A methodology to optimize nitrogen management by accounting for the trade‐offs between crop yield, available nitrogen and potential nitrogen losses in lettuce

Abstract Intensive vegetable production systems are of major economic relevance in many areas despite being associated with excessive use of fertilizers and nitrogen (N) losses to the environment. To improve N fertilizer management adapted to crop N demands, this work developed a methodology that requires the combined use of crop N status indicators with empirical relations that assess crop N dynamics, accounting for the trade‐offs between crop yield, total available nitrogen (TAN) and potential N losses (calculated as the sum of leaching and gaseous losses). Four lettuce crops were grown with spring cycles in open field conditions under a Mediterranean climate, with seasonal N fertilizer rates ranging from 9 to 171 kg N ha −1 . A critical curve of %Nc = 3.39 · DMP −0.22 , where %Nc is the minimum total crop N content associated with maximum total dry matter production (DMP), was determined for lettuce. The maximum relative dry yield from the four lettuce crops was associated with an integrated nitrogen nutrition index value of 1.09, determined from a linear–plateau relationship ( R 2 = 0.81), and TAN of 132 kg N ha −1 . An optimal TAN (minimum TAN for maximum relative dry yield) of 110 kg N ha −1 was determined using a linear‐plateau model ( R 2 = 0.67). Nitrogen use efficiency (NUE), defined as the ratio between crop dry yield and TAN, decreased exponentially with increasing TAN ( R 2 = 0.90), and a NUE value of 20.7 kg kg −1 was associated with the optimum TAN. For the lettuce crops grown under the conditions of the study, N utilization efficiency of the four crops had a strong linear‐plateau relationship with crop N uptake ( R 2 = 0.93), suggesting crop N over‐consumption for N uptake values >67 kg N ha −1 . Total N losses increased linearly with increasing TAN above 68 kg N ha −1 and the potential N loss associated with optimal TAN was 22 kg N ha −1 . Above TAN of 68 kg N ha −1 , total N losses would reflect the environmental cost of lettuce production. However, recommending values lower than 110 kg N ha −1 would be at the expense of reducing crop yield. The methodology presented in this work suggested that there is appreciable potential to reduce N fertilization and increase nitrogen use efficiency, particularly if soil and water N supply (as TAN) are included in the calculations, without affecting yield.