Combined effects of the biochar/biochar composites and water management strategies on the phyto-availability of arsenic in paddy rice soils

Combined effects of the biochar/biochar composites and water management strategies on the phyto-availability of arsenic in paddy rice soils

Rice (Oryza sativa L.) is the main staple carbohydrate source for more than 50% of the world’s population. Rice production needs to be increased by 70% by 2030 to meet the demand of an ever-increasing population worldwide. It is reported that millions of people around the world are at risk of health problems because of ingestion of arsenic (As) through rice consumption. The application of pristine/modified biochar (BC) would be a sustainable way of improving rice yield and decreasing the bioavailability of As in the rice rhizosphere. Taking these facts into account, this PhD investigated, for the first time, the integrated effects of pristine/modified BC-water management approaches (flooded and intermittent) on plant growth parameters and As phyto-availability in As-contaminated paddy rice soils with respect to potential mechanisms. In the first phase, the effects of pristine rice hull BC supplementation to As-contaminated paddy soils under different water management practices were investigated. The incorporation of rice hull BC to As-contaminated paddy soils has increased rice yield by 11%-19% in rice hull BC-intermittent and -flooded treatments compared to the conventional flooded treatment. Inorganic As concentration in rice roots, shoots, husks, and unpolished rice grains and abundance of Fe(III) reducing bacteria in the rice rhizosphere decreased by 10%-83% and 40-70%, respectively, in rice hull BC-flooded, -intermittent, and intermittent treatments compared to the conventional flooded treatment. Concentrations of essential elements such as Fe, Mn, Zn, Mg, and Ca in unpolished rice grains increased by 45%-329% in rice hull BC-flooded and -intermittent treatments compared to flooded treatment. The steady release of Si, SO and essential elements and the adsorption of As species, following the incorporation of rice hull BC in paddy soils could contribute to a decreased As accumulation in rice tissues, while increasing the concentration of essential elements in unpolished rice grains. In the second phase, the effects of birnessite modified rice hull BC supplementation to As-contaminated paddy soils under different water management practices were evaluated. Rice yield in both selected rice varieties (Jayanthi and Ishikari) increased by 10%-34% under birnessite modified rice hull BC-flooded and birnessite modified rice hull BC-intermittent treatments compared to the conventional flooded treatment.

In most cases, inorganic As concentration in rice roots, shoots, husks, and unpolished grains in both rice varieties was significantly (p ≤ 0.05) decreased by 20% - 81%, 6% - 81%, 30% - 75%, and 18% - 44%, respectively, under birnessite modified rice hull BC-flooded, birnessite modified rice hull BC-intermittent, and intermittent treatments over flooded treatment. Incremental lifetime cancer risks associated with consumption of both rice varieties were also decreased from 18% to 44% under birnessite modified rice hull BC-flooded, birnessite modified rice hull BC-intermittent, and intermittent treatments compared to the conventional flooded treatment. Formation of Mn plaque on rice roots, physi- and chemisorption of As species to birnessite modified rice hull BC, following the supplementation of birnessite modified rice hull BC in As-contaminated paddy soils could involve the reduction of bioavailability of As species in the rice rhizosphere. In the third phase, the effects of Fe-modified rice hull BC addition to As-contaminated paddy soils under different water management practices were examined. Compared to conventional flooded water management, rice yield per pot under Fe-modified rice hull BC-intermittent and Fe-modified rice hull BC-flooded treatments increased by 24%-39%. The supplementation of Fe-modified rice hull BC has decreased the As/Fe ratio and the abundance of Fe(III) reducing bacteria by 57%-88% and 24%-64%, respectively, in Fe-modified rice hull BC-flooded and Fe-modified rice hull BC-RBC-intermittent treatments compared to the conventional flooded treatment. Fe-modified rice hull BC-intermittent treatment has also significantly (p ≤ 0.05) reduced the accumulation of As in rice roots, shoots, husks, and unpolished rice grains by 62%, 37%, 79%, and 59%, respectively, compared to the conventional flooded treatment. The steady release of Si and Fe to paddy pore water, promoting Fe plaque formation, adsorption of As species onto Fe-modified rice hull BC could contribute to a decreased As accumulation in rice tissues under Fe-modified rice hull BC-intermittent treatment compared to other treatments. Overall, this PhD research paves the way to a sustainable-integrated approach of pristine/modified rice hull BC-intermittent water supply management strategy that can be adapted for rice grown in As-contaminated paddy rice soils. The results of this approach would be to improve the quality of rice, to increase rice yield for the demand of ever-increasing populations worldwide.