Simultaneous Decrease Of As And Cd Availability In Paddy Field And Its Associated Mechanisms By Water Management Coupling With Passivation

The present and future challenges of arsenic（As）and cadmium（Cd）contamination of soil represent a serious threat to sustainable agricultural productivity,food quality,and reduction of suitable land for farming resulting in food insecurity worldwide,particularly in the south and southeast Asia.Among all the crops,rice efficiently depositing Cd and As into grains has made it a major source of Cd and As in the diet,posing a direct risk to human health and livelihoods.Most of the prior studies have been proposed for the management of Cd and As in contaminated paddy soil but still not enough comprehensive strategy and knowledge gap for remediation of Cd and As in the current co-contamination situation.Although some research focused on the knowledge about the co-contamination status but still in the individual and experimental stage,not for a worldwide status,and the knowledge gap exists in the comprehensive understanding of remediation approaches.As a consequence,an effort was made to reveal the systematic review work about the advanced knowledge of global current co-contamination of As and Cd as well as factors affecting the biogeochemical behaviors of As and Cd in agricultural soils,and ways or strategies to simultaneously alleviate As and Cd over-accumulation in edible parts of agricultural food crops.Furthermore,soil ridge cultivation is a sustainable soil management strategy that might provide a variety of benefits,especially in cold waterlogged paddy fields.Soil Eh markedly improved,p H decreased and soil moisture might be altered by soil ridge cultivation.On the other hand,Fe,Mn,S,and other associated elements with As and Cd availability may also fluctuate to some extent.However,until now,little research has focused on the possible contribution of soil ridge cultivation to the mitigation of metal（loid）mobility and rice uptake in the co-contamination paddy soils.But this research is still a limitation on the grain Cd increase,especially when soil ridge cultivation is employed in paddy soils with higher Cd contents.For this reason,we further studied the soil ridge cultivation together with the passivation application decreasing Cd in soil encourages the lower grain Cd.This research study empirically demonstrated a new advantage of traditional soil ridge cultivation and a promising strategy of simultaneously decreasing As and Cd in paddy soil and showed that it could contribute to the safe production of rice even in paddy soil co-contaminated with As and Cd.（1）The review paper described a comprehensive note on the co-contamination of As and Cd in agricultural soils around the world and the risk of As and Cd accumulation in edible plant parts;explores the combination of promising methods and mechanisms that simultaneously minimize the bioavailability of As and Cd in co-contaminated paddy soil;biogeochemical behavior and its properties and their influencing factors;future research needs to eliminate the over-accumulation of contaminated As and Cd in food crops.This review also provides an update comparing different techniques for simultaneous removal of As and Cd from different contamination sites around the world.But specifically,the applicability and promotion of these innovations or guidelines depends on the location of cultivation,soil climate,soil pollution levels,and socioeconomic circumstances.For heavily polluted soils to reduce As and Cd accumulation in crop foods,we strongly recommend a combined approach such as cultivar selection,soil management,and/or irrigation with fertilizer and subsequent agricultural planning.In addition,another key strategy to control point pollution and harmful and/or input sources is due to strict regulatory guidelines and rules for paddy soil-rice systems.One more approach is that the avoiding untreated wastewater and sewage sludge,the accumulation of heavy metals in food crops may be reduced considerably.Besides,rapid and precise mapping of soil contamination is necessary to avoid the transport of metallic pollutants into the food chain in paddy fields.Therefore,this review could hardly afford to summarize some scientific research literature in order to provide a comprehensive understanding of the simultaneous remediation of co-contamination of As and Cd in paddy fields around the world.（2）Soil ridge cultivation is an established agronomic measure that has been practiced since antiquity.In the field study,we set up various soil ridge height treatments with Eh ranging from-225 to 87 m V and p H ranging from 6.3 to 4.1.Grain and porewater As and Cd significantly decreased and increased,respectively,with increasing soil ridge height.At～11 cm ridge height,p H 4.6 and Eh 43 m V,inorganic As decreased at maximum by 48%;grain DMAs was 55%lower than that of the control without soil ridge(0.15 mg kg^-1).The over-accumulation of husk dimethylarsinic acid（DMAs）induces the rice straight-head disease,which threatens rice production worldwide.Grain Cd(0.14 mg kg^-1)increased but was still below the limit(0.2 mg kg^-1)in China,and the cost of ridging is acceptable.There were definite correlations among porewater As,Cd,Fe,S,and Mn contents across various Eh and p H values.Soil ridge cultivation significantly（P<0.05）diminished the copy number of As-reducing（harboring ars C and arr A）,As-methylating（harboring ars M）,and sulfate-reducing（harboring dsr A）bacteria.Moreover,soil ridge cultivation shifted the ars M-harboring microbiota.In response to ridge height increase,the abundance of the bacterial biomarker phylum Euryachaeota declined and the families Halorubrum and Planctomyces were gradually replaced by Sandaracinus in paddy soil.Therefore,traditional paddy soil ridge cultivation could effectively mitigate rice grain i As and Cd at a low level for slightly co-contaminated paddy soils and q PCR study revealed that soil ridge cultivation significantly diminished the copy number of As reducing bacteria especially inhibit As methylation in a sustainable and cost-effective manner.（3）Soil ridge cultivation plus passivation is an important cultivation technique for simultaneous remediation of As and Cd in paddy soil.In response,two passivators such as rice straw biochar（RSB）and calcium-magnesium and phosphate（CMP）were taken into consideration.RSB and CMP are highly efficient passivation for Cd remediation because both have a large surface area and the ability to elevate p H,and Ca and Mg have an antagonistic affinity for Cd.Furthermore,as soil p H improved,RSB and CMP significantly reduced grain Cd levels.The tissue of As and Cd content in this investigation was in the following order:root>straw>husk>grain.In this study,the maximum As concentration was reduced by56%and Cd expanded by 34%for the IIyou28,and As and Cd were decreased by 62%and 0.50%for the Ruiyou 399 cultivar,separately but this varietal difference was not statistically significant（P>0.05）with the application of RSB plus ridge height treatment.Similarly,the application of Ca-Mg-P combined with ridge height treatment reduces As by 47%while increasing Cd by 36%for the IIyou28 cultivar,and reduces As by 59%while increasing Cd by 20%for the Ruiyou 399 cultivar.Despite the fact that the Cd concentration was higher than the control,these did not differ substantially（P>0.05）.We concluded the comprehensive note that combining soil ridge with treatment control As and Cd content in the soil,resulting in decreased grain As and Cd in the rice paddy soil environment.This dissertation focused on innovative ideas new knowledge gained from old agronomic practices and application of passivation to simultaneously decrease As and Cd in paddy soil.Traditional paddy soil ridge cultivation could effectively mitigate As and Cd at a low level in a cost-effective manner for the soil with low Cd content.Therefore,all observations will enlarge the knowledge of researchers on the measures to simultaneously alleviate the As and Cd co-contamination in agricultural soil in a sustainable way and it is underlying mechanisms.