Nitrogen and phosphorus release from dehydration products of urine mixed with pine bark feedstock, biochar types and ash

Human urine contains large quantities of nitrogen (N) and phosphorus (P), yet most of it is lost to the environment untreated, resulting into nutrient loss and environmental pollution. Although several studies have evaluated nutrient recovery from urine, little is known about how pine bark (PB) and its derivatives (biochar and ash) influence N and P release from dehydrated urine products in soil. This study evaluated effects of various products derived from dehydration of urine, mixed with solid materials from pine bark (PB) on mineral nitrogen (N) and phosphorus (P) availability in soil. The PB materials included feedstock, biochar (pyrolysed at 350 and 650 °C) and ash. The mixtures of urine and these materials were dehydrated at 45 and 60 °C in the oven. Others from PB were also dehydrated in a glasshouse (G). In the experiments, the dehydration products were applied at the recommended N (100 kg ha^{- 1}) rate for spinach. The soils (100 g) were incubated at field capacity and 25 °C temperature. Destructive sampling was done at days 0, 7, 14, 21, 28, 42, and 56 for the first and second experiment, with analysis of pH, ammonium- and nitrate-N and extractable P per sampling day. Nitrate-N increased with a decrease in ammonium-N. In experiment 1 and 2, ash dehydrated at 45 °C (ash 45) and ash dehydrated at the glasshouse (ash G) maintained the highest nitrate-N (37.2 and 38.7 mg kg^{- 1}, respectively), suggesting efficient N recovery and release. Overall, ash was the most effective in increasing N availability. The results shows that the composition of recovery media including its pH strongly influence nutrient recovery and release of nutrients from the urine-based products.