Optimal water and nitrogen strategy for watermelon under CO2 enrichment by considering multiple indicators

The accurate and efficient management of water and nitrogen is essential for the effective intensive agricultural production. To investigate the coupling effect of water and nitrogen under the enriched CO2 concentration, and realize a precise water and fertilizer management mode, we conducted an experiment over two study years in 2020 and 2021, including two CO2 concentrations, C1 (400 ppm) and C2 (800 ppm), two nitrogen levels, N1 (644.0 kg/ha) and N2 (1288.1 kg/ha), and three irrigation levels, I1 (80% evaporation (Ep)), I2 (100% Ep), and I3 (120% Ep). The results showed that CO2 enrichment had a significant effect on nitrogen and potassium content in fruits, while increasing nitrogen fertilizer inhibited the absorption of fruit potassium. Elevated CO2 alleviated the negative effect of low water on element contents, especially improving fruit phosphorus content by 10.0% and 26.0% in the two study years, respectively. Elevated CO2 increased total soluble sugar content in fruits, while increased irrigation promoted yield. The interaction of CO2 × I significantly increased free amino acids by 6.2% and 9.4% in the two study years, and the interaction of CO2 × N × I had a significant effect on quality indexes except for free amino acids of 2020. Based on correlation and path analysis, fruit potassium content had a significant positive contribution to lycopene, and fruit nitrogen content had an important contribution to yield, with direct path coefficients of 0.755 and 0.904 for the two study years, respectively. Considering yield, quality, and efficiency using TOPSIS model, the optimal combination was determined as nitrogen of 644.0 kg/ha and irrigation of 957.3-1159.6 m3/ha under CO2 800 ppm. The results could provide a theoretical and practical basis for water and nitrogen management in watermelon production under the increasing CO2 concentration in the future.精准高效的水氮调控是保障集约化农业高效生产的关键基础。为探究CO₂富集条件下水氮耦合效应并实现精准水肥管理模式，本研究于2020年和2021年开展了为期两年的试验，设置2个CO₂浓度水平：C1（400 ppm）、C2（800 ppm）；2个施氮水平：N1（644.0 kg/ha）、N2（1288.1 kg/ha）；3个灌溉水平：I1（80%蒸发量（Ep））、I2（100% Ep）、I3（120% Ep）。研究结果表明，CO₂富集对果实氮、钾含量具有显著影响，而增施氮肥会抑制果实对钾元素的吸收。CO₂富集会缓解低水分胁迫对矿质元素含量的负面影响，在两年试验中分别使果实磷含量提升10.0%和26.0%。CO₂富集可提升果实可溶性总糖含量，而增加灌溉量可促进产量提升。CO₂浓度与灌溉量的交互作用在两年试验中分别使果实游离氨基酸含量提升6.2%和9.4%；CO₂、氮素与灌溉量的三重交互作用对除2020年游离氨基酸指标外的各项品质指标均具有显著影响。通过相关分析与通径分析可知，果实钾含量对番茄红素具有显著正向贡献，果实氮含量对产量具有重要贡献，两年试验对应的直接通径系数分别为0.755和0.904。采用优劣解距离法（TOPSIS）综合考量产量、品质与生产效率后，确定的最优组合为：在CO₂浓度800 ppm条件下，施氮量为644.0 kg/ha，灌溉量为957.3~1159.6 m³/ha。本研究结果可为未来CO₂浓度升高背景下的西瓜生产水氮调控提供理论与实践依据。