Modeling organic matter and nitrogen dynamics in integrated aquaculture/agriculture systems: Effects of cycling pathways on nitrogen retention and system productivity

A computer model was developed to investigate the effects of organic matter and nitrogen cycling pathways on nitrogen retention and productivity in integrated aquaculture/agriculture systems and to identify the processes responsible for increased nitrogen retention in integrated systems. The model, developed using StellaTM modeling software, consists of a series of mass balance calculations for fish growth, phytoplankton biomass, dissolved oxygen, pond and terrestrial soil organic matter and nitrogen, and crop biomass growth. Solar radiation, temperature and wind speed are included in the model as driving variables. The model allowed for the simulation of sporadic water inflow and outflow that occur during harvest, irrigation and seepage. The model was calibrated using literature and measured data from the Pond Dynamics/Aquaculture Collaborative Research Support Program (PD/A CRSP) database. The model performance was evaluated using sensitivity analysis and model verification procedures that compared model simulations to measured data from three sites. Sensitivity analysis results suggest that priority areas for future research in integrated aquaculture/agriculture systems are stocking density effects, sediment processes and water management. Model verification results showed that the model performance was satisfactory with respect to fish growth, crop growth, nitrogen and organic matter simulations. However, the modeling of phytoplankton biomass was less satisfactory and the results suggested the need for more site-specific calibration of exogenous factors. A modeling study designed to investigate the effects of different cycling pathways on nitrogen retention and productivity showed that the recycling of plant wastes to aquaculture ponds had a major effect in reducing system nitrogen losses and increasing system productivity. Sediment organic matter processes were identified as the major determinants of system nitrogen retention as measured by the nitrogen retention index. Results of the modeling study suggested that the system nitrogen status may be more important in determining the number and type of cycling pathways that should be incorporated in integrated aquaculture/agriculture systems. The long term behavior of the nitrogen retention index was similar to that observed in natural systems suggesting that the index could be a useful measure of system nitrogen retention and cycling in managed agroecosystems.