Study On Leaching Characteristics Of Magnesium And Regulation Of Magnesium Deficiency In Citrus Orchard

With the rapid development of global citrus industry,citrus has become the fruit crop with the highest yield and the widest planting area in the world.Among them,China ranks first in the world with a planting area of 2.87 million ha and total citrus output of 51 million tons.However,there are still some problems in the process of citrus production in China,such as soil acidification,imbalance of fertilizer input and neglect of the application of medium and trace elements.Magnesium deficiency in citrus orchards has increasingly become an important limiting factor for sustainable citrus production.Magnesium（Mg）is an indispensable nutrient element in the process of plant growth.Under the combined action of factors such as low Mg content of soil parent material,large rainfall,easy leaching of magnesium and ion antagonism in the main citrus producing areas in south China,and Mg leaching loss may be an important link leading to Mg deficiency in citrus orchards.Therefore,clarifying the characteristics of Mg leaching loss in citrus orchards and establishing Mg deficiency control measures based on this can not only provide new ideas for solving the general Mg deficiency in citrus producing areas in China,but also provide technical support for the rational application of Mg fertilizer in citrus orchards.Therefore,this study took citrus orchards in southwest area Meishan City,Danling County with typical acid and Mg deficiency soil as the research object,quantified the characteristics of Mg leaching loss through different Mg fertilizer dosage experiments,and explored the best Mg fertilizer dosage in citrus orchards;Then,the field application effects of different types of Mg fertilizer were compared,the most suitable types of Mg fertilizer were selected,and the physiological mechanism of Mg application to improve Mg nutrition in citrus leaves was discussed;Finally,by establishing the fertilization mode of optimizing the input of nitrogen,phosphorus and potassium nutrients and increasing the application of Mg fertilizer,this study explores its comprehensive impact on the growth,yield,quality and environmental cost of citrus,in order to provide theoretical basis and technical support for the application of Mg fertilizer in citrus orchard.The main results of this study are as follows:（1）The leaching was the main way of Mg loss in citrus orchards.During the experiment period,the annual average total amount of Mg loss（calculated as pure Mg）under different Mg fertilizer amount treatments was 65.5 kg ha^-1,73.5 kg ha^-1,76 kg ha^-1 and 91.5 kg ha^-1,respectively.And then,the annual average Mg loss rates of Mg fertilization treatments were 29.5%,19.3%and 23.7%,respectively.The application of Mg fertilizer significantly increased the yield of citrus.Compared with no Mg application（Mg₀）treatment,the average yield of citrus increased by 4.77 t ha^-1.At the same time,Mg fertilization could effectively improve fruit quality and promote fruit color transformation.The reason is that Mg fertilizer application can effectively balance soil nutrients and improve citrus tree Mg nutrition.To sum,in the Mg0 and low Mg fertilization(Mg₄₅)and Mg₉₀ treatment,the soil Mg pool is deficiency due to leaching loss and fruit removal.Therefore,the application amount of Mg fertilizer in acid soil citrus orchard should be greater than 90 kg Mg O ha^-1.（2）In different Mg fertilizer field expeirment,the Mg fertilization treatments increased the citrus yield by 5.90 t ha^-1 on average,and improved the fruit quality,leaf Mg nutrients and soil nutrient.The application effect of the four types of Mg fertilizer in acid soil citrus orchards was magnesium carbonate（Mg CO₃）>magnesium oxide（Mg O）/modified magnesium sulfate（M-Mg SO₄）>magnesium sulfate（Mg SO₄）.Therefore,Mg CO₃ was preferentially recommended in acid soil citrus orchard in this area to achieve acid reduction,Mg supplement,high yield and high quality.The important reason of applying Mg fertilizer（Mg CO₃）to increase production and improve quality is that it changes the Mg nutrition and related metabolic processes in new and old leaves at the physiological,transcriptional and metabolic levels.Under the Mg deficiency in citrus orchards,the concentration of Mg in citrus leaves was decreased,and the content of sugar was accumulated in source leaves.There were 5831 differentially expressed genes（DEGs）in the transcriptome under four control treatment groups,3091 up-regulated DEGs,2740 down-regulated DEGs and 23 common DEGs.A total of 748differential metabolites（DMs）were found in the metabolic group,of which 54 were metabolic DMs.At the same time,KEGG pathways such as carbon sequestration,galactose metabolism,wax and cutin synthesis and glycolysis/gluconeogenesis of photosynthetic organisms in leaves were affected,while citrus leaves increased thickness and flavonoid synthesis to deal with Mg nutrient deficiency.In the new and old leaves of citrus,it is significantly enriched in the ABC transporter enrichment pathway,indicating the reuse and transport of Mg in the new and old leaves of citrus.（3）Compared with the conventional fertilization treatment of farmers,the optimized fertilization treatment reduces the amount of chemical fertilizer by 45%on average,which will not reduce the yield of citrus on the basis of reducing the amount of chemical fertilizer.At the same time,the Mg fertilization treatments increase the yield of citurs by 8.54%on average,and has no significant reduction trend in citrus fruit quality,fruit peel color and nutrient content.At the same time,in the process of reducing the input of nitrogen fertilizer and other chemical fertilizers,it can effectively alleviate the decrease of soil p H and the imbalance of nutrients in the soil.In the conventional fertilization treatment of farmers,the greenhouse gas emission potential,acidification potential and eutrophication potential are 207 kg CO₂-eq t^-1,2.75 kg SO₂-eq t^-1 and 1.60 kg PO₄-eq t^-1,respectively.Under the optimized fertilization treatment,the environmental costs were reduced by 35.8%,38.4%and 39.3%,respectively.While under the optimized fertilization and Mg fertilizer treatment,the environmental costs were reduced by 54.3%on average.Therefore,in Mg deficiency citrus orchards,optimizing the input of nitrogen,phosphorus and potassium nutrients and increasing the application of Mg fertilizer can jointly realize the improvement of acid soil,soil nutrient balance,tree nutrition,increase yield,improve quality and reduce environmental costs,which is conducive to the sustainable production of Mg deficiency citrus orchards in China.To sum up,the lack of Mg in citrus orchards has become an important factor limited the development of citrus industry.Combined with the leaching characteristics of Mg in citrus orchards and the verification of the application effect of Mg fertilizer,the optimal application amount of Mg fertilizer in citrus orchards and the types of Mg fertilizer suitable for citrus orchards in this area are determined.At the same time,reducing the amount of chemical fertilizer and increasing the application of Mg fertilizer can promote the yield,quality,income and environment cost of citrus.Therefore,leaching loss is the main way of Mg deficiency in citrus orchards in Mg deficient soil.In order to alleviate the problem of Mg deficiency,slow acting Mg fertilizer dominated by Mg CO₃ should be applied,which is better than water-soluble Mg fertilizer.When the input amount of Mg fertilizer should be greater than 90 kg Mg O ha^-1,it can meet the balance of soil nutrients in citrus orchards.Optimizing the input of nitrogen,phosphorus and potassium nutrients and increasing the application of Mg fertilizer can improve the yield and quality,and contribute to the green and sustainable production in Mg defiency citrus orchards.