The Status Quo Of Phosphorus Accumulation In Chongqing Citrus Orchard Soil And The Mechanism Of Efficient Utilization

Citrus is the world’s most important cash crop for fruit trees.By the end of 2018,China’s citrus production area and yield reached 3.46 million hectares and 65 million tons,respectively,ranking the first in the world.Chongqing is located in the superior core area of the citrus industry in the middle and upper reaches of the Yangtze River,and is one of the most suitable ecological areas for citrus growth in China.At present,there are some problems in China’s citrus production,such as high fertilization,low yield,and low quality.These problems will seriously restrict the sustainable development of China’s citrus industry.How to make targeted improvements to the production problems in the citrus industry is the biggest challenge facing the citrus industry.To this end,this article used a combination of investigation,field experiment and pot experiment.Through investigation and sampling analysis,the current status and existing problems of phosphorus accumulation in the citrus orchards in Chongqing were clarified.Through field positioning experiment,studied the transformation of soil phosphorus forms by combined application of organic and inorganic fertilizers.The local high-intensity phosphorus supply method was used to clear the growth of phosphorus,the absorption of phosphorus and the adaptation mechanism of roots to low phosphorus stress,and explore the efficient phosphorus application methods in citrus orchards.Finally,through field verification,it was clear that the appropriate amount of phosphorus and the method of phosphorus application for citrus.To provide theoretical and practical basis for the sustainable development of the citrus industry in Chongqing and China.The following major results were achieved:A survey and sampling analysis was conducted on five citrus producing areas in Chongqing.The results showed that the total amount of phosphorus applied in Chongqing citrus orchard is on average 303 kg/ha,which was much higher than that in the world.The orchard applied chemical fertilizer rather than organic fertilizer.The average organophosphorus input was only 26.9%of the total nutrient input,while the average inorganic phosphorus input was 221.8 kg/ha,which was higher than 400 kg/ha in some production areas.The average content of available phosphorus in citrus orchard soil was 48.27 mg/kg,the soil Olsen-P content was generally high.The surface phosphorus content in citrus orchard was about 120 mg/kg,and the phosphorus accumulation accounts for 75%.The accumulation of surface phosphorus was serious,and the space of soil phosphorus and citrus root system was not matched.The soil average Olsen-P content was 1.77times of the critical value of soil phosphorus risk.The environmental risk of the citrus orchards was greater.There were"three highs and one low"in the application of phosphate fertilizer in Chongqing Orange Garden,that is,the problems of high phosphorus application,high accumulation,high environmental risk and low fertilizer utilization rate.In view of the low carbon-to-phosphorus ratio in citrus orchards,organic fertilizers were applied to citrus orchards to increase the carbon content.The results showed that inorganic phosphorus content in citrus orchard soil is Ca-P>O-P>Fe-P>Al-P,and as the soil layer deepens,Fe-P tended to migrate downward.This showed that long-term continuous fertilization,soil phosphorus could be different degrees of downward migration,especially the application of organic fertilizers.The order of organophosphorus morphology content in each treatment was residual-P（NaOH-P_o+HCl-P_o）>（NaHCO₃-P_o+H₂O-P_o）.Combined application of organic and inorganic fertilizers increased the phosphorus content of soil soluble phosphorus（NaHCO₃-P_t and H₂O-P_t）,and the application of organic fertilizers helped to transform the phosphorus into HCl-P_o.Therefore,it indicated that the contents of Olsen-P,inorganic phosphorus and organophosphorus in soil increased to different degrees after long-term application of chemical phosphate fertilizer and organic and inorganic fertilizers,among which combined application of organic and inorganic fertilizers was more conducive to the improvement of soil phosphorus absorption and its effectiveness.The chemical action of organic matter to promote the transformation of phosphorus forms was clarified.Aiming at the problems of wide and rare root system of fruit trees and spatial dislocation of phosphorus,in this paper,two citrus varieties representing Sweet orange and Hybrid citrus seedlings were grown in specially designed split and stratified soil cultures with a low-P purple soil.The results showed that the local supply of phosphorus may affect the morphology and structure of the roots,thereby affecting phosphorus uptake.Local phosphorus supply（Vertically meet 1/3 root phosphorus needs）met citrus phosphorus