Continuous leachate irrigation as an ammonia source enhancing methane oxidation in landfill biocover soils

Nitrogen amendment can help recover high methane (CH₄) oxidation activity in landfill cover soils experiencing a post-peak decline. Herein, the feasibility of leachate used as an ammonia-rich liquid to restore CH₄ oxidation activity was assessed in landfill biocover soils in comparison with ammonia addition under both batch and continuous application modes. Although batch ammonia and leachate additions at 400 mg kg^-1 NH₄⁺-N similarly boosted the initial CH₄ oxidation rate, continuous leachate addition achieved 1.17-2.61 times greater accumulative CH₄ consumption than the others at the end of the incubation. The added ammonia could be rapidly converted into organic nitrogen and deposited in the landfill biocover soils. The continuous ammonia and leachate supplements were superior for microbial growth compared to the batch treatments. Type I methane-oxidizing bacteria (MOB) including Methylocaldum, Methylomicrobium and Methylobacter, and type II MOB Methylosinus were the major methanotrophs in the landfill biocover soils. The total abundances of type II MOB ranged from 9.06 × 10⁷ to 1.26 × 10⁸ copies g^-1 in control and the treatments with continuous ammonia or leachate supplement at the end of the incubation, 10.0-29.2 times higher than in the batch treatments, indicating a potential enhancement in N₂-fixing activity of MOB under low-bioavailable nitrogen conditions. Partial Least Squares Path Modeling showed that the continuous ammonia and leachate addition primarily enhanced CH₄ oxidation activity by increasing microbial and MOB abundances in the landfill biocover soils. These findings indicated that continuous small-scale leachate irrigation could serve as an ammonia source for operating cover systems to reduce CH₄ emissions from landfills.