The effect of land-use conversion from agriculture to perennial biomass crops and nitrogen fertilizer on soil organic carbon stock in southern Ontario, Canada

Switchgrass [Panicum virgatum, (SG)] and miscanthus [Miscanthus spp., (Mis)] are perennial biomass crops (PBCs) commonly grown in Ontario, Canada. By the integration of PBCs on marginal/degraded agricultural (Ag) fields, a positive gain in soil organic carbon (SOC) sequestration has been reported in the scientific literature. Therefore, using PBCs to enhance SOC sequestration can potentially contribute to Canada's goal to reach net-zero greenhouse gas emissions by 2050. However, long-term field research (>10 years) to clearly demonstrate this trend in Canada is limited. In the above context, this study compared the current SOC stock (Mg C ha(-1)) in SG and Mis to that of baseline Ag SOC obtained from three different locations, namely Elora (2008-2019), Guelph (2009-2020) and Burlington (2016-2020), Ontario, Canada. SOC stock at the time of land-use conversion from Ag to SG and Mis was considered as baseline. Woodlots (WLs), undisturbed natural forests, were used as reference SOC (potential maximum SOC stock) to predict the potential SOC sequestration in the future by PBCs at the respective locations. Results showed that SOC stocks in all SG fields, Mis fields and WLs were higher compared to respective baseline Ag fields. SOC stocks (Mg C ha(-1)) in PBCs were significantly higher in Elora (SG: 96.3 +/- 3.44, Mis: 100.0 +/- 1.48) and Guelph (SG: 88.5 +/- 5.72, Mis: 87.9 +/- 6.43) over 11 years, whereas SOC stock in Burlington (SG: 87.5 +/- 5.04, Mis: 94.7 +/- 4.86), showed no significant difference over 4 years compared to their respective baseline Ag fields (77.6 +/- 3.09, 59.3 +/- 1.17 and 84.1 +/- 3.39). All WLs had significantly higher SOC stock compared to their respective baseline Ag fields, showing the potential for future SOC stock gain by PBCs. SOC stock values in SG, with different nitrogen (N) fertilizer application rates (0, 40, 80 and 160 kg N ha(-1)), were not significantly different. Conversely, SOC stock at all N rates were significantly higher in SG than baseline Ag SOC stock values. For Mis, only N fertilizer rates 40, 80, 160 kg N ha(-1) significantly increased SOC stock. However, within the four N fertilizer rates, SOC significantly increased only at 80 kg N ha(-1). Increases in SOC stock suggest that converting marginal lands to PBCs over the long-term (>10 years) could create additional terrestrial C sinks. In Canada, there are close to 10 million ha of degraded Ag lands that can potentially be repurposed for PBCs, therefore the conversion of these unproductive Ag lands to PBCs could significantly contribute to Canada's climate mitigation strategies.