Soil microbial community responses to pulp mill and municipal biosolid residuals in smelter-impacted soils

Context Pollution from historical mining and smelting practices around Sudbury, Canada, resulted in acidified, metal toxic soils and semi-barren landscapes unlikely to recover without intervention. The Sudbury Regreening Program successfully addresses this with application of lime + fertiliser and planting trees, but alternative amendments may be superior. Land application of biomass-derived residuals (pulp mill sludge, wood ash, and alkaline-treated biosolids (ATBs)) can increase pH and provide organic carbon and nutrients. Aims We investigated the suitability of biomass-derived residuals as amendments on smelter-impacted soils. Methods We applied eight treatments, including controls and standard lime + fertiliser, on two semi-barren sites, and measured changes in soil characteristics and the microbial community over three years. Key results Amendments did not significantly increase soil organic matter. Despite application at liming equivalency, amendments with ash and ATBs increased pH more than the standard treatment. Other significant changes were associated with pH: residual-treated soils had increased available phosphorus and sulfur, and decreased available copper and nickel. The pH was positively correlated with bacterial diversity and richness. Soil bacterial community structures grouped based on pH rather than amendment, and higher pH was associated with an increase in the relative abundances of Proteobacteria, Gemmatimonadota, Bacteroidota, and Verrucomicrobia, and a decrease in Chloroflexi and WPS-2. Fungal richness and diversity were not affected by treatment or pH, and displayed a weaker association between community structure and pH, but high pH appeared to suppress relative abundances of Leotiomycetes and Lecanoromycetes, and low pH appeared to suppress Pezizomycetes and Sordariomycetes. Conclusions Because most amendments increased pH more than standard treatment, they showed greater improvements in nutrient availability and increases in bacterial diversity, though overall planted-tree survival was unaffected. Implications Long-term studies are required to determine persistence of treatment effects, but alkaline-treated biosolids and amendments with ash and pulp mill sludge or fertiliser may improve outcomes for regreening smelter-impacted lands compared to lime + fertiliser.