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_min: NH₄ ⁺ and NO₃ ^-) from compost, which we hypothesize reduces N₂O formation. However, associating N_min captured by biochar with the dynamics of N₂O 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 (CO₂, CH₄, N₂O) emissions, N_min content in compost and biochar particles (sequential extractions), and biochar surface transformations (SEM-EDX and ¹³C-NMR spectroscopy) along composting. Biochar did not significantly reduce or increase GHG emissions and N_min content (mg kg^-1) in compost. However, the final NO₃ ^- amount (g compost pile^-1) in the Bc treatment was significantly higher (54%) compared to the Control, indicating lower NO₃ ^- losses. Despite the high aromaticity and minimal contribution of carboxyl C to the biochar structure, biochar retained NH₄ ⁺, 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 NO₃ ^- content in biochar increased continuously during composting. In the final compost, the NO₃ ^- content extracted from biochar was 164 (37%, easily extractable), 80 (19%, moderately extractable), and 194 mg NO₃ ^--N kg^-1 (44%, strongly retained). Although N_min retention in biochar was not accompanied by lower N₂O emissions, contradicting our hypothesis, we demonstrated the efficacy of biochar to recover N_min from organic waste without stimulating GHG emissions.