Closed-Loop Crop Cascade to Optimize Nutrient Flows and Grow Low-Impact Vegetables in Cities

Area of Science:

• Urban agriculture
• Hydroponics
• Nutrient management

Background:

• Inefficient food supply chains and limited urban food sovereignty necessitate innovative agricultural solutions.
• Urban agriculture systems offer potential for improved environmental performance through effective nutrient management.

Purpose of the Study:

• To evaluate a cascade system using tomato crop leachates for irrigating successive lettuce cycles.
• To determine the agronomic performance and nutrient metabolism of this system.
• To assess its potential for nutrient depletion avoidance and eutrophication mitigation.

Main Methods:

• A rooftop greenhouse in Barcelona was used for the study.
• Tomato crop leachates (January-July) served as the primary irrigation source for five sequential lettuce cycles.
• Agronomic performance and nutrient uptake were monitored.

Main Results:

• Low lettuce yields (<130 g/plant) occurred when using early-stage tomato leachates due to insufficient nutrients (e.g., N at 62.4 mg/plant).
• Lettuce N content increased with leachate nutrient concentration, reaching control levels (4.4% N).
• High electrical conductivity (near 3 dS/m) was observed in leachates late in the donor crop cycle.

Conclusions:

• Cascade systems can utilize residual nutrients for local vegetable production, mitigating nitrogen (N) and phosphorus (P) runoff.
• Optimizing nutrient flow scaling between donor and receiving crops is crucial for maintaining yield and quality.
• The study proposes strategies to enhance nutrient flow efficiency in urban agriculture cascade systems.