Closing the loop on human urine: Plant availability of zeolite-recovered nutrients in a peat-based substrate

Recycling mineral nutrients from household wastewater is a central step in the development of a circular economy based society. The objective of this study was to evaluate plant availability of mineral elements and plant performance in a peat substrate containing nutrient-enriched zeolite (NEZ) obtained by nutrient recovery from human urine in a source separated wastewater system. Substrate content of potentially available mineral nutrients was determined by CaCl₂/DTPA-extraction during a 12 weeks incubation experiment for 20:80 (R20) and 30:70 (R30) volume % of NEZ:sphagnum peat, limed R20 (R20L), and 20:80 vol% of unloaded zeolite:sphagnum peat (Z20). Plant availability of mineral elements from R20, R20L, R30 and Z20 was compared with conventionally fertilised sphagnum peat (P100) for sunflower (Helianthus annuus L.) cv. 'Topolino' in a pot experiment. Recovery of nutrients in a potentially available form in the R20 substrate after 12 weeks was 3% (K), 23% (N, P), 34% (Mg) and 90% (S). Liming increased the recovery of mineral N to 39%, suggesting that nitrification was an important driver for the release of NH₄⁺. For R20, estimated recovery of urine-derived N in sunflower shoots was 30-36%. Shoot biomass was similar in R20 and in conventionally fertilised peat (P100). However, P100 plants had more leaves and flowers+buds. Initial addition of ammonium phosphate or supplemental fertilisation with a complete nutrient solution increased flower+bud number in R20. For the NEZ-treatments, Cu and B shoot concentrations were in the low or marginal range while Zn and Mn were high or in excess. Shoot growth and nutrient uptake of sunflower were highly restricted in the unloaded zeolite control (Z20). We conclude that 20% NEZ in a peat substrate was effective as a macronutrient source for sunflower, producing similar biomass as in conventionally fertilised peat. However, micronutrient balance and early P supply may need to be adjusted for optimal plant performance.