Effect Of Water And Nitrogen Coupling On Watermelon Growth Under CO₂ Enrichment

At present,the CO₂ concentration in the environment is increasing year by year,and the unreasonable use of water and fertilizer resources has seriously affected the production process of facility vegetables.The gift watermelon was used as the test material to explore the effect of the coupling of three factors of greenhouse CO₂ concentration,nitrogen fertilizer and irrigation on the comprehensive growth of watermelon.The experiment was carried out in three crops: spring 2019（May-July）,spring 2020（May-July）and spring 2021（April-June）.A split-plot test design was adopted,with CO₂ concentration as the main split plot,nitrogen fertilizer as the secondary split plot,and irrigation again,with two CO₂ concentration levels（C1: 400 ppm and C2: 800ppm）,two nitrogen fertilizer levels（N1: 644.04 kg/hm2 and N2:1288.09 kg/hm2）and three irrigation levels（I1: 80% ET,I2: 100% ET and I3: 120% ET）,a total of 12 treatments.In three consecutive croppings,three growth indicators of leaf number,leaf area,and dry matter accumulation were measured;five photosynthetic indicators,net photosynthetic rate,transpiration rate,intercellular CO₂ concentration,stomatal conductance,and chlorophyll;hectare yield;five quality indicators of sugar,soluble protein,vitamin C,lycopene;two efficiency indicators of irrigation water use efficiency and nitrogen fertilizer use efficiency.In 2020 and 2021,two quality indicators of free amino acid and nitrate were also measured;three nutrient absorption indicators of fruit nitrogen content,fruit phosphorus content,and fruit potassium content were measured.The regulation effect of CO₂ concentration and water-nitrogen coupling on each indicator was analyzed,and the TOPSIS method was introduced to comprehensively evaluate 12 characteristic indicators of watermelon growth.Finally,by analyzing the comprehensive evaluation value,the optimal CO₂ concentration enrichment was determined.The specific results of the water and nitrogen management model are as follows:（1）CO₂ concentration,nitrogen fertilizer and irrigation all had a significant effect on the number of leaves.Increasing the application of CO₂ and nitrogen fertilizer significantly promoted the number of leaves.Among them,increasing the application of CO₂ significantly improved the number of leaves under N1,increasing by 13.5%,11.5% and 6.8% respectively in three years.The increased application of CO₂ significantly inhibited the leaf area,which decreased by 37.0% and 25.7% in 2019 and 2020,respectively.In both 2019 and 2020,it was found that increased application of CO₂ promoted the accumulation of dry matter,and irrigation also had a significant effect on the accumulation of dry matter,and the overall performance of I2 level was the best.（2）The CO₂ concentration significantly affected all photosynthetic indicators,among which the net photosynthetic rate,the intercellular CO₂ concentration and the chlorophyll content were higher at the concentration of 800 ppm,while the transpiration rate and stomatal conductance were inhibited when CO₂ was added.The increase of nitrogen fertilizer only significantly promoted the intercellular CO₂ concentration,and the N2 level increased by12.3%,17.6% and 12.2% respectively in three years.With the increase of irrigation amount,net photosynthetic rate showed a trend of first increase and then decrease;transpiration rate and stomatal conductance showed an upward trend,and the best performance was at the I3 level.Among the interaction effects,CO₂×I and CO₂×N×I had significant effects on chlorophyll content.（3）Yield was significantly affected by irrigation,which showed a gradual increase with the increase of irrigation amount.CO₂×N showed a significant interaction in 2019 and 2021,and increased CO₂ application improved yield at N1 level.At a CO₂ concentration of 400 ppm,the yield was higher at the N2 level;while at a CO₂ concentration of 800 ppm,the yield at the N1 level was better.（4）The increase of CO₂ had a significant promoting effect on the total soluble sugar,which increased by 2.7%,14.0% and 3.2% respectively in three years.Increased nitrogen application significantly decreased lycopene and increased nitrate accumulation,but promoted free amino acids.With the increase of irrigation amount,vitamin C and total soluble sugar first increased and then decreased,while soluble solids and soluble protein decreased gradually.CO₂×N and CO₂×N×I had significant interaction effects on vitamin C and lycopene,CO₂×I had significant effects on vitamin C and free amino acids,and I×N had significant effects on soluble solids.（5）The concentration of CO₂ significantly affected the element content of fruit,and enrichment of CO₂ could promote the accumulation of nitrogen content and potassium content in fruit;the content of phosphorus in fruit increased slightly under N2 level,but it was not significantly different from N1 level.Irrigation significantly affected the content of nitrogen,phosphorus and potassium in fruit.With the increase of irrigation amount,the content of nitrogen,phosphorus and potassium in fruit first increased and then decreased,with the maximum accumulation at the I2 level.Except for CO₂×N,other interactions had significant effects on fruit nitrogen content;fruit phosphorus content was significantly affected by all interaction effects;and fruit potassium content was only significantly affected by I×N.（6）Nitrogen fertilizer and irrigation have significant effects on irrigation water use efficiency and nitrogen fertilizer use efficiency.With the increase of nitrogen fertilizer dosage,the irrigation water use efficiency gradually increased,while the nitrogen fertilizer use efficiency gradually decreased.（7）The TOPSIS method was used to analyze the 12 indicators of leaf number,leaf area,dry matter accumulation,net photosynthetic rate,yield,soluble solids,soluble protein,soluble total sugar,lycopene,vitamin C,irrigation water use efficiency,and nitrogen fertilizer use efficiency.MATLAB software was used to optimize the comprehensive evaluation results,and the optimal water and nitrogen application interval for watermelon comprehensive growth after CO₂ enrichment was determined.In 2019,it was（1177.20¹394.78m3/hm2,664.04⁷26.44kg/hm2）.In 2020,it was（1120.04¹148.76m3/hm2,664.04⁹44.86kg/hm2）.In2021,it was（1252.37¹391.52m3/hm2,664.04⁷88.85kg/hm2）.Therefore,under the environment of high CO₂ concentration in the future,nitrogen fertilizer application can be appropriately reduced in watermelon production,saving resources and reducing environmental pollution.