Effects of fulvic acid application on drip irrigation processing tomato yield, quality, and soil nitrogen balance

High nitrogen (N) application can dominate the processing of tomato (Solanum lycopersicum L.) under drip irrigation in Xinjiang, China. The risks are associated with the low nitrogen use efficiency (NUE) and significant environmental pollution. In this study, the potential of fulvic acid (FA) application was determined to mitigate these negative impacts under the prevailing high N regime. A systematic investigation was also made to evaluate the effects of the FA application rates on the soil-crop continuum under the drip irrigation. The optimal FA rate was identified to enhance the tomato yield, quality, and NUE. A field experiment was conducted over two consecutive growing seasons (2023 and 2024). Five application rates of the fulvic acid were tested as the CK (0), FA1 (10 kg/hm²), FA2 (25 kg/hm²), FA3 (50 kg/hm²), and FA4 (75 kg/hm²). All treatments received the same high rate of nitrogen fertilizer. The impacts of FA were assessed on: (1) Soil properties: Nitrate-N (NO₃⁻-N), ammonium-N (NH₄⁺-N), total nitrogen (TN), and total carbon (TC) content in the 0~60 cm soil profile. (2) Crop growth and physiology: Plant height, stem diameter, leaf area index (LAI), aboveground biomass, leaf SPAD (soil and plant analyzer development) value (indicating chlorophyll content), and photosynthetic performance. (3) Yield and water use efficiency: Processing tomato fruit yield and irrigation water use efficiency (IWUE). (4) Fruit quality: soluble sugar, soluble solids, organic acid, vitamin C content, lycopene content, and sugar acid ratio. (5) Nitrogen dynamics: Plant N uptake efficiency, aboveground N accumulation, partial factor productivity of nitrogen (PFPN), and N surplus in the soil-crop system. A total of 16 key indicators were measured over these categories. The results showed that the optimal FA application rate was determined to construct the evaluation model using both the Analytic Hierarchy Process (AHP) and Principal Component Analysis (PCA). Specifically, the FA application significantly increased (P<0.05) the concentrations of NO₃⁻-N, NH₄⁺-N, TN, and TC in the 0-60 cm soil layer, compared with the CK control. Significant improvements (P<0.05) were observed in the plant height, stem diameter, LAI, aboveground biomass, leaf SPAD value, and photosynthetic parameters in the FA-amended plots. The FA3 and FA4 treatments significantly increased the processing tomato yield by 8.39%-16.33% and irrigation water use efficiency by 8.39%-16.32%, respectively, compared with the CK treatment. At the same time, the better performance was achieved in the soluble sugar, soluble solids, fruit vitamin C, lycopene content, and sugar acid ratio quality parameters. In addition, the FA3 and FA4 treatments significantly increased the plant nitrogen uptake efficiency, aboveground nitrogen accumulation, and nitrogen partial factor productivity, and reduced nitrogen surplus in the soil-crop system by 9.93%-10.67% (2023) and 7.20%-7.67% (2024) (P<0.05), compared with the CK treatment. The evaluation model was integrated with the 16 indicators after AHP and PCA. The FA3 treatment (50 kg/hm²) was consistently identified as the optimal application rate of the fulvic acid under the conventional high N regime. The high nitrogen fertilization was realized with the fulvic acid. Particularly, the highly effective strategy was provided for the tomato production under drip irrigation at the optimal rate of 50 kg/hm². FA application significantly improved the soil nitrogen and carbon status, particularly for the crop growth and physiological performance, the yield, and irrigation water productivity. The key attributes of the fruit quality were promoted more efficient nitrogen utilization, thereby substantially reducing the risk of nitrogen loss and environmental pollution. These findings can also provide a robust theoretical and practical solution to increase the yield and quality of the nitrogen use efficiency in tomato cultivation.