CRISP-crayfish rice integrated system of production. 5. Simulation of nitrogen dynamics

In the lower Mondego river valley, part of the agricultural lands have been converted into rice fields during the recent years. Meanwhile, an imported and non-native species, the Louisiana Red Crayfish (Procambarus clarkii) has invaded the fields and are hampering the establishment of especially the young rice plants. The species is lowering the potential income of the farmers and is, therefore, considered a pest. A project financed by EU was set up to define a ‘best possible stragety’ for the management of crayfish populations in rice fields aiming to make possible the integration of the two crops in the same system. This paper presents the first outlines of a model for the nitrogen cycle in a freshwater, aquacultural farming system with combined growth and harvest of rice and crayfish. The fertilization of the rice-fields is connected with high inputs of nitrogen as artificial fertilizers. Not all the nitrogen added as fertilizers actually ends up improving the rice-growth. Usually, a major part enters the general biogeochemical N-cycle of the field. The quantities added mean that this may also play a major role in determining the water quality of the rice field. Maintaining an adequate water quality is important for the integration of the rice field with aquaculture of P. clarkii. The model has passed the phase of verification and reached a state of calibration and will be integrated with other submodels of the system. Ammonia is more abundant than the nitrate during the wet season, whereas the nitrate dominates during dry season, indicating that removal of nitrogen by denitrification will take place during this time of the year.