Land application of pulp and paper mill sludge may reduce greenhouse gas emissions compared to landfilling

Land application of pulp and paper mill sludge (PPMS) may replace landfilling in certain jurisdictions. However, the consequences of this change in management on greenhouse gas (GHG) emissions from PPMS are unknown. Land application may also entail temporary storage by landfilling due to a timing mismatch between PPMS production and application. We quantified direct GHG emissions from landfilled PPMS (pilot scale; N-rich and N-poor types) over two snow-free seasons, and after application of previously landfilled PPMS (LPPMS; < 6-month landfilled) to a clay loam soil cropped to wheat over one snow-free season. Landfilling of N-rich PPMS induced the highest GHG emission factor (EF; 0.26 Mg CO2-equivalent Mg-1 dry PPMS) and generated substantial N2O emissions (2% of landfilled N), which are neglected by the current assessment methods. Landfilling of N-rich PPMS also resulted in the highest CH4 emissions (0.6% of landfilled C). Replacing urea with LPPMS to fulfill wheat N requirements did not change field N2O-N EFs (1.1-3.5% of applied N) and did not generate CH4. Shortterm (1-2 years) GHG (CO2-equivalent) EFs with landfilled N-rich PPMS were 0.54 to 4.48 times the EFs reported in the literature for land-applied PPMS. Long-term EF estimates suggest that landfilled PPMS may increase GHG emissions by up to three times compared to land application. Further research at the industrial scale is warranted to reduce uncertainty on GHG emissions from PPMS landfill sites, fulfill sustainable development goals and determine accurate carbon credits associated with changes in PPMS management practices.