An analysis of ecological succession, microbial communities, and plant responses to guide reuse and restoration of steel slag substrates; case studies from Algoma Steel Inc.

Abstract

Steelmaking operations generate large deposits of steel slag; a highly alkaline, metal-rich, and nutrient-poor substrate, with very low water-holding capacity. These conditions limit early plant establishment and microbial colonization, yet slag deposits also undergo progressive weathering and biological development over time. Understanding plant-fungal interactions across different stages of slag development is critical for evaluating its potential use in ecological restoration and as a soil amendment. To assess how fungal communities respond to slag weathering, I characterized slag chemistry and fungal assemblages at “young” (<5 years) and “old” (>30 years) slag deposits at Algoma Steel Inc. I hypothesized that older slag would exhibit reduced alkalinity, greater organic matter accumulation, and increased representation of mutualistic fungal guilds. Within-site (alpha) diversity did not differ at either slag site, but fungal community composition shifted with slag age. Young slag was dominated by stress-tolerant groups, whereas old slag supported more functionally diverse communities enriched in mycorrhizal fungi. These results demonstrate that weathering and organic inputs gradually reduce abiotic constraints, allowing more complex plant–microbe interactions. I then conducted a greenhouse experiment to test how slag concentration, sterility, and fertilizer application affect plant performance and belowground symbioses. Using romaine lettuce (Lactuca sativa) as a model species, I hypothesized that moderate slag concentrations, when paired with nutrient supplementation, would improve plant growth, while high slag concentrations would suppress it due to alkalinity, micronutrient imbalance, and low water retention. Plants grown in 10–30% slag with half-strength Hoagland’s solution showed substantial increases in biomass and height, whereas those grown in 100% slag exhibited severely reduced growth, very low water-holding capacity and near complete loss of arbuscular mycorrhizal fungal colonization. Overall, slag suitability for plant growth depends strongly on its degree of weathering, physical characteristics, and nutrient availability Weathered slag and low-percentage slag-soil mixtures, particularly when supplemented with balanced fertilizers, can support plant establishment and microbial symbioses, informing both ecological understanding and applied restoration practices.