| With the rapid development of large-scale breeding industry,biogas project has become an important method for the treatment of livestock and poultry manure.The agricultural benefits of the use of fermented product biogas slurry in the field and the engineering benefits of rapid resource treatment make this method widely used.The composition of the biogas slurry makes the benefits and risks of its return to the field coexist.On the one hand,it is feasible to return biogas slurry to the field,which can improve crop yield and quality,improve soil structure and so on.On the other hand,the bearing capacity of soil system to biogas slurry is limited to a certain extent.Unreasonable,excessive and long-term application will cause certain pollution risks to soil and water environment.At the same time,there are differences in soil types,raw materials for fermentation of biogas slurry,climatic conditions,and types of crops,which makes it difficult to effectively apply the amount of biogas slurry that has been studied to the field.In this study,the purple soil and pig farm biogas widely distributed in the southwest were selected for field return research.The soil column simulation experiment of periodic irrigation was used to regularly sample and monitor the relevant indicators of soil and leachate.Pot experiment was used to monitor crop biomass and quality.It was used to explore the effect of high load biogas slurry on nutrient accumulation and leaching,heavy metal accumulation,soil microorganism and crops planting in purple soil.Finally,the bearing capacity of high load biogas slurry on purple soil was obtained.The main results are as follows:(1)According to the soil column experiment of periodic biogas slurry irrigation,with the increase of soil depth,the pH value of each treatment soil gradually increased.Although the excessive application of biogas slurry and compound fertilizer will make the surface soil acidified,the application of biogas slurry will make the acidification occur later and lighter.The application of biogas slurry with equal amount of nitrogen could increase or stabilize the pH value of soil in the early stage and delay the occurrence of acidification.Irrigation biogas slurry could increase the organic matter content of the soil by about 2.64%～15.56%,and the topsoil generally increased first and then decreased with the increase of irrigation time,and the middle and lower layers of soil increased greatly in the middle and late irrigation.Fertilization could significantly increase the total nitrogen and total phosphorus content of the topsoil.With the increase of soil depth,the total nitrogen and total phosphorus content of each treatment decreased.As the number of irrigation days increased,nutrients in the fertilization treatment gradually migrated downward,the total nitrogen in the middle and lower layers increased to a certain extent,and the total phosphorus content of the soil increased less.Long-term or excessive fertilization will threaten the groundwater,and the application of compound fertilizers will more easily cause downward migration and leaching of nitrogen.Biogas slurry could alleviate migration and leaching to a certain extent,and the effect of a small amount of multiple irrigation was better.(2)Through periodic biogas slurry irrigation soil column tests,it was found that the soil has a certain buffering,purification,and bearing capacity for biogas slurry.After application of biogas slurry,the soil nitrogen leaching loss was reduced by about0.13%～10.34%compared with compound fertilizer.A small amount of multiple irrigation biogas slurry would further reduce the amount of nitrogen leaching by0.08%～19.78%.The utilization ratio of soil nitrogen was higher after biogas slurry application than that of compound fertilizer.A small amount of multiple irrigation biogas slurry could delay the migration of nitrogen to groundwater and reduce the risk of nitrogen leaching.(3)High load biogas slurry return to the field could increase the Cu and Zn contents of the top soil respectively by 7.42%～8.07%and 10.82%～14.05%,and promoted its downward migration,and the increase was 0.34%～0.99%,3.33%～6.56%higher than that of compound fertilizer,respectively.The content of Cu and Zn in each layer of soil did not exceed the screening value of copper and zinc pollution risk of agricultural land soil in the Standard for Soil Environmental Quality and Risk Control of Agricultural Land Pollution(Trial)(GB 15618-2018).However,the soil for long-term biogas slurry return to the field should pay attention to the risk of Cu and Zn pollution and its bioavailability to determine the safe application period and risk prevention measures.(4)The total abundance of Proteobacteria,Acidobacteria and Actinobacteria in soil was more than 60%.The application of biogas slurry and compound fertilizer changed the abundance,diversity and community structure of microorganisms in the soil.Planting crops(ryegrass)could improve the diversity change caused by fertilization.Different kinds of fertilization had different effects on soil microbial community,resulting in their own unique species.Biogas slurry treatment had less effect on soil microbial community than compound fertilizer.The application of biogas slurry could significantly reduce the abundance of Acidobacteria in the soil,significantly increase the abundance of Proteobacteria,Actinobacteria,and Patescibacteria in the soil,but the degree of fluctuation was lower than that of compound fertilizer.The application of compound fertilizer or biogas slurry could significantly increase the abundance of soil Gemmatimonadetes,but the difference in Gemmatimonadetes between different fertilizers was not significant.(5)With the increase of the application amount of biogas slurry,the seed germination was promoted first and then inhibited.The direct soaking of biogas slurry could promote the germination of seeds more than the application of biogas slurry in the soil at low concentration(117.5 kgN/hm~2),and the inhibition was stronger at high concentration(167.5 kgN/hm~2).The biomass of pakchoi and radish increased by0.17～1.75 and 0.49～2.58 times respectively when biogas slurry was returned to the field.With the increase of fertilizer application,the nitrate content in pakchoi and radish leaves gradually increased.Because of the proportion of high ammonium nitrogen,containing a variety of trace elements and a large amount of organic matter,the biogas slurry was more effective to reduce the nitrate content in vegetables compared with the compound fertilizer.Biogas slurry could promote chlorophyll increase of pakchoi and radish more than compound fertilizer.The increase mainly depended on chlorophyll a.After excessive fertilization,chlorophyll decreased.(6)The suitable bearing capacity of purple soil for biogas slurry returning to the field under high load was 1188 kgN/hm~2.After planting ryegrass purple,the bearing capacity increased to 1296 kgN/hm~2.Phosphorus was a slow and continuous accumulation process in the soil.The long-term return of biogas slurry to the field should pay attention to the risk of phosphorus leaching.Limited to time,the bearing capacity is only the result of the simulation test,which needs to be corrected in combination with the results of further field tests.At the same time,it can further explore the correlation between soil environmental factors and biogas slurry components on microorganisms after the biogas slurry is returned to the field,and identify microorganisms that may affect the ecological balance of the soil. |
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