| The substitution and reduction of chemical fertilizer is the national strategic demand for the high-quality development of agriculture and environment during the 14th Five-Year Plan period in China.Biogas slurry,a by-product of the anaerobic fermentation of livestock and poultry manure,which is rich in nitrogen and various nutrients need by crops growth,can be regarded as a liquid organic fertilizer and replace chemical fertilizers for sustainable agriculture under circular economy in China.However,due to the biogas slurry contains trace heavy metals,long-term and excessive application may increase the potential ecological risk of heavy metals in the soil.Biochar,a carbon-rich porous material created by biomass in hypoxia under the condition of high temperature pyrolysis,which has high specific surface area,porous and rich oxygen containing functional groups such as characteristic,can reduce availability of soil heavy metal by adsorption,chelating.At present,it has been the most common passivator to remediate farmland heavy metal contaminated soil.Numerous studies on the effect of biogas slurry application on heavy metals of soil or grain,but there were few researches about soil heavy metal in content-bioavailability-crop roots-aerial part-grain,moreover what extent biochar effect on heavy metal of farmland fertilized with different dosage of biogas slurry and chemical fertilizer substitution by biogas slurry remains unclear.In present paper,the biogas slurry(CSTR process)produced by large-scale farms of COFCO Jia Jia Kang(Jiangsu)Co.,Ltd.was taken as an example to carry out field experiments in coastal saline soil agricultural area of Jiangsu Province.The experiment set:(1)0(no fertilization)and low(recommended N amount applied in local farmland,:250 m~3/ha for rice season,150m~3/ha for wheat season),medium and high(local actual N application range,500-750 m~3/ha for rice season,300-450 m~3/ha for wheat season),four biochar dosage levels and 0 and 15 t/ha biochar dosage,a total of eight treatments;(2)under the premise of maintaining the same total amount of N(225 kg/ha in paddy field and 150 kg/ha in wheat field),four biogas slurry levels(0,33%,66%and 100%)were set to replace chemical fertilizer,two biochar amounts(0 and 15 t/ha)were used for 8 treatments.The contents and availability of six heavy metals,including Cu,Zn,Pb,Cd,Cr and Cd were determined in 0-20 cm soil.The contents of heavy metals in roots,stems and leaves of crops and grains were analyzed simultaneously.The environmental risks of heavy metals were evaluated based on potential ecological risk index method and human health risk model(dietary exposure).The objectives of this study were:(1)to investigate the effect of chemical fertilizer substitution by biogas slurry on the availability and risk of soil heavy metals,(2)provide theoretical and practical basis for the sustainably application biogas slurry.The main results and conclusions of this thesis are as follows:(1)Under different biogas slurry dosage,whether biochar was added or not,the contents of Cu,Zn,Pb,Cd,Cr and As in the soil under low biogas slurry dosage.When the amount of biogas slurry increased to medium and high biogas slurry application,the contents of Cu and Zn in the soil of rice-wheat cropland increased by 9.0%-15.4%and 9.7%-15.0%,respectively.Therefore,when the dosage of biogas slurry more than 500 m~3/ha for rice season,more than 300 m~3/ha in wheat season,the accumulation of Cu and Zn in the soil would be lead.(2)Whether biochar was added or not,chemical fertilizer substituted by biogas slurry substituted had no significant effects on the contents of Cu,Zn,Pb,Cd and As in soil of rice-wheat field,the content of Cr in the soil of wheat field increased by 9.2%-16.8%under100%chemical fertilizer substituted by biogas slurry.(3)The availability of heavy metals in soil as follows:before adding biochar,application of low amount of biogas slurry had no significant effect on the mass fractions of Cu,Zn,Pb,Cd,Cr and As in weakly acid extracted state(F1)in rice-wheat field soil,but the mass fractions of Cu,Zn and Pb-F1 in rice-wheat field soil were significantly increased under medium and high biogas slurry application(P<0.05).After adding biochar,the contents of Cu and Pb in rice and wheat soil decreased significantly under medium and high biogas slurry dosage.In conclusion,when the biogas slurry applied in paddy field was 500-750 m~3/ha,and the biogas slurry applied in wheat field was 300-450 m~3/ha,the bioavailability of Cu,Zn and Pb in the soil of rice-wheat field was significantly increased,and the addition of biochar could reduce the bioavailability of Cu and Pb.(4)Before adding biochar,biogas slurry replacing chemical fertilizer(33%-100%)significantly reduced soil Pb and F1 mass fraction in rice-wheat field(P<0.05).After biochar was added,the mass fraction of Pb-F1 in rice-wheat farmland decreased significantly when biogas slurry replaced chemical fertilizer with 33%,and there was no significant difference in the F1 mass fraction of six heavy metals in rice and wheat fields when biogas slurry replaced chemical fertilizer with 100%.(5)Before adding biochar,the contents of Cu in rice and wheat grains increased by10.8%-116.5%under three biogas slurry dosages,while the contents of Pb and Cr in wheat grains decreased by 33.7%-112.5%.After adding biochar,the Cu content in rice and wheat grains under medium and high biogas slurry dosage decreased by 9.8%-37.5%compared with that before adding biochar.Although Cu content in rice and wheat grains increased under different biogas slurry dosages,it was lower than the standard limit of contaminant in food(GB 2762-2017).(6)Whether biochar was added or not,biogas slurry replacing chemical fertilizer(33%-100%)could reduce Pb content in rice and wheat grains,and the decrease was19.4%-39.9%in rice grains and 6.3%-27.0%in wheat grains.(7)Cu,Pb,Cd,Cr and As were all enriched in crop roots with or without biochar.The difference is that Zn is enriched in stem and leaf in rice,while it is enriched in grain in wheat.(8)The evaluation results of potential ecological risk index method showed that the risk degree of soil heavy metal pollution under different dosage of biogas slurry and biogas slurry replacing chemical fertilizer was slight.Among the six heavy metals,Pb has a relatively high pollution risk.In comparison,the results of human health risk assessment(dietary exposure)showed that the rice and wheat grains of each treatment did not threaten human health.However,due to the relatively high pollution risk entropy values of As and Zn,sufficient attention should be paid to the safety assessment of grains in the future.In conclusion,the chemical fertilizer substitution by biogas slurry does not cause the pollution of soil heavy metal in coastal rice and wheat fields,and the rice and wheat grains are all at a safe level.Therefore,it is recommended that 100%chemical fertilizer should be substituted by biogas slurry(225 kg·N/ha for rice field and 150 kg·N/ha for wheat field).From the perspective of heavy metal pollution control,the addition of biochar can significantly reduce the bioavailability of Cu and Pb. |
