Associate Professor Ohio University - Athens, OH Athens, Ohio, United States
Background/Question/Methods In the US, most of the nutrients used in agriculture require large quantities of fossil fuels during manufacturing. Alternatively, a portion of the nutrient inputs for agriculture could come from recycling wastes, such as food waste, septage, and manure. How much of our agricultural nutrients could be derived from human waste systems? If this waste is turned into hydrochar, what are the impacts of applying it to agricultural soils? Can carbon sequestration of crops increase with hydrochar application? To answer these questions, a nutrient inventory was completed to assess the potential of human waste to meet agricultural nutrient demands, and then waste-derived fertilizer was tested experimentally. Hydrochar made from septage was mixed into soils from experimental field plots at Ohio University. The soils were incubated for a 6-month period to observe the change in soil greenhouse gas flux over time. A pot study was also completed to evaluate the effects of hydrochar fertilizer on the growth, photosynthesis, and biomass yield of Spinacia oleracea (spinach). The study provides useful information for farmers and land managers about the best time to fertilize fields with hydrochar to maximize carbon sequestration and the optimum application rate for the highest yields.
Results/Conclusions A nutrient inventory of human waste systems was used to estimate the amount of plant essential nutrients in organic waste from food, feces, and urine. The amount and cost of nitrogen (N) per pound was estimated, with costs based on the market value of common fertilizers used in the agricultural industry ($1.15 per kilogram on average). The fertilizer equivalent value of N in waste from the population in the State of Ohio was estimated to be $61,768,305, accounting for 15% of the total agricultural N costs. After hydrothermally treating waste, a hydrochar fertilizer tested on field soils resulted in a 34% reduction in soil CO2 emissions. In the soil incubation study, significantly greater methane uptake occurred in soil treated with hydrochar (p < 0.05). Thus far, no significant differences in photosynthesis (p >0.05) or biomass (p >0.05) were observed in spinach treated with hydrochar relative to reference fertilizer applications, but further measurements in the summer of 2022, will resolve the plant responses of varying hydrochar application rates. The combined results from the nutrient inventory, soil incubation study, and pot study provide information to guide alternative practices for fertilization, making use of nutrient resources available from human waste.