Impact of winter lettuce crop management on nitrate pollution and quality:Diagnosis and modelling approach

A major challenge for creating sustainable greenhouse winter lettuce croppi ng systems is to produce high quality lettuces of the required weight and n itrate content, while being kind to the environment (i.e. limiting N pollut ion). The aim of the research was to design crop management programs that a chieve this objective by adopting the dual approach of integrating (a) both experiments and an agronomic diagnosis, to achieve an understanding of the main factors determining the behaviour of the cropping system and how the farmer manages it; and (b) simulation modelling to test the effects of crop management on product quality and N pollution. The diagnosis and experimen ts showed that the farming techniques used under plastic create widely diff erent growing conditions in different parts of the greenhouse. This makes i t harder to control production and increases the likelihood of N pollution and lower product quality. A model of the cropping system's behaviour was built to integrate this stru ctured heterogeneity of the greenhouse environment, the parameters set, and the model validated against independent data. Simulation results show that the system is highly sensitive to input quantities of water and nitrogen, and the model has been used to explore different ways of managing irrigatio n and nitrogen applications (including the balance-sheet method, management according to the state of the system, and fertigation). The model could al so be used to test decision rules for managing organic matter and greenhous e climates. This would require further research to extend its scope. In the long run, it could be incorporated in a decision aid for technical managem ent of lettuce crops.