UC Merced

Fertilizer production and application to global agricultural land, along with incomplete nutrient removal during human excreta treatment, exerts a considerable strain on global biogeochemical cycles. There is increasing interest in the safe recovery of nutrients in human excreta for reuse in agriculture, often termed Ecological Sanitation (EcoSan). Often employed in EcoSan systems is the source-separation of urine and feces, which allows for distinct treatment processes that may reduce greenhouse gas emissions and water use compared to traditional sewerage, while conferring high nutrient recovery efficiencies. EcoSan may also provide a local source of plant-available nutrients to smallholder farmers, who are a foundational, yet underserved, part of the global food system. While EcoSan is gaining traction in terms of both research and operation, there is limited information on the biogeochemical outcomes of EcoSan end-use product fertilizer application. Particularly, soil and plant nutrient dynamics following the application of urine and urine-enriched biochar is understudied. Additionally, agronomic research on the application of feces-derived compost on agricultural land near EcoSan systems that produce such compost, is lacking. In this dissertation, we address these research gaps to further the field of EcoSan. In the first project, we investigated urine-enriched biochar as a nitrogenous fertilizer for tomato growth. We prepared urine-enriched biochar with three types of biochar combined with human urine stored in three realistic conditions. We found that the < 500-μm biochar particle size fraction retained significantly more nitrogen (N) than larger particles across biochars, and that urine-N in fresh urine had higher sorption affinities for > 500-μm biochar particles compared to urea-hydrolyzed urines. We also showed that urine-N applied alone is more immediately plant-available than urine-N sorbed to biochar. In our second project, we investigated the agronomic relevance of a suite of EcoSan fertilizers by assessing nitrogen (N) and phosphorus (P) mineralization of urine, urine-enriched biochar, and feces-derived compost in a 90-day amended soil incubation. We showed that urine applied alone is an excellent source of immediately plant-available N, while urine-enriched biochar application supplied approximately half of the N applied. We observed that feces-derived compost application stimulated substantial mining of native soil-P and led to a moderate, slow release of plant-available N. In the third project, we investigated the effect of repeated feces-derived compost application on sorghum production and soil health indicators over two consecutive cropping cycles in an agroecosystem in northern Haiti. We found that feces-derived compost, particularly when applied at 150% of the sorghum N demand at the start of the growing season, led to significant increases in bioavailable soil macro- and micronutrients by the second cycle of management. One application of feces-derived compost resulted in decreased soil bulk density. Moderate increases in soil carbon (C) and N in the topsoil were observed for compost applied at 150% of the N demand by the end of the second cropping cycle. We also provide a chapter reflecting on the unique challenges and opportunities inherent to EcoSan research. This dissertation shows that human excreta is an important, largely untapped resource that can provide nutrients to agricultural soils and improve their health. EcoSan is a powerful tool that can help ameliorate the negative impact of human activity on global biogeochemical cycles, which is urgent to preserve our Earth system.