There is a long-standing tradition of using seaweed (SW) as a fertilizer in coastal areas across the globe. SW acts as a slow-release fertilizer/ soil conditioner, decomposing to release major (e.g., nitrogen (N)) and minor nutrients. Moreover, slow N release from SW and bromoform in SW (i.e., methane (CH4) inhibitor) may reduce nitrous oxide (N2O) and CH4 emissions, respectively, from amended soils. In this study, we determined the key biogeochemical processes governing N cycling and greenhouse gas (GHG) emissions from sugar kelp (SK) amended podzols. Bioreactors containing either soil (control, n=3) or soil + SK (n=3) were saturated and exposed to 3 alternating oxidizing/ reducing cycles by sparging with air (7 days) and N2 (21 days), respectively. Aqueous, gas, and solid phase samples were analyzed for various chemical species, including GHGs, anions, dissolved organic carbon (DOC), and total nitrogen (TN). The biodegradation of SK increased bioavailable DOC and TN relative to controls which fueled microbial processes, increasing carbon dioxide (CO2) and CH4 production while also reducing N2O production. Higher Fe(II) concentrations (208 µM) were observed in soil + SK bioreactors relative to the controls (25 µM) and are attributed to microbial Fe(III) reduction which delayed CH4 production (i.e., lower energy-yielding metabolism). The results from this study build on the ingenuity of past generations to demonstrate the potential GHG mitigating properties (i.e., N2O reduction and delayed CH4 production) of SK-based fertilizers and, in doing so, advances our ability to make evidence-based recommendations for GHG mitigation in agriculture.