Optimizing Nutrient Recycling to Achieve Zero–Discharge Management for Controlled Environment Agriculture

Controlled environment agriculture (CEA) allows for human control of one or more environmental factors affecting crop production. Optimizing nutrient management in such systems allows for translation and predictions of sustainable management strategies in the field. These systems will be used to supply fresh food as public and private space agencies are planning to send humans to the Moon and Mars in the near future. Food resupply becomes increasingly difficult as the distance from Earth increases. Plants can be grown off-Earth as a food source, but their growth must be carefully managed to ensure long-term sustainability. I describe a model used to determine human nitrogen needs, a validation of a part of this model using lettuce, and an expansion of this framework to additional essential elements for plant and human nutrition. Nitrogen is poorly managed in crop systems on Earth, but inefficiencies must be reduced to ensure long-term recyclability of nutrients. Following the flow of nitrogen through humans and the processes required to convert it among bioavailable forms helped determine which steps could be optimized. Lettuce was used as a model crop to determine if the efficiencies predicted in the previous model could be achieved in a real-world scenario. Plants were grown repeatedly in the same substrate to simulate replanting methods. Yield was optimized when both nitrate and ammonium nitrogen were provided to the plants. The optimal fertilizer concentrations for all nutrients were calculated from the desired amount in the plant and the efficiency by which the plants could take up the nutrients. This approach was validated using lettuce in a liquid hydroponics system. An initial trial with twenty-five repeated plantings without discarding any nutrient solution showed steady-state nutrient management was possible with a few modifications. Subsequent trials with refined recipes led to tighter nutrient management control had comparable yields to plants grown using a standard dump and refill approach.