Human urine -derived fertilizers: implications to soil microbial abundance, crop growth, pharmaceutical uptake and human health.

Abstract

The recovery of nitrogen (N) and phosphorus (P) from human urine and their use for crop production closes the nutrient loop, thus reducing waste and lowering dependency on manufactured mineral fertilizers. However, it is unlikely that human urine-derived fertilizers (HUDF) could be produced in sufficient quantities to meet crop nutrient demands. Therefore, there is a need to develop new ways of using them in combination with mineral fertilizers. Information on using combinations of mineral and HUDF for crop production and their potential interaction effects in soils is limited. Few studies have reported the effect of HUDF on soil microbial abundance. The potential plant uptake of pharmaceuticals in human urine and the risks to human health are poorly understood. This study was undertaken to (i) evaluate the use of different combinations of mineral and HUDF on crop growth and yield and determine their interaction effects on two contrasting soils, (ii) assess the effects of HUDF on soil biological properties as indicators of soil quality, and (iii) determine plant uptake of antiretrovirals on a range of soil-plant systems. The effects of 15 combinations of mineral and HUDF on maize (Zea mays) growth and yield on a high organic matter, clay loam and a sand soil were evaluated in a tunnel study. A laboratory incubation study was conducted to evaluate the effects of stored urine, nitrified urine concentrate (NUC), and dried urine on microbial biomass C and N, carbon dioxide and nitrous oxide emissions, soil enzyme activity and microbial abundance over a 60-day period. A pot experiment was conducted to evaluate the uptake of nine target antiretrovirals (ARVs) by pepper (Capsicum annum), ryegrass (Lolium perenne) and radish (Raphanus sativus) grown in the same two soils and fertilized with stored urine, NUC and struvite. The estimated daily dietary intake (DDI) of ARVs by consumption of the pepper and radish fertilized with stored urine was compared to Threshold of Toxicological Concern (TTC) values based on the Cramer classification tree to assess direct human exposure. Additionally, to identify and reduce risks and optimise benefits associated with the use of HUDFs in agricultural production, the World Health Organization's Sanitation Safety Planning technique was applied in a South African setting. In the tunnel study, NUC and dried urine as N sources combined with single superphosphate as a P source showed the best performance in both soils with respect to biomass, grain yield, and yield-related variables. The results of the incubation study showed increased soil enzyme activity and microbial abundance in response to the application of dried urine and stored urine while a reduction in enzyme activity was observed in the NUC treatment. In the pot experiment, nevirapine was the only ARV detected in crops grown with NUC and struvite on both soils. Plants fertilized with stored urine absorbed lamivudine, ritonavir, stavudine, emtricitabine,

nevirapine, and didanosine. The calculated daily dietary intake for all ARVs were about 300- 3000 times lower than the Threshold of Toxicological Concern values for class III compounds. The sanitation safety plan revealed minimal risk associated with urine collection and treatment and the application of HUDF. The application of dried and stored HUDF has the potential to improve soil microbial abundance, and the effect of combinations of minerals and HUDF on crop growth is affected by soil type. Plants fertilized with human urine have the potential to absorb ARVs. However, the daily consumption of the tested crops fertilized with stored urine does not pose a health risk to the consumer. Long-term and field-based trials are recommended for future studies to understand the realistic effects of HUDFs on crop growth and soil microbial properties. Additionally, it would be beneficial to create a priority list of pharmaceuticals with the largest plant uptake potential under field settings so that research and assessment may be more focused.