Fate of nitrogen and phosphorus from source-separated human urine in a calcareous soil

Abstract

Human urine concentrates 88% of the nitrogen and 50% of the phosphorus excreted by humans, making it a potential alternative crop fertilizer. However, knowledge gaps remain on the fate of nitrogen in situations favouring NH 3 volatilization and on the availability of P from urine in soils. This study aimed at identifying the fate of nitrogen and phosphorus supplied by human urine from source separation toilets in a calcareous soil. To this end, a spinach crop was fertilized with 2 different doses of human urine (170 kgN ha−1 + 8.5 kgP ha−1 and 510 kgN ha−1 + 25.5 kgP ha−1) and compared with a synthetic fertilizer treatment (170 kgN ha−1 + 8.5 kgP ha−1) and an unfertilized control. The experiment was conducted in 4 soil tanks (50-cm depth) in greenhouse conditions, according to a randomized block scheme. We monitored soil mineral nitrogen over time and simulated nitrogen volatilization using Hydrus-1D and Visual Minteq software. We also monitored soil phosphorus pools, carbon, nitrogen and phosphorus (CNP) in microbial biomass, soil pH and electrical conductivity. Only an excessive input of urine affected soil pH (decreasing it by 0.2 units) and soil conductivity (increasing it by 183%). The phosphorus supplied was either taken up by the crop or remained mostly in the available P pool, as demonstrated by a net increase of the resin and bicarbonate extractable P. Ammonium seemed to be nitrified within about 10 days after application. However, both Visual Minteq and Hydrus models estimated that more than 50% of the nitrogen supplied was lost by ammonia volatilization. Overall, our results indicate that direct application of urine to a calcareous soil provides available nutrients for plant growth, but that heavy losses of volatilized nitrogen are to be expected. Our results also question whether long-term application could affect soil pH and salinity.

Unveiling the Impact of Human Urine Fertilization on Soil Bacterial Communities: A Path Toward Sustainable Fertilization

“…These tanks were each divided into four experimental units and placed within a tunnel greenhouse exposed to natural sunlight in Montpellier, France (43 • 38′51.05"N, 3 • 52′26.13″E). The detailed experiment is related in Rumeau et al (2023). Briefly, the soil used was a loamy clay soil rich in carbonates (45 %) with a pH of 8.7, and spinach plants (Spinacia oleracea L.) were cultivated in soil tanks from May to July 2020.…”

“…The 16S rRNA region amplification, sequencing and read data analysis were performed following the same procedure as the soil DNA samples (see Section 2.3 for method description). The urine's chemical composition is given in Rumeau et al (2023). Briefly, the urine contained 4341 mg L − 1 of ammonium, 206 mg L − 1 of inorganic P and 57 mg L − 1 of organic carbon.…”

“…Urine applications modified soil chemical properties by reducing soil pH (likely through ammonia volatilization and nitrification) and increased soil conductivity and soil mineral nitrogen content (Rumeau et al, 2023). Despite these changes, the soil bacterial community structure was similarly resistant to urine as it was to synthetic fertilizer even under excessive dose of urine application that only showed a slight and non-significant divergence in community structure.…”

“…Recycling human urine as a fertilizer has been identified these past years as a real opportunity to enhance agricultural sustainability, reduce wastewater pollution, wastewater energy consumption, and decrease dependency on synthetic fertilizer, among other benefits (Karak and Bhattacharyya, 2011;Wald, 2022). Fresh urine is composed of 95 % water and the 5 % remaining are amino compounds (such as urea or creatinine), organic anions and inorganic salts making it a source of bioavailable nutrients and micronutrients for plant growth (Akpan-Idiok et al., 2012;Pradhan et al, 2009;Rumeau et al, 2023).…”

“…Their effects were compared with those of a synthetic fertilizer treatment, as well as a water treatment with no fertilization. We measured bacterial community composition that we analysed in regard with previously published data on plant growth, soil physico-chemical properties (pH, conductivity, nutrients) and soil microbial biomass (Rumeau et al, 2023). The hypotheses were that 1) stored human urine would have a depleted microbiota, 2) the taxonomic composition of the soil bacterial communities would remain relatively resistant to the addition of urine, although 3) urine fertilization would favour bacteria involved in nitrification and salt tolerance.…”