Molecular Toxicology of the Use of Coal Fly Ash as Soil Amendment for the Production of Bioenergy Feedstock, Switchgrass (Panicum virgatum L.)

This study assessed the toxicity of coal fly ash (CFA), the toxicological implications of the use of CFA as soil amendment in cultivating a bioenergy crop, switchgrass (Panicum virgatum L.), the fate of heavy metals (HMs) in CFA-amended soil, and the downstream effects of CFA exposure on the yield of glucose and bioethanol from switchgrass biomass. The roles of arbuscular mycorrhizal fungi (AMF) and exogenous glutathione (GSH) in modulating the CFA effects were also investigated. To determine CFA toxic, Salmonella enterica Typhimurium (TA100) was exposed to 0, 5, 10, 15, 20 and 25% CFA-water extracts (w/v). This was followed by a 90 days pot experiment conducted in the greenhouse of Tennessee State University, with switchgrass grown on a CFA-amedned Armour silt loam (ASL) soil (0, 7.5 and 15% w/w CFA/ASL soil). To separate portions of the treatments, AMF (3% substrate containing Rhizophagus clarum) and GSH (0.65 mM) were introduced. Pots were modified for leachate collection. At harvest, switchgrass morphological parameters were measured including dry weights and lengths of root and shoot to assess biomass yield, mycorrhizal dependency and tolerance indices. Levels of oxidative stress biomarkers including superoxide dismutase (SOD), glutathione peroxidase (GPx), malondidialdehyde (MDA) and genomic double strand deoxyribonucleic acid (ddDNA) in switchgrass were determined by spectrophotometry. Priority pollutant metals in the CFA-amended soil, switchgrass tissues and leachate were determined using ICP-OES. Plant biomass were hydrolzed and fermentented to determine glucose and bioethanol yield. Results showed that the CFA used in this study induced mutagenicity against TA100 at rates ≥ 20% w/vol CFA/water. It significantly increased the levels of the physicochemical parameters and mineral elements in the ASL soil. It increased plant metal uptake, induced oxidative stress, reduced tolerance and biomass yield while it significantly enhanced the downstream concentrations of glucose with lesser bioethanol yield. Mycorrhiza reduced oxidative stress and enhanced biomass yield while GSH reduced oxidative stress at a higher rate of CFA amendment. Both AMF and GSH enhanced bioethanol yield. The use of CFA as an amendment should be with great caution to avoid or minimize any adverse effects of CFA. Assessing the toxic limits of CFA will be important to avoid high levels CFA that may impair the health of a living system. Also, plant tolerance and oxidative stress biomarkers (SOD, GPx, MDA and ddDNA) will serve as important tools for biomonitoring CFA effects when used for agricultural production while it may be beneficial to couple CFA amendment with AMF to help minimize CFA impacts on the exposed crops. The study is significant because it elucidates on the morphological and molecular responses of switchgrass to CFA coupled with the protective roles of AMF and exogenous GSH