Blended municipal compost and biosolid materials for mine reclamation: long-term field studies to explore the impacts of biosolid additions to compost covers on metal mobility, soil fertility and microbial communities

Application of stable soil amendments is the key to successful phytostabilization and reclamation programs on mine tailings and microbial guilds are primary drivers of many geochemical processes promoted by these amendments. Long term 4- and 10- year field studies (2014-2018 and 2008-2018) were set up in Onaping, Ontario to study blends of lime stabilized municipal biosolids at nine different rates with one-meter thick municipal compost covers planted with agricultural crops. Based on biogeochemical variability of the substrates four and ten years after application of the initial compost cover, the experimental plots could be classified into three categories: “Low” rate (0-100 t ha-1 biosolids), “Medium” rate (200-800 t ha-1), and “High” rate (1600-3200 t ha-1) treatments. The addition of biosolid materials to the thick compost cover at rates higher than 100 kg ha-1 significantly reduced C:N ratio of the substrates, available phosphorous, and some of the nutrient cations while notably increasing inorganic carbon and solubility of Ni and Cu. This suggests that increasing biosolid application rates may not equivalently ameliorate soil quality and geochemical stability. Correspondingly, microbial communities were altered by biosolid additions, further intensifying the negative impacts of biosolids on long-term efficiency of the compost cover. Abundance of cellulose, hemicellulose, and lignocellulose decomposers (as key drivers of mineralization and humification) was significantly reduced by “Medium” and “High” rate treatments. Most DNA sequences with high affinity to denitrifiers were detected in “High” rate treatments where geochemical conditions were optimal for higher microbial denitrification activities. These findings have implications for improving long-term efficiency of reclamation and environmental management programs in mine tailings of northern temperate climate.