Biochar captures ammonium and nitrate in easily extractable and strongly retained form without stimulating greenhouse gas emissions during composting.

During composting of organic waste, nitrogen is lost through gaseous forms and ion leaching. Biochar has been shown to capture mineral nitrogen (N: NH and NO ) from compost, which we hypothesize reduces NO formation. However, associating N captured by biochar with the dynamics of NO and other greenhouse gas (GHG) emissions during composting remains unstudied and was the aim of this work. We composted (outdoor for 148 days) together kitchen scraps (43.3% dw, where dw is dry weight), horse manure (40.9% dw), and wheat (Triticum aestivum L) straw (15.8% dw) without (Control) or with biochar (Bc, 15% compost dw). The biochar consisted of hardwood and softwood pieces pyrolyzed at 680°C and exhibited 60% of particles with 4-8 mm. We monitored compost GHG (CO, CH, NO) emissions, N content in compost and biochar particles (sequential extractions), and biochar surface transformations (SEM-EDX and C-NMR spectroscopy) along composting. Biochar did not significantly reduce or increase GHG emissions and N content (mg kg) in compost. However, the final NO amount (g compost pile) in the Bc treatment was significantly higher (54%) compared to the Control, indicating lower NO losses. Despite the high aromaticity and minimal contribution of carboxyl C to the biochar structure, biochar retained NH , mainly in easily extractable form (55%), in the first 2 weeks of composting and mainly in strongly retained form (75%) in the final compost. The NO content in biochar increased continuously during composting. In the final compost, the NO content extracted from biochar was 164 (37%, easily extractable), 80 (19%, moderately extractable), and 194 mg NO -N kg (44%, strongly retained). Although N retention in biochar was not accompanied by lower NO emissions, contradicting our hypothesis, we demonstrated the efficacy of biochar to recover N from organic waste without stimulating GHG emissions.