| Alfalfa(Medicago sativa L.)has the advantages of high yield,high feed value and excellent ecological value.Auto-toxicity is one of the main limiting factors for alfalfa cultivation,which seriously reduces the seed germination,seedling growth,forage yield and quality,leading to the deterioration of the soil environment and the increasing of incidence of diseases.Rotation could effectively reduce the auto-toxicity effect as well as improve soil nutrients,microbial diversity and control of soil-borne diseases.At present,the mechanism of auto-toxicity of alfalfa and wheat/maize rotation is still unclear.In this study,the contents of phenolic acids(feric acid,p-hydroxytoluene,caffeic acid,chlorogenic acid and coumarin)under different planting years of alfalfa(1,4,5,6 and 7a)were determined,and the effects of soil extract and exogenous phenolic acids on seed germination were analyzed to verify the auto-toxicity of alfalfa and to select the most suitable tillage years.After that,rotation experiment,including 5a alfalfa-wheat(AW),5a alfalfa-corn(alfalfa-corn,AC),5a alfalfa-wheat-wheat,(alfalfa-wheat-wheat,abbreviation AWW),5a alfalfa-corn-corn(ACC),was carried out using 5a alfalfa grassland as control(CK)to explore the improvement effects of rotation crop on the contents of autotoxic substances,soil nutrients,ecological factors such as quantity and community structure of microbes in soil planted alfalfa.Meanwhile,the relationship between the mitigation dynamics of phenolic acids and soil micro-ecological factors under rotation conditions was explored,and the following main results were obtained.1.The auto-toxicity intensity of alfalfa was related to the planting years.The soil extract of 1a alfalfa promoted the seed germination rate and seedling growth,while the 5a alfalfa soil extract inhibited the seed germination and seedling growth,showing obvious auto-toxic effect.The contents of p-hydroxybenzoic acid,coumarin,chlorogenic acid,caffeic acid and ferulic acid in 5a soil were 1.2,3.4,2.3,2.5 and 2.0 times higher than those in 1a soil(P < 0.05),respectively.The auto-toxicity increased with the increase of alfalfa planting years and concentration.The 7a soil had the strongest auto-toxic effect,and the most suitable tillage time of alfalfa was 5a.The auto-toxic effect of chlorogenic acid,ferulic acid and p-hydroxybenzoic acid showed the tendency of “low concentration promotion,high concentration inhibition”,and the auto-toxic effect on roots was stronger than seed germination.2.Alfalfa and wheat\maize rotation slowed the auto-toxicity and promoted seed germination and seedling growth.There were two modes of the detoxification effect.One mode was the superimposed effect of crop rotation and rotation years,that is,under the same soil extract concentration treatment,the seed germination rate of rotation treatment was higher than that of CK(P<0.05).The order in the alleviation of self-toxicity was AWW > ACC > AW > AC,AWW showed the best alleviation of self-toxic effect of 5a alfalfa.Another mode was the concentration effect-the effect of soil extract on the germ and radicle of alfalfa with the increasing of the concentration of soil extract,and the lengths of embryo and radicle in rotation treatment were higher than those in CK(P<0.05).The contents of soluble sugar and the soluble protein in alfalfa seedlings treated with each rotation mode were significantly increased(P<0.05).3.Alfalfa rotation of wheat\maize could significantly reduce the contents of self-toxic substances as well as improve soil microbial population structure and soil nutrient utilization rate.The total contents of auto-toxic substances in wheat/corn soil was lower than that in continuous cropping soil(P<0.05).The total amount of AC and AW in the continuous cropping soil decreased by 17.90% and 27.50%,respectively,the ACC decreased by 16.24% compared with AC,AWW decreased by 24.18% compared with AW,suggesting the effect of rotation on the self-toxic effect of alfalfa;the order of the amount of soil bacteria was AWW > AW > ACC > AC,which increased by 138.04%,69.00%,42.62% and 34.75 compared with continuous cropping soil,respectively.The order of the amount of soil fungi was AC > ACC > AW > AWW,and the number of fungi in AC,ACC,AW and AWW treatments decreased by 74.91%,66.72%,65.19% and 48.96%,respectively.4.Using Illumina MiSeq high-throughput method,the study of soil microbial diversity under different rotation patterns showed that alfalfa-wheat/maize rotation increased soil bacterial diversity and reduced soil fungal diversity.Under alfalfa-crop rotation conditions,the dominant bacteria were Proteobacteria,Actinobacteria and Firmicutes.The dominant fungi of the fungus were Basidiomycota and Ascomycota.Alfalfa-wheat\maize rotation increased relative content of Proteobacteria,Acidobacter,Gliophage,Rozellomycota,Glomus and Ciliophora,and reduced the relative content of Actinomycetes,thick-walled bacteria,Bacteroidetes,Fusarium,Bufosporium,Basidiomycetes and Ascomycete.The order of soil bacterial diversity was ACC >AWW> AW > AC>CK,and the order of fungal diversity was CK>AWW>AC>ACC>AW.5.Through typical correlation analysis,the number of soil bacteria and fungi was significantly correlated with the contents of five auto-toxic substances.The number of actinomycetes was significantly correlated with the amount of chlorogenic acid,p-hydroxybenzoic acid,coumarin content and total amount of self-toxic substances.These results indicated that soil microbes could regulate the content of self-toxic substances and reduce the concentration or release rate of auto-toxic substances through microbial metabolism,thereby regulating the duration and effect of self-toxicity.Soil nutrients had no significant effect on soil auto-toxic substances. |
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