Soil organic carbon credits from turf and clover planting beneath solar panels

• First SOC credit methodology combining solar panels and ground vegetation in Japan. • A spray-seeding technique enhances vegetation establishment under solar panels. • SOC quantification: Loss on ignition, Tyurin method, and automated dry combustion. • Net SOC sequestration of 85.785 tCO₂e achieved through turf-clover establishment. • Annual SOC increase of 0.927 tC·ha⁻¹·year⁻¹ (3.397 tCO₂e·ha⁻¹·year⁻¹). Although Japan's forests cover approximately two-thirds of its land area and offer potential for improved management, new large-scale afforestation efforts face substantial constraints, highlighting the need for alternative methods to enhance soil organic carbon. This study investigates whether planting turf and clover beneath solar panels can increase soil carbon and reduce erosion. Over two years in Okayama Prefecture, seeds and a binder solution containing fiber and fertilizer were applied to bare ground. While three measurement methods were tested, the loss on ignition method was selected for its simplicity. Analysis at 24 sampling sites revealed measurable carbon gains in the surface soil, estimated at approximately 3.4 tonnes of carbon dioxide equivalent per hectare per year. These findings suggest that under-panel vegetation could be a viable option for climate mitigation in Japan's constrained land-use context.