THE SHORT- AND LONG- TERM EFFECT OF ZINC AND COPPER CONTAMINATION ON SOIL MICROBIAL FUNCTIONS
Soil microbial functions are vital for maintaining soil health and are therefore also essential for the provision of services required to support human and ecological health. The inadvertent release of trace metals into soil ecosystems through their extraction, processing, and use of metal ores has led to elevated soil trace metal concentrations worldwide. To preserve soil ecosystem functions and soil health, soil regulatory limits that define acceptable limits of trace metals in the environment need to be set. This is done to reduce the exposure of the soil microbial community to potentially hazardous and toxic substances. However, no single concentration of trace metals can be used to ensure the universal protection of all soil ecosystems. There is therefore a need to define the safe limits of exposure to trace metals in different land-uses. Considerable evidence shows soil microbial functions are affected by trace metal contamination in the short term but are restored over time. However, the long-term effect of trace metals on soil microbial functions is poorly understood. Much of the available data on adaptation is derived from studies focused on agricultural soils in Europe. This study aimed to examine soil enzyme activity across a range of different soil types and determine the long-term effects of zinc and copper contamination, focusing on artificially managed and natural systems, such as agricultural, urban, and anthropic soils, grassland, and forest soil. Initially this investigation validated new methods to determine soil enzyme activity. Standard methods of assessing soil enzyme activity require significant quantities of soil to be collected to then assess contaminated sites and determine toxicity. This experiment aimed to reduce the overall quantity of soil required by an order of magnitude. The assessment of soil nutrient cycling forms a part of all site investigations carried out by Canadian risk assessors. One of the key factors influencing the cost and therefore the scope and size of any potential investigation is the transportation of soils from a site to laboratories for experimentation. Therefore, a low soil requirement assay would significantly expand the potential for site investigations into nutrient cycling. This work demonstrates the validity of a low soil requirement nutrient cycling enzyme (phosphorus, sulfur, and carbon) assay and functional (nitrification) assay. These assays were applied to investigate different soil remediation treatments as part of a site investigation. Heterotrophic and autotrophic nitrification responded differently to lime addition over a decade and the assay demonstrated the effectiveness of different soil treatments (biochar, smectite) and hydro-seeding on soil enzyme activities. The central research of this work focused the soil enzyme activities of 18 soils from across Western Canada. Providing a clear demonstration of the capability of soil enzyme assays for terrestrial eco-toxicity assessment and how this method meets the requirements of the CCME for the assessment of contaminated sites and determining soil quality criteria. This experiment also demonstrated the importance of soil properties and land-use. The experiment provides a range of eco-toxicity data and the first steps towards making soil enzyme assessment a part of the Canadian regulatory regime. This study used North American soils from a range of land-uses to identify differences in adaptation rate and sensitivity, and aimed to evaluate methods used to assess the hazards from trace metals using spiked soils. First, soils (n=18) were spiked with zinc (Zn) and copper (Cu) applied at eight nominal concentrations (0, 300, 1000, 2000, 3000, 5000, 10,000, and 20,000 mgkg−1. Dose-response curves were then determined for enzyme activities (nitrification, dehydrogenase, arylsulphatase, and acid phosphatase). Land-use was shown to be a factor (P < 0.01) affecting the toxicity of zinc and copper. Boreal forest soil nitrification rates had the most sensitive end-point to zinc (EC50 = 202 ± 44 mg kg−1) and copper (EC50 = 197 ± 23 mg kg−1). Across all land-uses and soil types, a similar pattern of sensitivity was noted: potential nitrification > dehydrogenase > arylsulfatase > acid phosphatase. Soils were then monitored to identify adaptation of these microbial functions over 180 days. All soils enzyme activity responses showed adaptation to both zinc and copper over time. The relative sensitivity of each enzyme activity remained consistent after adaptation had occurred. Finally the long term consequences of the adaptation to trace metals by the soil microbial community were investigated. This study tested whether adaptation to a severe stress (trace metals) affects resistance or resilience to a subsequent mild stress. The resistance, resilience, and relative soil stability index (RSSI) of soils were determined for heat (60°C for 24h) and moisture stress, applied as short- term secondary stresses. Soils that had adapted to zinc and copper were not less resilient to additional stresses after adaption, than before recovery. The results of this experiment suggest that activity is a good indicator of soil microbial health but metal concentration is not. Metal speciation was determined using synchrotron-based X-ray Absorption Spectroscopy (XAS) at the Canadian Light Source Inc. Specifically Zn k-edge near-edge structures (XANES) data were collected at the HXMA beamline (06-ID1). Overall these studies provide new insights into the long-term consequences of Zn and Cu in soils. This study showed that soils dosed with zinc in the form of ZnSO4 did not undergo the changes in metal speciation to become less available, which may have been expected. Soil zinc speciation remained consistent overtime and across soils either over time or soil in the most available form of zinc (aqueous zinc). In conclusion, detailed analysis of soils artificially contaminated with zinc and copper in this study allowed the long term consequences for soil microbial functioning to be studied in greater detailed than they have been before. This gave an improved understanding of the effects of zinc and copper to a range of soils. It also improved the understanding of how soils artificially contaminated with metals for hazard testing differ from field contaminated soils. This study provides an improvement in the understanding of how microbial data can be incorporated into soil quality guidelines for Canadian soils.
Soil, Microbial, Toxicity, Zinc, Copper, Land-use
Doctor of Philosophy (Ph.D.)