requirements.There were two main strategies:increasing the number of roots in high-phosphorus areas,increasing acid phosphatase activity and mycorrhizal infection rate,so as to obtain and meet the phosphorus demand of citrus.The analysis of citrus root system found that there were more roots in the high-phosphorus areas,and the main roots were fine roots.Under low phosphorus stress,citrus grew more roots in high-phosphorus areas to increase mycorrhizal infection and phosphatase activity to obtain more phosphorus from the soil to meet the citrus growth requirements to ensure normal plant growth.Therefore,local application of phosphate fertilizer,such as strip or hole application,could reduce the fixation of phosphorus in the soil,reduce the amount of phosphate fertilizer,and improve the utilization efficiency of phosphate fertilizer,which had important theoretical and practical significance.To clarify the suitable phosphorus supply intensity and suitable phosphorus application method of citrus,pot experiment of homogeneous and heterogeneous phosphorus supply was conducted respectively.The results showed that there was a parabolic relationship between the different phosphorus supply levels and newhall navel orange biomass.The root biomass,aboveground biomass and total biomass were highest when the phosphorus supply levels were 25 mg/kg and 45mg/kg,and plant growth Stronger than other phosphorus treatment.When the phosphorus supply level was lower than 5 mg/kg or higher than 105 mg/kg,the ratio of root to shoot was increased,and the assimilation material in the aerial part was relatively small,inhibiting the growth of the aerial part.Citrus roots had the strongest root vitality and the ability to absorb water and nutrients when treated with 25 mg/kg and 45 mg/kg.The test of heterogeneous citrus phosphorus supply showed that the optimum phosphorus input in homogeneous phosphorus supply was 50 mg/kg,and excessive fertilization was not conducive to citrus growth.Phosphate fertilizer input increased,citrus roots grew,but phosphorus absorption in the aboveground did not increase significantly.Therefore,the suitable soil phosphorus supply intensity of citrus orchards should be 45-50 mg/kg,which could maximize the biomass,root vitality and phosphorus absorption of citrus seedlings.In order to clarify the high-efficiency application method of phosphorus fertilizer,the hole-type fertilization method was used in the field instead of the farmer’s customary fertilization,and the field verification test of the combined application of phosphorus reduction and organic fertilizer on the citrus orchard.The results showed that the relative yield of citrus showed a linear plus platform yield pattern.When the Olsen-P in the soil was higher than the critical value,the yield did not increase significantly,and it also caused environmental risks.The threshold range of soil available phosphorus in Chongqing orangery considering production and environment was 35.6～46.2 mg/kg,and its corresponding phosphorus application range was 156.7～240 kg/ha The citrus was treated with 50%phosphorus reduction for two years,and the yield and stored phosphorus did not decrease significantly.It could be seen that changing farmers’surface application and fertilization habits to local phosphorus supply,phosphorus reduction combined with organic fertilizer application could meet the nutrient requirements of citrus production,achieve citrus yield and phosphorus storage and farmers’habits have no significant difference,conducive to citrus production,and reduce environmental risks.In summary,this study identified the outstanding problems of phosphorus fertilizer application in Chongqing citrus orchards,that is,the“three high and one low”problems,that is,high phosphorus fertilizer application,high soil phosphorus accumulation,high phosphorus fertilizer environmental risk and low phosphate fertilizer utilization efficiency.It was clear that the most suitable soil phosphorus supply intensity for citrus is 45-50 mg/kg.Satisfying the phosphorus supply of 1/3 root system of citrus,which could meet the nutritional requirements of citrus phosphorus,and provided scientific support for"local application"instead of"full layer mixed application".Clarified the role of organic matter in promoting the transformation of phosphorus forms.It was clear that the threshold range of soil available phosphorus in Chongqing citrus orchards considering production and environment is 35.6～46.2 mg/kg,and its corresponding phosphorus application threshold is156.7～240 kg/ha.It had been verified that reducing the inorganic phosphorus by 50%based on the traditional habits of farmers can still meet the normal yield and growth requirements of citrus.In short,this study provides a very important theoretical and practical basis for the Chongqing citrus orchards phosphate fertilizer to"reduce phosphorus and increase efficiency"to achieve scientific application.