UC Riverside

Soil contamination with metals and metalloids is a serious environmental and public health issue. Contaminated sites cover 22 million acres in the United States and 96,000 acres in California in California, exposing nearby communities to metals and other contaminants via entrained dust, surface water and groundwater. Conventional remediation involves excavation, capping, and disposal of contaminated soils, but is cost prohibitive and has environmental and social downsides. Biological remediation utilizes plants, fungi, and microbes to remediate soils and is more cost-effective than soil excavation plus has additional environmental and social co-benefits. However, bioremediation strategies using combinations of plants, especially native plants, and fungi, have not been tested in field conditions, particularly in semi-arid/arid regions. In this dissertation, a series of studies were carried out to explore the role of arbuscular mycorrhizal fungal-plant systems in metals dynamics in bioremediation systems using a multi-scale, field and greenhouse-based approach including a field survey of plant and microbial life on seven contaminated sites in Southern California, a controlled greenhouse pot experiment identifying controls on plant-AMF chromium (Cr) uptake under drought conditions, and a field-based phyto-mycoremediation study on three contaminated sites (brownfields) in Los Angeles. In Chapter 2, we report the findings from a field survey of contaminated sites throughout Southern California, where several California native plant accumulators of metals such as lead (Pb), chromium (Cr), copper (Cu), arsenic (As) and nickel (Ni), were identified. Microbial taxa previously identified as contaminant remediators were seen at all study sites, as well as some yet untested taxa which may be candidates for use in bioremediation. Most of the variation in microbial community composition was explained by differences across sites; while water-extractable organic carbon, silt and sand content, and cationic exchange capacity were the most significant drivers of microbial community composition within brownfields. Our findings identified potential plant and fungal candidates for further exploration in bioremediation applications suited to Southern California’s climate. In Chapter 3, we provide results from a greenhouse study that investigated the interactions between AMF and Cr in Medicago sativa under two different soil moisture regimes, drought and optimal moisture, in unsterilized field soils collected from a contaminated brownfield. Chromium translocation to Medicago sativa shoots was reduced by AMF inoculation and significantly increased by soil moisture and soil organic carbon content. Medicago plants grown under drought stress showed reduced Cr accumulation in plant shoots. AM fungal inoculation reduced soil Cr concentration by 45.8% and the greatest Cr reductions were seen in droughted soils with higher soil carbon concentrations and AMF inoculation. Inversely, significant increases in Cr(VI) concentrations (123% on average) occurred when there was no AMF inoculation and at low AMF colonization percentages (<20%) as well as at higher SOC percentages. These findings suggest it may be beneficial to apply AMF for the phytoremediation of Cr and Cr(VI) contaminated soils, especially in semi-arid regions, where similar drought or redox-fluctuating conditions may exist. Finally, in Chapter 4, we evaluated a novel biological soil metal remediation approach utilizing California native plants with different fungal treatments including arbuscular mycorrhizal fungi (AMF) inoculum and Pleurotus ostreatus inoculum in the form of spent mushroom blocks on three brownfields. Metals uptake by plant-fungal systems were examined with different levels of organic and inorganic contaminants at brownfield sites under irrigated and unirrigated water regimes. To our knowledge, this was the first field study of phyto-mycoremediation in Southern California, which was carried out in partnership with community groups, city, and regulatory agencies. Treatment with plants and fungi reduced soil concentrations of all metals of concern, and produced statistically significant reductions for Pb, Cu and As, when compared to control plots. Although the native plants tested were not as performant in metal accumulation and extraction into aboveground parts as the control plant, Chrysnopogon zizanioides (dryland remediator Vetiver), they had much greater survival and biomass production in both irrigated and unirrigated plots. Heterotheca grandiflora had the highest mean Pb uptake of the native plants tested, but was not significantly different than Erigonium fasciculatum, suggesting both may be suitable native plant options for phytoremediation efforts in Southern California. Irrigation and inoculation with AMF significantly increased metal accumulation in all plants tested, although AMF-mediated effects on Pb, Cr, and As translocation from plant roots to shoots was plant species dependent. Unexpectedly, significant increases to plant germination, survival, biomass, and metal uptake were observed with Pleurotus ostreatus (Oyster mushroom) inoculation which has not been reported in past studies. Study results suggest that longer timelines are necessary for Pb reductions in highly contaminated brownfield soils using plant-fungi combinations. Regional pilot-scale studies of the best plant-fungal treatment combinations could be explored over longer timeframes for in situ brownfields bioremediation.