Effects And Mechanism Of Nitrogen And Phosphorus Reduction Combined With Selenium On Growth Of Guanxi Pomelo And Soil Nitrogen Utilization

China is the fastest growing country in terms of citrus cultivation area and production,and among them,the cultivation industry of Guanxi pomelo is gradually expanding.In Pinghe County,Fujian Province,as the main production area for the cultivation of Guanxi pomelo,according to a investigation,the local nitrogen and phosphorus fertilizer dosage was as high as 1255.37 kg/hm² and 917.99 kg/hm²,leading to increasingly serious runoff and groundwater pollution in the orchards there,and a gradual decline in fruit quality and yield.In order to reduce fertilizer inputs and environmental pollution,a variety of ways can be taken to balance fertilizer application and improve fertilizer utilization efficiency,such as micronutrient matching and organic and inorganic fertilizer matching.Selenium is one of the essential nutrients for human body and a beneficial element for plant growth.In this paper,we used a questionnaire survey combined with sampling analysis to clarify the fertilizer application status and nutrient abundance and deficiency situation in Fujian Pinghe Guanxi pomelo orchard;on the basis of this,we used field trials and drenching pot experiments to study the effects of nitrogen and phosphorus reduction with selenium,organic fertilizer and chitosan on the fruit yield and quality of Guanxi pomelo,and to clarify their effects on improving yield and quality;and we also used macro-genomic technology to analysis microbial ecological The effects of selenium and chitin on the transformation,leaching,distribution and utilization of different sources of nitrogen fertilizers in the soil and their microbiological and enzymatic mechanisms were investigated in depth.The main results are as follows:（1）The orchard soil in the main producing area of Guanxi pomelo in Pinghe County,Fujian Province was acid,the utilization rate of nitrogen fertilizer was low,and the application of phosphorus fertilizer was excessive.Farmers in Guanxi pomelo main production area in Pinghe County,Fujian Province mainly apply organic fertilizer and compound fertilizer,and the application rates of nitrogen,phosphorus and potassium were1255.37 kg/hm²,917.99 kg/hm² and 1063.15 kg/hm² respectively,which is much higher than the reasonable fertilization rate of pomelo in China（N 350-450 kg/hm²,P₂O₅200-250 kg/hm²）.Among them,the average fruit yield per kilogram of nitrogen fertilizer is only 43.67 kg.90%of the orchard soil in the main Guanxi producing area of Pinghe County,Fujian Province was acidic and strongly acidic,and the average p H value was only 4.50.The alkali hydrolyzable nitrogen content of orchard soil was generally low,93.34%of orchard soil alkali hydrolyzable nitrogen content was lower than the appropriate range（100 mg/kg）,and the total nitrogen content in 60%of leaves is deficient.There are 21 orchards with extremely rich content of available phosphorus in the soil,accounting for 70%of the total orchards,with the highest content of 155.7 mg/kg.The average content of exchangeable magnesium in orchard soil was only 51.61 mg/kg and27.74 mg/kg,which were lower than the appropriate concentration range.（2）The reduction of nitrogen and phosphorus combined with the application of organic fertilizer,chitin and selenium significantly increased the fruit yield and optimized the fruit quality of Guanxi pomelo.Compared with conventional fertilization,the fruit yield of Guanxi pomelo with reduced nitrogen and phosphorus fertilization increased by 9.31%（2018）and 20.99%（2019）respectively.Among them,the highest yield of Guanxi pomelo in 2018 was reduced fertilization combined with chitin and selenium treatment,which was 96.74 kg/plant,which was 13.98%higher than the conventional fertilization;The highest fruit yield in 2019 was the treatment of nitrogen and phosphorus reduction combined with selenium application,in which the fruit yield of reduced fertilization treatment was only 35.04 kg/plant,and the fruit yield increased by 39.66%after selenium application.On the basis of conventional fertilization,nitrogen and phosphorus reduction fertilization can reduce the thickness of pericarp and improve the content of soluble solids,single fruit weight and solid acid ratio.Compared with the reduction of nitrogen and phosphorus,the solid acid ratio,vitamin C content and juice yield increased by 11.52%,10.28%and 20.82%respectively.After the restoration of customary fertilization in 2020,the fruit yield of guanxi honey pomelo treated with two years of reduced fertilization combined with chitin and selenium was significantly higher than that without exogenous substances,with an increase of 49.07%,and the single fruit weight increased by 55.58%;Soluble solid content,solid acid ratio and vitamin C content increased by 0.48%,9.06%and 37.45%respectively.（3）The combined application of chitin and selenium significantly reduced nitrogen leaching by affecting the microbial community structure and the abundance of functional bacteria related to nitrogen metabolism in soil.Using the leaching column test,the application of chitin can significantly reduce the content of nitrate nitrogen and ammonium nitrogen in the leaching solution under the condition of only applying nitrogen,phosphorus and potassium fertilizer;When NPK fertilizer was combined with chitin and selenium,the contents of ammonium nitrogen and nitrate nitrogen in the leaching solution could be significantly reduced by 51.13%and 27.04%.In the soil depth of 0-20 cm,selenium application can significantly reduce the content of nitrate nitrogen in the soil,and the effect of sodium selenite is more obvious than that of sodium selenite;It can also increase the total nitrogen content of leaves,up to 1.22%,with an increase of3.88%-22.73%.Thermophilic bacteria are the dominant bacteria in the soil with only nitrogen,phosphorus and potassium fertilizer,accounting for more than 50%of all bacteria.However,with the addition of chitin and sodium selenite,thermophilic bacteria are still the dominant bacteria in deep soil（20-40 cm）,while Xanthomonas become the most important bacteria in surface soil（0-20 cm）,accounting for 37.03%of all bacteria.The functional categories with the highest relative abundance in soil are metabolic pathway,biosynthesis of secondary metabolites and microbial metabolism in different environments,accounting for 57.41%,57.57%,57.47%and 57.41%of the total relative abundance,respectively.The gene abundance of nitrogen cycle function related to nitrification,denitrification and dissimilatory nitrogen reduction process is higher in the soil with the combined application of chitin fertilizer and selenium,so as to promote the absorption of nitrogen by plants and improve the utilization efficiency of nitrogen fertilizer.（4）Selenium with different valence can promote the transformation of nitrogen forms and improve the absorption of nitrogen by plants by stimulating the activity of enzymes in soil.Selenium application significantly promoted the absorption of nitrogen by Guanxi honey pomelo seedlings,with the highest total nitrogen content of 3.10%.Compared with nitrogen application only,the leaf nitrogen content was significantly increased by 29.17%.The effects of selenium（IV）and selenium（VI）were similar,and the leaf nitrogen content increased with the increase of selenium application,but selenium application had no significant effect on the soil total nitrogen content.The application of selenium（VI）could significantly increase the content of soil ammonium nitrogen by 97.42%,and the increase rates were 113.65%and 186.80%respectively.Similarly,nitrate nitrogen content increased with the increase of selenium content.The activities of catalase and phosphatase increased with the increase of selenium application;After adding 1.0 mg/kg sodium selenate,the sucrase activity was significantly higher than that of other treatments,and the urease activity was also significantly improved（only when the nitrogen source was nitrate nitrogen）.