Effects of nitrogen dose and N-NH4:Ntotal ratio on growth, yield, and quality of greenhouse lettuce across seasons

A factorial experiment testing the effects of different nitrogen dose applications (60, 80, and 120 kg ha–1) provided varying ratios of N-NH4 to the total amount of nitrogen supplied (0.4, 1.0) was conducted in two successive growing seasons (autumn-winter and spring) with the Lactuca sativa Lagarde F1. Phosphorus and potassium fertilizers were kept constant and uniform at all experimental plots, respectively 25 (P2O5) and 180 (K2O) kg ha–1. Root traits, growth parameters, yield, nitrogen use efficiency (NUE), and NO3 concentration in the lettuce leaves were measured and analyzed. N-NO3 concentration and NUE were the most sensitive traits to N dose applications and N-NH4:Ntotal ratio. The remaining traits, yield included, rather than on the N dose and its application forms, were subject to seasonal variation of environmental factors. A range of 60‒ 80 kg ha–1 N was the optimum for greenhouse lettuce fertilization. Further increase of N dose applications did not provide a higher yield, whereas it significantly increased the N-NO3 concentration in the plant and reduced the N use efficiency. The NO3 concentration in the lettuce leaves was reduced by increasing the ratio of N-NH4 to total N applied and extending the period of the latest N application before harvesting.