Effects Of Biogas Slurry Returning Years On Soil Nutrients,heavy Metals And Microorganisms In Purple Soil

Biogas fermentation is the mainstream technology of waste treatment and resource utilization in livestock and poultry breeding.For a long time,it has been widely believed that the biogas slurry contains a large amount of nitrogen,phosphorus and organic matter needed by plants,which can be used as a good organic fertilizer to improve soil fertility and improve the quality of agricultural products.However,biogas slurry returning to the field is a"double-edged sword",if not used properly,it will become a hidden danger to the ecological environment.Nitrogen,phosphorus and other elements are important factors leading to eutrophication of water.Heavy metals such As copper（Cu）,zinc（Zn）,cadmium（Cd）and arsenic（As）enriched in biogas slurry are even more threatening to soil environmental quality and the safety and quality of agricultural products.Therefore,the influence of different biogas slurry returning years on soil ecological environment is worth discussing.In this paper,Bishan is taken as the research area,and purple soil widely distributed in southwest China is taken as the research object.Vegesfield soil and orchard soil of similar size are selected after 1,3 and 8 years of biogas slurry irrigation and returned to the field.Meanwhile,the surrounding plots without biogas slurry are selected as the control.According to the principle of snake-like distribution,5 evenly distributed points were selected in each block.（1）With the increase of irrigation time,the p H value of each soil layer increased first and then decreased,and the p H value reached the maximum after irrigation for 1a.The organic matter content of vegetable soil increased with the increase of biogas slurry irrigation years.The soil organic matter content of vegetable field at 0-40 cm was the best improvement effect.Soil organic matter content of orchard increased significantly from 0 cm to 100 cm under biogas slurry irrigation.The contents of total nitrogen,nitrate nitrogen,total phosphorus,available phosphorus and total potassium in 0-20cm vegetable soil were positively correlated with the irrigation time of biogas slurry.（2）The maximum Cu content in 0～20 cm soil after 8 years of irrigation was40.92 mg·kg^-1 in vegetable soil.The soil of orchard was 25.56 mg·kg^-1.The Cu content in orchard soil was lower than that in vegetable soil.The influence of biogas slurry irrigation on the soil Zn content of vegetable field and orchard was mainly reflected in the range of 0～40 cm.From 0 cm to 60 cm,the soil Cd content of vegetable field increased firstly,then decreased and then increased with the increase of irrigation years.In the orchard soil,Cd reached the maximum value at 20～40 cm after 0 and 3 years of irrigation.In 0～20 cm,the soil As content of the vegetable field was the highest after 1 year of irrigation.The maximum As content in orchard soil was 8 years after irrigation.（3）At the distance of 0～40 cm,the soil urease activity of vegetable field increased with the increase of biogas slurry irrigation years.Irrigation for 8 a significantly increased the activities of soil urease and sucrase in 0～20cm orchard.Irrigation for 1 year significantly increased the activity of sucrase in 40～60 cm orchard soil.Biogas slurry irrigation could significantly increase the soil acid phosphatase activity at 0～60 cm.（4）One year of biogas slurry irrigation can improve the diversity and richness of soil microorganisms;When the irrigation time reached 8 years,the diversity and richness of soil microorganisms in vegetable field were lower than those in control.The diversity and richness of microbial community in orchard soil increased first,then decreased and then increased with the increase of irrigation years,and the diversity and richness of microbial community were the lowest for 3 years of irrigation.Principal component analysis showed that the microbial community structure was significantly different after biogas slurry application.After applying biogas slurry,the abundance of Actinobacteriota and Chloroflexi in vegetable soil increased year by year with the increase of biogas slurry irrigation years.The relative abundance of Proteobacteria increased first and then decreased slightly.The relative abundance of Acidobacteriota decreased.The relative abundance of Chloroflexi in orchard soil first increased and then decreased with the increase of biogas slurry irrigation years.The relative abundance of Acidobacteriota decreased with irrigation time longer than 1 year.（5）Soil microbial community structure was significantly affected by soil organic matter,total nitrogen,total phosphorus and total potassium in vegetable plots irrigated for 3 and 8 years.There was a significant positive correlation between the relative abundance of Patescibacteria and total potassium in orchard soil.Soil microbial community structure of vegetable field after 8 years of irrigation was mainly related to the contents of Zn and Cu.There were obvious synergistic effects among Cu,Zn,Cd and As in orchard soil.Zn had the greatest effect on soil microbial community structure after 8 years of irrigation.In vegetable soil,the relative abundance of Cu and Actinobacteriota was significantly positively correlated.The activities of soil urease,soil sucrase and soil acid phosphatase had important effects on soil microbial community structure of vegetable field after 3 and 8 years of irrigation.Soil microbial community structure was mainly affected by soil urease activity after 1 and 8 years of irrigation.Soil sucrase activity was significantly positively correlated with the relative abundance of WPS-2 and Cyanobacteria in vegetable plots.