Effects of Biochar Addition on Soil Properties and Greenhouse Gas Emissions in Permafrost Wetlands Under Different Culture Methods

Permafrost wetlands are important components of the global carbon cycle, functioning as carbon stores while also releasing methane (CH4). Although biochar amendment has been proposed as a strategy to mitigate greenhouse gas (GHG) emissions, its effectiveness in permafrost wetland soils under contrasting moisture regimes remains uncertain. We conducted a 100-day incubation experiment to examine the effects of pine-sawdust biochar produced at 300 or 800 °C and applied at five rates (0%, 1%, 3%, 5%, and 8%, w/w) on soil properties, enzyme activities, and CO2 and CH4 emissions under non-flooded and waterlogged conditions. Biochar addition altered soil pH, carbon and nitrogen pools, and enzyme activities, with responses varying between moisture regimes. Under non-flooded conditions, 300 °C biochar reduced soil pH and increased nitrate nitrogen (NO3−-N), whereas under waterlogged conditions, biochar treatments increased soil pH and were associated with higher ammonium nitrogen (NH4+-N) and lower NO3−-N concentrations. Biochar generally reduced CO2 emissions under non-flooded conditions and CH4 emissions under waterlogged conditions. The 8% amendment of 800 °C biochar under waterlogged conditions yielded the lowest reported global warming potential (GWP; 5.49 ± 0.15 g·kg−1). Enzyme responses also depended on moisture regime: dehydrogenase and urease activities generally declined under waterlogged conditions, whereas 300 °C biochar increased catalase activity. Pearson correlation and SHapley Additive exPlanations (SHAP) analyses identified NH4+-N (29.3%) and catalase activity (17.4%) as the two leading predictors of GWP in the fitted model. These findings suggest that biochar amendment can reduce GHG emissions during laboratory incubation, although the magnitude and direction of the response depend on pyrolysis temperature, application rate, and moisture regime. Among the treatments tested, the 8% amendment of 800 °C biochar under waterlogged conditions produced the lowest GWP.