Iron Status And Rhizosphere Bacterial Community Characteristics Of Citrus In Typical Coastal Saline Alkali Land

Citrus occupies a leading position in the world's fruits,and it is also an important economic crop in South China.However,the nutrient imbalance of trees frequently happened due to the lack of scientific guidance and management,which have become the main factors restricting the improvement of fruit quality and yield.In this study,the citrus ‘Hongmeiren'from Xiangshan,Zhejiang Province was selected to investigate the nutrient status of citrus as well as the soil.We used a combination of techniques including geographic information system(GIS),16 S r RNA gene sequencing and inductively coupled plasma mass spectrometry(ICPMS)to analyze the soil causes of leaf chlorisis.Based on the soil nutrient deficiency and the spatial distribution differences,we selected the ‘Hongmeiren' on coastal saline alkali land to investigate the status and causes of its deficiency and the characteristics of its soil bacterial community.In this way,we provided scientific basis for soil nutrient improvement in citrus orchards to improve the quality of ‘Hongmeiren'.The main results are as follows:1.Soil nutrients in 62 citrus orchards and fruit nutrients in 31 citrus orchards in Xiangshan County were investigated respectively,and the spatial distribution of nutrients was predicted by Inverse Distance Weight(IDW).It showed that the soil pH of the whole county showed a polarization trend.The proportion of strong acid(pH < 4.50)and strong alkaline(pH > 7.50)citrus orchards were 32.26% and 20.97% respectively,which were located in mountain land,flat land and tideland.In addition,the tideland soil had low organic matter,total nitrogen and available iron.The content of micronutrients in ‘Hongmeiren' citrus furit was lower in tideland than that in mountain land and flat land,thus revealing its sensitivity to nutrient deficiency.By analyzing the spatial variability of nutrients in Xiangshan citrus orchards,we found a lack of available boron in Xiangshan citrus orchards and the shortage of available iron in tideland citrus orchards.Moreover,‘Hongmeiren' was more easily affected by soil nutrients than Citrus unshiu.2.The nutrient status of soil,tree body and fruit of the tideland ‘Hongmeiren' orchards was systematically analyzed to explore the nutrient deficiency status and the cause.It showed that the deficiency of iron,zinc and other micronutrients was more serious in tideland orchards.That reflected in the leaf phenotype,soil,tree body and fruit content,among which the leaf and fruit were more sensitive to nutrient deficiency.Due to the alkaline soil and high salt content in the coastal saline alkali land,although farmers applied a large amount of fertilizer to make the whole amount of micronutrient too abundant,its availability was still insufficient.In addition,long-term cultivation under mulching facilities in greenhouses aggravated soil salinization,made salt accumulate to the soil surface,and inhibited citrus to absorb nutrients such as iron.Therefore,soil properties,such as high salinity and low available nutrients,as well as agronomic measures,such as long-term cultivation with film mulching and extensive application of fertilizers,were the main causes for the deficiency of micronutrient such as iron in citrus of ‘Hongmeiren'.3.Based on the knowledge of the cyclical deficiency of citrus Fe and other micronutrients,the response of the rhizosphere and bulk bacteria communities to soil environmental factors was further studied.The results showed that soil depth and citrus root distribution influenced the vertical and horizontal physical and chemical properties of soil,thus affecting the structure of rhizosphere and bulk bacterial communities.This study revealed the law of the change of bacterial community structure with depth in the tideland soil,clarified the regulation effect of the change of environmental factors such as salt and alkali in the vertical direction on the bacterial community.Then we found some specific bacteria that have tolerance to the stress of salt and alkali in the bulk.More importantly,we identified the regulatory role of citrus roots in soil bacterial community structure,further put forward the unique ecological network structure of rhizosphere bacteria and citrus nutrient utilization of inner link,and found some specialized bacteria associated with promoting plant nutrient uptake and high iron secretion by citrus root enrichment in tideland soil.We proposed the potential role of these specialized bacteria in enhancing the activity of nutrients such as iron in citrus rhizosphere soil under salt and alkali stress.