Mycorrhiza and Biochar for Remediation and Plant Production in Soils Polluted with Arsenic

Abstract

Arsenic (As) negatively affects the soil, and at high concentrations can cause biomass decrease, inhibition of photosynthesis and death of plants. Arsenic can enter the food chain via plant uptake and can be harmful to human health. Biological As-remediation is a process of using soil organisms or natural materials to reduce As concentration in soil and As toxicity in plants. Arbuscular mycorrhizal fungi (AMF) have a known role in enhancing plant growth and minimising effects of toxins. Magnetic biochar has recently gained interest for its capacity to adsorb pollutants, however research on its influence on accumulation of contaminants by plants and plant growth in contaminated soils is limited. Mycorrhizal fungi from different sources were tested for remediation of As in soil in two experiments. Commercial mycorrhizal inoculum was used to inoculate 8 species of vegetable plants (tomato, bean, capsicum, eggplant, lettuce, maize, okra and spinach) to study the effects on growth and on arsenic concentration and uptake in field contaminated soil. The plants were found to range in sensitivity to As. Arsenic reduced the growth of all species and the commercial mycorrhizal inoculum failed to improve the growth in contaminated soil. In another experiment mycorrhizal communities from contaminated and uncontaminated soil were propagated in pot culture and applied to maize in soil spiked with rates of arsenate from 0-75 mg kg-1. Both sources of mycorrhiza reduced arsenic concentration and uptake of maize, but inoculum from uncontaminated soil resulted in arsenic concentration and uptake that were half of those in plants inoculated with mycorrhiza from contaminated soil, although there was little difference in growth. Magnetic biochar, made from steam-activated coconut husk biochar and iron precipitate, and untreated biochar were applied to contaminated soil (156 mg kg-1 As), to determine effect on maize growth. The magnetic biochar adsorbed 3.8 times as much As per weight as raw biochar. Magnetic biochar reduced As concentration in shoot tissues by 42% and increased shoot dry weight by 40%. Raw biochar increased arsenic concentration in tissues and reduced shoot dry weight in contaminated soil. However, magnetic biochar reduced plant growth in uncontaminated soil, possibly due to excess iron. This study showed that, despite expectations, mycorrhiza from uncontaminated sites could be more effective for remediation than mycorrhiza from contaminated soils. It also showed for the first time that magnetic biochar can be used to remediate soil polluted with As. Mycorrhiza and magnetic biochar have potential roles in risk management for contaminated soil and in reducing arsenic concentration and uptake in plants, but further work is needed to improve methods of application.