Manganese concentration influences nitrogen cycling in agricultural soil

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.