Enhancement Mechanism Of Freeze-thaw Aging On Biochar Remediation Of DEHP Pollution In Black Soil

The pollution of phthalates(PAEs)in farmland soil is becoming more and more serious.PAEs not only have a negative impact on the growth of crops,but also can enter the human body through the food chain,posing a potential threat to human health.Di-(2-ethylhexyl)phthalate(DEHP)is one of the most common and refractory PAEs due to its unique properties.Therefore,it is particularly important to find an economical and effective way to reduce the accumulation of DEHP in crops and achieve food safety production.In this paper,fresh biochar(BC)was prepared from corn straw,and freezing and thawing aging biochar(FBC)was prepared by freezing and thawing cycle treatment with BC to simulate winter temperature in Northeast black soil region.Pot experiments were conducted to study the effects of freezethaw aging on the detoxification efficiency of Chinese cabbage in biochar enhanced DEHP contaminated black soil,the improvement of soil physical and chemical properties,the regulation of soil microbial community structure and functional genes,and to reveal the mechanism of freeze-thaw aging process on biochar enhanced black soil DEHP contaminated black soil.The specific results of this study are as follows:(1)Freeze-thaw cycle aging can help detoxify pakchoi under DEHP stress by biochar.Compared with CK treatment group,the contents of dry weight,total nitrogen,total phosphorus and total potassium of pakchoi after adding BC and FBC were increased in different degrees.The content of total phosphorus in FBC treatment group was increased most significantly,which was 57.94% higher than that in CK treatment group.These results indicated that the application of FBC and BC ensured the healthy growth and nutrient utilization of pakchoi.The results of ROS content and ROS staining in pakchoi leaves showed that BC and FBC significantly reduced ROS content in pakchoi,and reduced the risk of oxidative stress damage in pakchoi cells.The comprehensive evaluation radar analysis showed that the comprehensive evaluation value of cabbage in FBC treatment group was 1.02,and the evaluation effect was the best.The results of scanning and transmission electron microscopy of pakchoi root system showed that the root system of FBC treatment group was in the best condition,and the damage caused by DEHP stress was the lowest.After adding BC and FBC,the relative content of low molecular weight organic acids in the root exudates of pakchoi increased,which was helpful to the remediation of soil DEHP pollution.(2)Compared with CK treatment group,adding FBC increased soil organic matter content,total nitrogen content and total potassium content by 5.96%,9.64% and 5.76%,respectively.The change of soil total phosphorus content was not significant.These results indicated that FBC could promote soil nutrient content to some extent.Adding FBC significantly increased the activities of urease,β-glucosidase,dehydrogenase and polyphenol oxidase in soil.After adding FBC,soil microbial biomass carbon and microbial biomass nitrogen increased by 37.84% and 34.78%,respectively.These results indicate that freeze-thaw aging process is more helpful for biochar to improve soil quality by improving soil nutrients,soil enzyme activities and soil microbial biomass.(3)Compared with CK treatment group,soil DEHP content decreased significantly after adding FBC during the same period.This indicates that freeze-thaw aging process can help biochar improve the degradation efficiency of soil indigenous microorganisms on DEHP.Metagenomic analysis also showed that the addition of FBC increased soil microbial diversity and improved soil microbial community structure.Based on KEGG database,the effects of FBC on the abundance of DEHP functional genes degraded by indigenous microorganisms were explored.The results showed that the relative abundance of esterase,lipase,carboxylesterase and cytochrome P450 genes degraded upstream of DEHP increased from 527.38 ppm,144.95 ppm,132.62 ppm and 77.10 ppm to 644.45 ppm and 226.92 ppm,respectively ppm,145.23 ppm,and 99.80 ppm.FBC also promoted the metabolism of degraded phthalic acid(PA)in the downstream of soil DEHP.(4)Based on KEGG database,the effects of FBC on soil function(nitrogen cycle)of typical facility farmland were further explored.The results showed that the addition of FBC promoted the up-regulation of the gene abundance of nitrogen-fixing genes nif D,nitrification genes amo A and amo B,and decreased the gene abundance of denitrification genes nar H and nor B,suggesting that FBC promoted the nitrogen fixation and nitrification of soil microorganisms in DEHP contaminated soil,and inhibited the denitrification effect.This is of great significance for reducing soil nitrogen loss and soil nitrogen cycling.In conclusion,freeze-thaw aging process is more conducive to biochar to ensure the healthy growth of crops in DEHP-contaminated soil,improve soil quality,promote the degradation of DEHP in soil,and improve soil nitrogen cycling.This study provides a theoretical basis for the long-term remediation of DEHP-contaminated soil biochar in high latitude areas.