| Tea is an economic shrubby plant in tropical and subtropical regions of the world.China is the largest tea producing country.However,tea plant performance such as growth,quality and yield decrease over time after the establishment of tea plants in the same soil for several years.Therefore,addressing the indirect allelopathic problems related to long-term consecutively monocu lture tea fields are very critical to tea industry.In this study,field’s trials were conducted in tea garden consecutively ratooned and monocultured for 20-30 years compared with fresh 2-5 years and nearby uncultivated tea field.High performance liquid chromatography-electrospray ionization-mass spectrometry(HPLC-ESI-MS)was performed to identify and quantify the active allelochemicals in root exudates and leaves litters.High-throughput sequencing was used to determine the structural dynamics of root-associated bacterial communities.Quntatative PCR was used to confirm our hightroughput sequence results.Redundancy analysis(RDA)and in-vitro interactions were used to study the correlation among microorganisms,physio-chemical properties and allelochemicals in rhizosphere soil.When compared to a new tea field planted 2-5 years ago,the tea field which was consecutively monocultured for 20-30-years,showed ratooning/regeneration problems such as poor growth,chlorosis and wilting.Moreover,the quality of tea leaves was significantly lower in the consecutively monocultured tea field than in the newly planted tea field.New tea sprout’s length,content of the third leaf’s Chlorophyll,net photosynthetic rate(Pn),hundred-bud weight and yield of dry tea leaves were significantly low under consecutive monoculture system.Various factors,which contribute to continuous monoculture problems such as physiochemical properties of soil,allelopathy,microbial communities and their correlated mechanisms,were the main focus of our study.Although soil physiochemical properties depicted no significant differences of nutrients,but pH of tea garden was significantly decreased with increasing planting age.Moreover,long-term tea cultivation resulted in accumulation of allelochemicals in the rhizosphere including Epigallocatechin Gallate(EGCG),Epigallocatechin(EGC),Epicatechin gallate(ECG),Catechin(±C),Epicatechin(EC),Protocatechuic Acid(PCA)and Taxifolin(TF)in the form of root exudates and leaves litters.Meanwhile,phosphomonoesterase,sucrase and urease activity were significantly declined.However,the activities of catalase and polyphenol oxidase were highly increased in long-term 20-30-years consecutively monocultured tea field.Moreover,distinct distribution patterns of catechins degrading bacterial taxa were observed in all three rhizo-compartments(i.e,rhizosphere,rhizoplane and endorhizosphere)of 30-year monoculture tea mediated strongly by soil pH and catechin containing compounds.Redundancy analysis(RDA)depicted that the abundance of most bacterial phylum Proteobacteria and Bacteroidetes had a strong positive association with protocatechuic acid(PCA),taxifolin(TF),epicatechin(EC).Pyrosequencing results identified Pseudomonas and Burkholderia as the dominant groups at genus level while plant growth promoting bacteria such as Bacillus,Sphingomonas,and Prevotella were significantly lowered in long term monoculture degraded tea plantation.The qPCR results also confirmed that the whole bacterial contents in per gram degraded 30-year tea plants soil was significantly lowered as compared to fresh soil tea plantations.However,Pseudomonas population was significantly increased with increasing tea plantation.In-vitro study indicated that with increasing the concentration of these allelochemical,the growth of catechin degrading bacteria such as Pseudomonas was increased but the growth promoting bacteria such as Bacillus was significantly decreased.Furthermore,invitro-interaction results showed that the Pseudomonas genera are involved in the degradation of catechins into more phytotoxic allelochemicals such as EC and PCA.The pH of the broth with the accumulation of PCA was also decreased to 0.36-fold after 72hours.In conclusion,the dynamic shift of beneficial microbes and the increase of catechin degrading bacteria such as Pseudomonas and Burkholderia across a long-term monocultured 30-year tea plantation were related to catechins substrates from root exudates and leaves litters.Moreover,soil pH changed with the changing level of root exudates and leaves litters,ultimately causing a decline in plant growth promoting bacteria such as Bacillus,Sphingomonas,and Prevotella with continuous monocropping.The shift of beneficial microbes and the accumulation of active allelochemicals may be the cause of acidification,soil sickness and regeneration problem which may attribute to hindrance of plant growth,development and low yield in long-term monocultured 30-year tea plantation.These results helped to explore the reason why soil quality,fertility and pH are disturbed in continuous ratooning tea monocropping tea system,and to clarify the associated problems.Large quantity of chemical fertilizer application,root exudates and leaves litters contamination in long-term continuously ratooned and monoculture tea orchard can inevitably lead to soil acidification and fertility decline,which impact on the yield and quality of tea.Therefore,the restoration of soil fertility and the sustainable development of tea planting by organic fertilizers are critical for tea industry as well as for human health.In restoration study,field trials were conducted in tea garden continuously ratooned and monoculture for 20-30 years.Urea-based nitrogen fertilizer(NF),legume forage-based green manure(LF)and goat manure-based organic fertilizer(GM)were applied to determine their effect on acidity,soil fertility,microbial activity and community structure in tea rhizosphere soil and to solve the problem of shortage of winter pasture in the south region of China.Our main results showed that the green and goat manure applications attributed to the maintenance and improvements of pH,enhancing available nutrients status,improving the secretion of low molecular weight organic acids as well as balancing community structure in degraded 20-30 years tea orchard.Finally,the physiological traits of the new sprouts and the biochemical components of new tea leaves have statistically proven that intercropping tea crop with legume forage and browsing of tea plants by goat’s manures is good agriculture management practice,helping to promote plant growth,yield and increase nutrients in tea orchard as well as can also solved shortage of winter pasture in the south region of China. |
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