Regulating nitrate excess in lettuce-planted greenhouse soil with available carbon addition through irrigation

Nitrate excess is common in greenhouse soils, imposing environmental risks and degrading vegetable quality. In this study, the effectiveness of adding sucrose as available carbon through irrigation to cut nitrate excess in lettuce-planted soil was investigated under impacts of soil texture and irrigation type. In the pot experiment using two loam soils of same origin with different clay to sand ratios (50.2% and 39.8%) and nitrate excess (116.1 and 417.7 mg/kg N), three-time sucrose addition through flood irrigation was more effective in lowering net formation of nitrate-based inorganic N and increasing lettuce yield in the soil with the higher clay to sand ratio, and sucrose addition at 150-450 mg/kg reduced nitrate accumulation and leaching, and nitrate content of lettuce at harvest by 62.5-89.6%, 19.3-36.1%, and 11.4-76.0%, respectively. In the micro-plot field experiment with two-time sucrose addition at 0.6-1.2 g/L through furrow irrigation (42 mm) into two other soils of same origin with different clay to sand ratios (56.9%, 48.4%), nitrate accumulation at 0-30-cm depth at the prone-leaching furrow location at harvest decreased by 30.9-36.0% under the higher clay to sand ratio but increased by about 35% under the lower clay to sand ratio. The nitrate content and yield of ridge-planted lettuce was less affected in either soil. Hence, carbon addition rate, irrigation type, and clay to sand ratio all affected the effect of available carbon addition on nitrate accumulation in vegetable-planted soil, and their joint impacts need better quantification for cutting nitrate excess in soil and improving vegetable quality and even yield.