Manganese (Mn) can modulate nitrogen (N) transformations in soil, yet its role in agroecosystems remains understudied. We conducted a 51-day microcosm incubation with agricultural soils differing in long-term N history (N0, no added N; N1, added 225 kg N ha−1 for a duration of 27 years) and amended with soluble Mn at 0 (M0), 50 (M1), or 250 (M2) mg kg−1 and Glyceria striata (Lam.) residue. In N1 soils, Mn additions (both M1 and M2) lowered total mineral N by 25 % relative to N1M0 at day 51 and reduced 51-day cumulative N₂O by 32 % (N1M1) and 42 % (N1M2) vs. N1M0, whereas effects in N0 were negligible. Mn also depressed ammonia-oxidizing bacterial amoA gene transcripts at day 15 in N1M2 vs. N1M0 (2.5 fold change). This reduction was likely due to increased N loss via complete denitrification to N₂ through microbial pathways such as nitrate/nitrite-dependent manganese oxidation (NDMO), where bacteria directly used the added Mn2+ to reduce nitrate (NO3−) and nitrite (NO2−) to N2 or as Mn dependent-ammonia oxidation (Mnammox) where bacteria oxidized ammonium (NH4+) to N2, using Mn oxides as electron acceptors that formed from the oxidation of the added Mn. Other contributing mechanisms may include Mn-induced N immobilization, toxicity, and changes in the microbial community. These mechanistic results indicate that background Mn availability and redox dynamics can shape nitrification–denitrification pathways under N-rich conditions. We highlight how native Mn pools and redox state may help explain observed variability in N losses and greenhouse gas production across agricultural soils.
