Enhancing the transformation of nitrogenous organics to humification in composting: biotic and abiotic synergy mediated by phosphorus and magnesium modified biochar

Abstract This study developed phosphorus-modified biochar (BCP) and phosphorus-magnesium co-modified biochar (BCPM) to improve nitrogen retention and humification during composting. Systematically, this study elucidated the synergistic biotic-abiotic mechanisms by tracking nitrogen transformation, fluorescence spectral dynamics, functional genes and microbial succession. Results demonstrated that compared to conventional biochar (BC), the BCP/BCPM immobilized NH₄⁺ via an abiotic pathway (surface adsorption and struvite crystallization), mitigating NH₃ emissions by 21.29–27.99%, while upregulating nitrification genes ( amoA , hao , nxrA ) and enriching functional consortia ( Bacillaceae ) to enhance total nitrogen retention (by 3%) through a biotic pathway. The biotic-abiotic synergy elevated the humification index (P V,n /P III,n ) by 24.01–33.61%. The potential mechanism might be that a nitrogen retention supplied nitrogen skeleton and nitrogenous precursors for aromatic condensation reactions. Moreover, the enriched functional microbiota ( Thermobifida ) drove lignin degradation and protein-like conversion, redirecting toward precursors to stable humic-like substances. The phosphorus mainly mediated and enhanced the humification process (+ 7.74% vs. BCPM), while magnesium synergistically reduced more NH₃ emissions (–8.51% vs. BCP). Therefore, based on the phosphorus-magnesium co-modified biochar, increasing the phosphorus content loaded on biochar offers greater potential for humification. The spatiotemporal coordination of abiotic mineral interactions and biotic microbial specialization enabled simultaneous nitrogen retention and humification in composting. Graphical Abstract