Nitrogen and phosphorus release from organic wastes and suitability as bio-based fertilizers in a circular economy.

The drive to a more circular economy has created increasing interest in recycling organic wastes as bio-based fertilizers. This study screened 15 different manures, digestates, sludges, composts, industry by-products, and struvites. Nitrogen (N) and phosphorous (P) release was compared following addition to soil. Three waste materials were then 'upgraded' using heating and pressure (105°C at 220 kPa), alkalinization (pH 10), or sonification to modify N and P release properties, and compared in a second soil incubation. Generally, maximum N release was negatively correlated with the CN ratio of the material (r = -0.6). Composted, dried, or raw organic waste materials released less N (mean of 10.8 ± 0.5%, 45.3 ± 7.2%, and 47.4 ± 3.2% of total N added respectively) than digestates, industry-derived organic fertilizer products, and struvites (mean of 58.2 ± 2.8%, 77.7 ± 6.0%, and 100.0 ± 13.1% of total N added respectively). No analyzed chemical property or processing type could explain differences in P release. No single upgrading treatment consistently increased N or P release. However, for one raw biosolid, heating at a low temperature (105°C) with pressure did increase N release as a percentage of total N added to soil from 30% to 43%.

Pig slurry acidification and separation techniques affect soil N and C turnover and N2O emissions from solid, liquid and biochar fractions.

The combined effects of pig slurry acidification, subsequent separation techniques and biochar production from the solid fraction on N mineralisation and N2O and CO2 emissions in soil were investigated in an incubation experiment. Acidification of pig slurry increased N availability from the separated solid fractions in soil, but did not affect N2O and CO2 emissions. However acidification reduced soil N and C turnover from the liquid fraction. The use of more advanced separation techniques (flocculation and drainage > decanting centrifuge > screw press) increased N mineralisation from acidified solid fractions, but also increased N2O and CO2 emissions in soil amended with the liquid fraction. Finally, the biochar production from the solid fraction of pig slurry resulted in a very recalcitrant material, which reduced N and C mineralisation in soil compared to the raw solid fractions.