Invasive Phragmites australis is a significant threat to Great Lakes’ wetlands, creating monospecific stands that choke out native wetland plants and reduce habitat suitability for many animal populations. Applying soil amendments with high C/N ratios should favour nitrogen (N) immobilization, helping to reduce invasive species, including P. australis. These amendments could also increase soil carbon storage. We tested this strategy’s viability with a column experiment using 50 marsh sediment cores collected from Long Point, ON. Ten cores were destructively sampled to determine initial porewater concentrations of dissolved organic carbon (DOC), and dissolved organic and mineral N. Five treatments, with eight replicates, were applied to the remaining cores: 1) unamended control, 2) low (1 kg/m2) and 3) high (2 kg/m2) dose hardwood residue addition, and 4) low and 5) high dose softwood residue addition. We measured CO2, CH4 and N2O emissions weekly for four months, followed by porewater sampling. Ammonium dominated the mineral N pool, with a significant concentration decline over the experiment, except for the softwood high dose. Cores amended with hardwood residues had lower, but not significantly different, NH4+ concentrations than control. Amendment with softwood residue resulted in significantly higher DOC concentrations and 10–30 times higher organic N porewater concentrations. Greenhouse gas emissions were not significantly affected by amendments. Results indicate that carbon in hardwood residues is relatively stable in wetland sediment and has potential to slightly reduce mineral N availability. We recommend field experimentation to test high C/N ratio amendments, along with monitoring the plant community response.