| In recent years,obesity has become increasingly prominent,and calorie restriction is the key to solve obesity.Stevia(Stevia rebaudiana(Bertoni)Hemsl.)is native to South America and is widely cultivated for steviol glycosides with high sweetness and low calorie.As the demand for steviol glycosides as a natural food sweetener is continually increasing,the planting area of stevia rebaudiana also continues to expand.Although stevia has strong growth adaptability,its yield and quality vary greatly due to different regions and soil characteristic.In addition,continuous cropping obstacle is an important factor to limit high-level planting of stevia.At the current study,stevia of three main producing areas,including Jiangsu,Anhui and Xinjiang,is respectively used as the research objects to explore the correlation between yield or quality and environmental factors,investigate effects of continuous cropping and soil types on soil,stevia growth,yield and glycoside content,and reveal possible mechanisms of bio-organic fertilizers to alleviate continuous cropping obstacle.This study provides a scientific theoretical basis for the sustainable production of stevia.The results are presented as follows:(1)Based on the method of Partial least squares regression with variable importance for projection(VIP),the models of environmental factors affecting yield and quality of stevia was constructed to screen significant ones.The results showed that estimation performance of the model based on all environmental factors was better than that based only on soil factors,presenting a 11.20 % increasement in residual prediction deviation(RPD)and the greatest effect of soil alkaline phosphatase and sucrase.The soil factors affecting total glycosides were mainly alkaline phosphatase and total nitrogen.The prediction models of STV and RA were good(RPD > 2),and in this model,soil zinc and soil p H exhibited the largest VIP values.(2)After stevia cultivation,p H value of sandy loam soil was lower than that of clay soil,but nutrient content and bacterial diversity/richness of sandy loam soil were higher than that of clay.Compared with clay soil,sandy loam soil led to a higher relative abundance of Proteobacteria and a lower relative abundance of Acidobacteria.Some beneficial bacteria such as Dongia,SWB02,Chryseolinea,Bryobacter,Hirschia and Devosia were significantly enriched,while RB41,Haliangium,Ramlibacter and other bacteria that were not conducive to nutrient accumulation were less abundant.The results of redundancy analysis showed that changes in bacterial community composition were mainly driven by soil p H,organic matter and total nitrogen.(3)A total of 8 years continuous cropping of stevia in sandy loam soil for significantly increased the content of steviol glycosides in leaves.But yield of steviol glycosides was still significantly reduced because of inhibited stevia growth and reduced leaf weight induced by continuous cropping.After long-term continuous cropping,physical and chemical properties of soil became worse.Bacterial community richness did not vary in the different parts of roots,rhizosphere soil and bulk soil,but bacterial community diversity significantly did,and among them,rhizosphere soil bacterial communities are most affected by continuous cropping.The results of linear discriminant analysis effect(LEf Se)showed that the relative abundance of beneficial bacteria such as Sphingomonas,Devosia,Streptomyces and Flavobacterium in the rhizosphere soil of continuous cropping for 8 years was significantly decreased,respectively.Soil p H and nutrient content(especially organic matter)were significantly correlated with the relative abundance of dominant flora.(4)Bio-organic fertilizer rich in functional bacteria can alleviate soil deterioration caused by continuous cropping,increase soil enzyme activity and microbial biomass,significantly increase dry leaf weight and yield after continuous cropping for 2 and 3 years,and significantly increase the nutrient content and glycoside content of leaves for continuous cropping for 4 years.The correlation analysis results illustrated that stevia yield and steviol glycosides content were significantly positively correlated with soil organic matter,available phosphorus,urease,catalase,alkaline phosphatase,sucrase,microbial biomass carbon,microbial biomass nitrogen and leaf nitrogen,potassium and copper.In contrast,they were significantly negatively correlated with soil available potassium,polyphenol oxidase and leaf phosphorus,calcium,magnesium and manganese.Although the production and investment of bio-organic fertilizer applied continuous cropping for 4 years exhibited a greatest increase,it is advisable to apply bio-organic fertilizer as early as possible in production to alleviate continuous cropping obstacles.(5)Base on high-throughput sequencing and GC-TOF-MS platforms,this study combined soil microbiology with non-target metabolomics to explore how bio-organic fertilizers affect the microbial community and biological function of stevia soil microbial communities.The results showed that bio-organic fertilizers alter the structure and composition of soil bacterial and fungal communities.Beneficial bacteria such as Sphingomonas,Terrimonas,Bryobacter,and fungi including Aspergillus,Botryotrichum,and Sarocladium,which could improve stress resistance by producing secondary metabolites,were enriched after fertilization.Fungal function prediction showed the increased biosynthesis of fatty acids and lipids after application of bioorganic fertilizers.Metabolomic analysis showed bio-organic fertilizers changed the metabolic profiles of soils with different-year continuous cropping,and identified differential metabolites were mainly lipids and organic acid compounds.KEGG enrichment analysis showed that pathways of unsaturated fatty acids,valine,leucine and isoleucine were enriched after bio-organic fertilizer application that is consistent with the results of microbial enrichment analysis.Moreover,the correlation analysis further confirmed that microorganisms and metabolites are closely related to the accumulation of metabolites.In summary,environmental factors,especially soil factors,affected stevia yield and steviol glycosides content,and sandy loam soil was suitable for stevia growth.Long-term continuous cropping led to deterioration of soil characteristics,destroyed the structure of rhizosphere microbial community,and inhibited stevia growth and glycoside accumulation.Bio-organic fertilizers can improve rhizosphere microenvironment by regulating rhizosphere soil microbial community and metabolic profile that further alleviates stevia continuous cropping barriers and improves economic returns. |
