, some plant species may not survive. Most plants are more sensitive to chloride than sulfate. Halophytes are naturally tolerant of salinity and are found in naturally saline communities in prairie and boreal regions. However, number of species and diversity is limited.
There are a multitude of factors that influence plant response to salt affected soils including species, cultivar, providence, climate, precipitation, soil texture, soil water content, land management, type of salt, stage of development at application or contamination and plant response measure. Therefore, salt tolerance is a relative, not absolute, term. At one site with a specific type of contamination a plant species may be salt tolerant; However, at another site with the same contaminant, it may not be salt tolerant.
A variety of remediation methods have been employed to reduce soil salinity. Leaching is the most common approach and in a short period of time can make surface soil hospitable for plant establishment and growth. Amendments such as calcium and organic materials assist in alleviating poor soil properties caused by salts in the upper soil layers. Fertilizer can reduce negative effects of salts on plants by addressing nutrient deficiencies. Phytoremediation is a growing field of study, which involves use of halophytic plants to uptake sodium, reducing soil salinity. Plant growth promoting rhizobacteria can accelerate these processes. Subsoil salinity, however, is more difficult to address with current remediation options. Even if surface soil is successfully remediated, salts may remain in subsoil layers and rise to the surface under some environmental conditions.
Based on available scientific data, current guidelines for acceptable electrical conductivity values on reclaimed sites are not supported. Depending on type of salt, concentration, soil type, hydrologic regime and plant species, successful surface soil reclamation may be achieved at lower or higher values. Ranges of values are recommended versus absolute criteria. Guidelines for specific salts to be used in conjunction with electrical conductivity values may be worth investigating, as occurs in British Columbia. Based on studies reviewed, no conclusions can be drawn regarding effectiveness of the guidelines for subsoil salinity or effectiveness of current sodium adsorption ratio guidelines for surface or subsoil. The exception is that in boreal ecosystems, high electrical conductivity and sodium adsorption ratios at depth were not detrimental to trees in the short term. We recommend an experimental and scientifically rigourous approach to future research to address knowledge gaps.
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