Warm season perennial C4 grasses (WSGs), switchgrass (Panicum virgatum L.) and miscanthus species (Miscanthus spp.), have been reported to positively influence short-term (15–20 years) soil organic carbon (SOC). In this study, the DayCent model was used to predict changes in long-term SOC stocks under WSGs for moderate (Representative Concentration Pathway (RCP) 4.5) and high (RCP 8.5) warming climate change scenarios in southern Ontario, Canada, and to determine how long the enhanced SOC stock will last when WSGs are converted back to annual crop rotation. The model predicted that a consistent corn–corn–soybean–winter wheat (CCSW) rotation prevented SOC from depletion over the 21st century. Under WSGs, the model predicted high rates of SOC sequestration during the first 20–30 years which then tended to stabilize after 50–60 years. However, the rate of SOC sequestration over 90 years for RCP 4.5 was 0.26 and 0.94 Mg C ha⁻¹ yr⁻¹ for switchgrass and miscanthus, respectively. If 40-year stands of WSGs are converted back to CCSW, the model predicted SOC decline to the previous level in 40–50 years. DayCent predicted that under RCP 8.5 scenario in the second half of the 21st century and in the future, there will be a reduction in SOC stocks, especially under miscanthus stands.

Switchgrass ( Panicum virgatum) and miscanthus ( Miscanthus spp.) are C₄ perennial biomass crops (PBCs) commonly cultivated in Ontario. The ¹³C natural abundance technique was used to assess the following: (i) quantify the proportion of C₄-derived soil organic carbon (SOC) stock (Mg C ha⁻¹) within the SOC stocks of the whole soil, (ii) determine the C₄-derived SOC sequestration rate, and (iii) quantify C₄-derived SOC stocks in soil particle size fractions. This was achieved by analyzing the δ¹³C values in SOC of both the whole soil and various soil particle size fractions in PBC fields that were converted from agriculture. At Guelph after 11 years of PBC, C₄-derived SOC stock as a proportion of SOC stock in the whole soil accounted for 30% in switchgrass and 31% in miscanthus fields, while at Elora it comprised 41% in switchgrass and 50% in miscanthus. However, in Burlington after 4 years, C₄-derived SOC represented 10%–12% in PBC fields compared to 17% in agriculture fields. The C₄-derived SOC sequestration rates were 0.8, 0.7, and 0.4 Mg C ha⁻¹ year⁻¹ in switchgrass, miscanthus, and agriculture fields, respectively at the Guelph site. In miscanthus fields, lower C₄-derived SOC stock in macro soil particles were observed, yet higher percentages were observed in micro soil particles and the silt plus clay fractions compared to switchgrass and agriculture fields. Agriculture fields had higher C₄-derived SOC in macro soil particles, indicating lower SOC stability. Therefore, cultivating PBCs on low-productive agricultural lands could contribute to climate change mitigation practices due to increased SOC stability.