Nutrient Uptake Characteristics And Water And Fertilizer Precise Regulation Of Watermelon

China has achieved the top global ranking in watermelon production,surpassing other countries in terms of both planting area and yield.Watermelon cultivation has emerged as a significant economic sector for farmers in several prominent producing regions,serving as a means to augment their financial resources.Survey findings in this study indicated that farmers still used traditional irrigation and fertilization methods,such as high basal fertilizer rates（particularly,the rate of nitrogen fertilizer application was approximately 80%）and low top-dressing fertilizer rates in drip fertigation.Excessive fertilization and irrigation in watermelon production may result in increased soil nutrients accumulation and may impose serious ecological risks.Scientific fertilization approach was developed on the basis of nutrients uptake characteristics of plants.However,existing literature shows considerable variation in the nutrient uptake characteristics of watermelon,and there is a lack of systematic studies in this regard.Significant challenges are being posed to reduce fertilizer application rates and increase their use efficiency in the watermelon industry in China.Therefore,we conducted a four-year field study to reduce chemical fertilizer rates using automatic and accurate water and fertilizer control in a drip fertigation system.Moreover,we also conducted a pot experiment using the ¹⁵N isotope method.We combined the data of multi-plot trials and the literature.The objectives of this study were:i)to determine the nutrient uptake characteristics of watermelon;ii)to evaluate the effect of different nitrogen fertilizer application stages on the nitrogen uptake and translocation rate of watermelon;iii)to analyze the effect of successive fertilizer reduction on the watermelon production;and iv)to assess the fertilizer reduction potential and propose key scientific fertilizer measures in different watermelon production areas based on survey data of China.The following main results were obtained:1.The demands for N,P₂O₅,and K₂O to produce 1000 kg watermelon tended to be stable with increasing yield in big,medium and small watermelon fruits;the mean values of these demands were 2.0,0.83,and 3.3 kg,respectively.Nutrients demands at these rates were necessary to the yield of watermelon greater than the national average yield（42.3t/hm²）.The dry matter accumulation and N,P₂O₅,K₂O uptake dynamics in the seeding,vine-stretching stage,fruit-setting,fruit-expanding and maturing stages were similar.The average N,P₂O₅,and K₂O uptake rates in the fruit-expanding stage were 2.76,1.47 and 5.72kg/hm²/day,which were 7.90,11.4 and 11.9 times higher than those during the vine-stretching stage,and 1.74,2.58 and 2.08 times higher than those during the fruit-setting stage,respectively.The proportions of the N,P₂O₅,and K₂O uptake in the seedling-stage to the whole stage were 0.20%,0.15%,0.08%;and 11.6%,8.72%,7.94%during the vine-stretching stage;and 19.6%,14.5%,16.9%during the fruit-setting stage;and 67.2%,72.3%,70.8%during the fruit-expanding stage.2.The results of the ¹⁵N tracer-based pot experiment indicated that the ¹⁵N uptake and translocation rates were 15.8%-25.9%,18.1%-28.4%,28.5%-41.7%,and 49.5%-57.4%,respectively,when the nitrogen fertilizer applied in basal fertilizer,vine-stretching,fruit-setting and fruit-expanding stages.Meanwhile,the contribution rate to the fruit were17.1%-17.6%,18.8%-19.9%,11.7%-12.7%,and 50.7%-51.4%,respectively.The ¹⁵N translocation rate from watermelon vegetative organs to the fruit was only 9.39%,and the contribution rate to fruit was 7.63%.The chemical fertilizer nitrogen measured in fruit was the result of simultaneous nutrition absorption and assimilation.During watermelon growth,the fruit-expanding stage was crucial for fertilization due to its peak nutrient uptake efficiency.To maintain a balance between vegetative growth and reproductive growth,it’s essential to adjust the fertilizer application by reducing the basal fertilizer rate and increasing it in the fruit-expanding stage.3.The results of the field trials in four consecutive seasons indicated that the intelligent drip fertigation（IF）improved and maintained watermelon yield and quality with an average reduction of 30.8%in irrigation water rate and 46%,72%,and 57%in N,P₂O₅,and K₂O rate,respectively.The N,P₂O₅,K₂O use efficiency,irrigation water use efficiency,partial factor