Microwave and biochar soil treatment alleviates arsenic phytotoxicity in wheat and rice
AuthorKabir, Md Humayun
AffiliationAgriculture and Food Systems
Document TypePhD thesis
Access StatusOpen Access
© 2020 Md Humayun Kabir
Abstract Arsenic (As) is a toxic metalloid, which is carcinogenic i.e. cancerous to humans. Besides the drinking water, accumulation of As in food grains through plant uptake, when cultivated in As contaminated soils, is a potential route of human dietary As exposure. This has inspired research into alleviating grain As accumulation, despite there being already existing strategies with major disadvantages such as low efficiency, high costs, and usage being restricted to smaller-scale operations. Therefore, pre-sowing microwave (MW) soil heating and sawdust biochar were used to investigate if they can reduce As concentration in wheat and rice. Microwave is a form of electromagnetic radiation, which can produce heat in the soil by inducing the rotation of the dipoles of polar molecules (e.g. water). Microwave heating depends on the dielectric properties of the soil. Therefore, a study was conducted to determine the dielectric properties of different types of soils with different moisture content. The results showed that the soil moisture was the major contributor to the dielectric behavior of soil since dielectric properties increase as soil moisture increases. Soil types also had an influence as the dielectric properties of sandy soil were much lower than the other soils such as clay and loam soil. To investigate the effect of MW and biochar on wheat and rice grain As concentration, both the wheat and rice soils were spiked with five As concentrations (As-0, As-20, As-40, As-60, and As-80 mg kg-1 soil). In addition to MW, biochar was used to reduce rice grain As accumulation since biochar has been gaining attention for its heavy metal immobilization capacity. After As application, three levels of biochar (BC-0, BC-10, and BC-20 t ha-1 soil) were added only in the rice-growing soil. Then, soils were treated for 0, 3, and 6 minutes (MW-0, MW-3, and MW-6) in an MW chamber to achieve the soil temperature of around room temperature, 60 oC, and 90 oC respectively. The crops were grown in a completely randomized design with four replications in a glasshouse during 2017 (wheat) and 2018 (rice). The results demonstrated that, in both the wheat and rice, MW soil treatments, especially the MW-6, alleviated As phytotoxicity and facilitated less grain total As concentration compared with the MW-0 treatment across all the soil As concentrations. Also, MW treatment significantly reduced the concentration of arsenite [As(III)], the most toxic form of As. Decreased grain As concentration in rice was recorded at BC-10 in lower levels of soil As concentrations (As-20 and As-40) while, a negative impact was observed at BC-20 across all the soil As concentration, compared with BC-0 treatment. Furthermore, rice grain As(III) concentration increased significantly in BC-20 treatment. Thus, MW-6 treatment could be used for the alleviation of grain As concentration in wheat and rice grain, whereas more study is needed for the best biochar application rate. However, understanding the residual effect of MW and biochar is crucial for the sustainability of the treatment. Therefore, the same varieties of wheat and rice were grown in the following year, using the same pots, without the addition of further MW or biochar treatment. The results revealed that, 360 days after MW soil treatments there was still the potential to alleviate grain As concentration in both wheat and rice. A similar result was observed for biochar treatment in the residual year with a positive effect at BC-10 and a negative effect at BC-20 treatment. Furthermore, it is unclear whether MW soil treatment is just a heating effect or if there is some other effect of the electromagnetic wave involved. Therefore, a glasshouse pot study was designed to investigate the effect of MW and conventional electric oven (EO) soil heating on As phytotoxicity alleviation in rice. The soil was spiked with three levels of soil As concentration (As-0, As-40, and As-80 mg kg-1) prior to applying MW and EO heat treatments, to achieve the soil temperature of around 80 - 90 oC. The results showed that, there was no statistically significant difference between MW and EO treatments regarding As phytotoxicity alleviation. However, the positive effect was more in MW treatment than the EO treatment. Significantly less total energy required in the MW to treat the soil than the EO. Besides the As phytotoxicity alleviation, the effect of MW soil heating on soil microorganisms, particularly bacteria, was a topmost concern and investigation was needed to ascertain that MW soil heating does not affect it drastically. Therefore, an experiment was designed to investigate the effect of MW heating (80 - 90 oC) on the soil bacterial community in As contaminated (As-0, As-40, and As-80 mg kg-1 soil) soils. The 16S rRNA bacterial gene copy numbers decreased significantly after MW soil heating but recovered back to its previous number 42 days post treatment. The bacterial diversity also decreased significantly in MW treated soils but did not recover even after 56 days from MW heating. However, there was no noticeable effects of soil As concentration on bacterial community were observed. Furthermore, relative abundance of some beneficial bacteria such as Bacillus and Symbiobacterium were significantly higher in the MW treated soils. Thus, MW soil heating at 80 - 90 oC can potentially be applied for As phytotoxicity alleviation without significantly destroying the ecologically important taxa. Overall, pre-sowing MW soil heating could be applied as a novel technique to alleviate As phytotoxicity in wheat and rice with lower As accumulation in the grain. Thus, application of the MW technology in the As contaminated area like Bangladesh could add another feather in the crown of the As remediation techniques and help to reduce the human health risk through As contaminated food grain. However, further research needed before adopting the MW soil heating technique where different aspects should be explored such as response of MW technology in the field condition, scaling up the MW equipment for field application, cost of MW application in the field at farmers level, long-term effect of MW treatment on soil nutrient dynamics, soil organic matter and soil biota and sustainability of the MW technology in field condition. Also, sawdust biochar could be used in combination with MW soil heating for As phytotoxicity alleviation; however, more study needed to set the appropriate rate of biochar application.
KeywordsArsenic; Phytotoxicity; Remediation; Microwave; Biochar; Wheat; Rice
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