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

Abstract

Human urine concentrates 85% of the nitrogen and 65% 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 NH3 volatilisation and on the availability of P from urine. 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 + 8.5kgP.ha-1 and 510 kgN.ha-1 + 25.5 kgP.ha-1) and compared with a chemical fertiliser treatment (170 kgN.ha-1 + 8.5kgP.ha-1) and a water treatment. 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 volatilisation using Hydrus-1D and Visual Minteq softwares. We also monitored soil phosphorus pools, microbial biomass CNP, 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 in the available P pool. 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 volatilisation. Overall, our results indicate that direct application of urine to a calcareous soil is possible without harming soil properties, but that heavy losses of volatilised nitrogen are to be expected.