productivity and agronomic efficiency were significantly improved in the IF.The irrigation water use efficiency reached 135,92.7,75.7,and 51.3 kg/m³,respectively,and the partial factor productivity and agronomic efficiency of N,P₂O₅,and K₂O reached 507,1217,553and 114,274,129 kg/kg,respectively.The IF treatment increased water and nutrient uptake and translocation capacity of the root system and improved water and nutrient use efficiency by significantly increasing the fine root and medium root length,root surface area,root volume and root tips of the watermelon.Moreover,compared with the FP treatment,IF resulted in a lower apparent soil balance of N,P₂O₅,and K₂O.The NO^3--N accumulation of IF treatment in the 0-200 cm soil layer was 441 kg/hm²,which was 53.6%lower than that of FP treatment（951 kg/hm²）.The results indicated that the intelligent drip fertigation reduced the environmental risks.4.The results of plant ¹⁵N uptake rates and soil mineral nitrogen rates in the different growth stages of watermelon indicated that the nitrogen mineralization rates of manure（commercial manure）were 12.1%,11.7%,6.43%,and 0.22%in the vine-stretching,fruit-setting,fruit-expanding and maturity stages,respectively.Overall,the mineralization rate was 30.5%throughout the whole planting season.It was recommended to calculate the effective nitrogen rate of manure when applying large quantities of manure to greenhouses.The M+ON treatment significantly improved watermelon yield,quality and nitrogen fertilizer use efficiency,and significantly decreased the residual and loss rates,the plant uptake use efficiency was 56.1%,and the residual rate was 29.8%,and the loss rate was 14.1%.5.According to the survey conducted in this study,the average watermelon yield in the northern and southern areas of China were 45.6 t/hm²,and 32.1 t/hm²,respectively.Yield differences in different regions were related to regional climate,soil,and cultivation season and methods.The total watermelon yield was the result of a combination of these factors.The average fertilizer input of N,P₂O₅and K₂O were 256.6,230.7 and 262.1 kg/hm²,respectively.The scientific fertilizer rates were recommended based on watermelon yield level.The average fertilizer rates of N,P₂O₅,and K₂O applied by farmers in different regions were 1.3-3.0,1.9-5.5,and 0.3-2.5,respectively,compared with the recommended rates.The fertilizer inputs of nitrogen and phosphorus in the main watermelon producing areas were excessive;and for potassium fertilizer,both excessive and insufficient existed.The fertilizer reduction potential of N,P₂O₅,and K₂O were 43.0%,68.7%,and 12.2%respectively,and which was greater in southern China than in northern China.We have proposed some recommendations according to the characteristics of cultivation methods and problems with water and fertilizer management in the main production regions.Some key production methods,such as disease-resistant varieties,greenhouse or rain-sheltered cultivation methods,precise control methods of water and fertilizer in the drip fertigation system,and application of slow/controlled release fertilizer in field production were recommended in southern China.Some key production methods,such as soil testing for fertilizer formulation,manure alternatives and deep application of chemical fertilizers in the field,precise management of water and fertilizer in the drip fertigation system,soil remediation by manure and straw in the high temperature closed greenhouse were recommended in northern China.Following these recommendations,the goals of fertilizer reduction,higher efficiency and green sustainable and healthy development of watermelon industry can be achieved.To sum up,intelligent fertigation（IF）is a sustainable solution for greenhouse agriculture to reduce the demand for water and fertilizer resources and adverse environmental impacts,and is recommended to be promoted and applied in greenhouse watermelon production.Farmers should pay attention to the reasonable application ratio of organic fertilizer and chemical fertilizer,considering the effective nutrients rate in organic fertilizer when applied a large number of manure in greenhouses.The watermelon’s vegetative and reproductive growth should be balanced by reducing the basal fertilizer rate and increasing it in the fruit-expanding stage.This will reduce chemical fertilizer rate and increase fertilizer use efficiency.Finally,the goals of developing a green and sustainable and healthy industry will be achieved.