Distinct material traits shape soil-dependent N₂O responses to Fe-, Ca- and Se-modified biochars

Modified biochars are widely proposed as amendments for regulating soil nitrogen cycling, yet the material traits that underpin their contrasting effects on nitrous oxide (N₂O) emissions remain unclear.

We compared unmodified biochar with Fe-, Ca-, and Se-modified biochars in loess and black soils during a short-term incubation. Material characterization was combined with N₂O flux measurements, soil pH and inorganic-N dynamics, bacterial community profiling, and qPCR quantification of nosZ, nirK, and nirS.

XPS, Raman, and FTIR analyses showed clear differences in surface elemental composition, carbon structure, and functional groups among the modified biochars. FeBC consistently reduced cumulative N₂O emissions in both soils, whereas SeBC increased cumulative N₂O emissions in loess soil; all biochar treatments reduced N₂O emissions in black soil. Among the measured chemical variables, NH₄ ⁺-N showed the closest association with cumulative N₂O emissions, while pH and NO₃ ^--N reflected the broader N-transformation environment. FeBC was associated with bacterial community shifts linked to lower N₂O emissions, whereas CaBC increased nosZ and nirS abundances in loess soil.

The results show that Fe-, Ca-, and Se-modified biochars regulate N₂O emissions through distinct material-soil-microbial pathways, with their mitigation potential strongly shaped by soil background. These findings provide a trait-based basis for the soil-specific screening of modified biochars for N₂O mitigation.