Closing the poop loop: Recycling organic waste streams to improve soil and crop health using ecological-based sanitation

Background/Question/Methods: Broken nutrient and carbon cycles in food systems waste vast quantities of organic resources and contribute to climate change, food insecurity, and soil degradation. Closing these cycles may contribute to climate change mitigation goals and foster resilient agroecosystems. The capture and transformation of human organic waste is an enormous and largely untapped resource stream. Emerging ecological-based sanitation (EcoSan) systems deploy container-based toilets to collect and compost human feces using an aerobic, thermophilic composting technology. Our research aims to quantify the ecological impacts of the reuse of EcoSan compost on agricultural soils. We hypothesize that amending soils with EcoSan compost reduces nutrient losses compared to alternative fertilization practices by improving soil conditions and acting as a slow-release fertilizer. We tested this hypothesis using a controlled, replicated greenhouse experiment using soils (Oxisols) collected from an agricultural area on Oahu, Hawaii. Treatments included an unfertilized control, urea, and three fecal-derived organic amendments: EcoSan compost, anaerobically digested biosolids, and thermal hydrolyzed biofertilizer. All treatments were applied at a rate of 100 kg PAN/ha, only one-time immediately prior to the first planting. Six consecutive cropping cycles of radishes were grown and harvested to determine legacy effects of amendments on crop, water, and soil nutrient dynamics.

Results/Conclusions: In the first growth cycle, crop yields from organic amendments were not significantly different from each other with increases 15±2.2-fold compared to control and 2.6±0.36-fold compared to urea. In subsequent cropping cycles, crop yields declined for all treatments but at different rates. By the second cropping cycle, yields in the urea treatments declined abruptly and were similar to the control. The legacy effect of organic amendments on crop production was strongest for EcoSan compost, which resulted in elevated crop production throughout all cropping cycles. EcoSan compost and biosolid amendment improved soil water retention, with 12±2.2% of water lost through leaching, compared to an average loss of 17±0.27% from other treatments. Leachate nitrate and ammonium concentrations from EcoSan compost amended soils were higher, however overall nutrient losses were less due to significantly lower rates of leaching. Biosolids resulted in the highest emissions of soil nitrous oxide, with highest fluxes occurring immediately following application. We also found that EcoSan amendment increased the amount of soil organic nitrogen stored in physically protected soil fractions. These findings suggest that closing the poop loop through ecological sanitation can be an effective strategy for improving soil health, tightening nutrient cycles, and promoting resilient agroecosystems.