The scientists discovered that even when applied in high doses, one-year stored urine had little impact on soil bacterial communities and produced minimal change in soil pH and salinity.

However, the researchers did discover that urine fertilisation increased the relative amounts of nitrifying and denitrifying groups compared to synthetic fertiliser – implying that more nitrogen oxides could be emitted when fertilising with urine.

Publishing their findings in Applied Soil Ecology, the researchers have called for further studies to be done on the long-term effects of urine fertilisation – particularly regarding nitrogen oxide production and soil salinity.

“Our research highlights the potential of recycling human urine to enhance agricultural sustainability, reduce wastewater pollution, and decrease reliance on synthetic fertilisers,” said co-author Manon Rumeau, from the University of Birmingham. “Stored urine can be safely applied to a plant-soil system without negatively impacting the soil microbiome.”

Fresh urine is composed of 95 % water with the remaining 5% made up of amino compounds, such as urea or creatinine, organic anions and inorganic salts making it a source of bioavailable nutrients and micronutrients for plant growth.

There has been great interest in re-using human urine as a crop fertiliser, however prior to this study more understanding was required on how urine could affect soil functions and microbial communities.

Scientists fertilised a spinach crop with two different doses of a source-separated and stored human urine – comparing these with a synthetic fertiliser treatment and a water treatment without fertilisation, conducted across four soil tanks in greenhouse conditions.

After 12 months of storage, urine had a depleted microbiome but contained few common strains of urine. Thud storing urine for several months, with the resulting increase in its pH value (about 9 rather than 6.5 for fresh urine) and its free ammonia concentration is considered sufficient to inactivate most human pathogenic bacteria and break down extracellular DNA. Soil bacterial communities were resistant to urine fertilisation with only 3% of groups of organisms impacted. The urine’s high salt concentration had little discernible effect on the bacterial community.

‘Unveiling the impact of human urine fertilization on soil bacterial communities: A path toward sustainable fertilization’ by Manon Rumeau, Chiara Pistocchi, Nassim Ait-Mouheb, Claire Marsden, Brigitte Brunel is published in Applied Soil Ecology.

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