Abstract: Sunken green spaces offer important advantages for managing runoff and preserving water quality, which are commonly used in urban areas. However, their potential for carbon sequestration is not well understood. We investigated the Nakao River Wetland Park in Nanning, employing plot surveys and laboratory tests to evaluate carbon content and storage in both vegetation and soil of sunken green spaces-including bioswales, rain gardens, and water-purified terraced fields-in contrast to horizontal green spaces, such as traditional lawns and layered tree-shrub-grass environments. The results showed that: (1) In layered tree-shrub-grass configurations, the carbon density in sunken green spaces was higher than that in horizontal green spaces (P<0.05). Specifically, vegetative carbon densities for rain gardens and water-purified terraced fields were 2.42 and 1.43 kg·m^-2 respectively, which were 2.2 and 1.3 times greater than those of tree-shrub-grass landscapes. For that with single-layer structure, the carbon density of bioswales was at 0.15 kg·m^-2, making it 1.2 times that of conventional lawns. (2) In the 0-20 cm soil layer, carbon densities in rain gardens and water-purified terraced fields reached 5.39 and 2.15 kg·m^-2, respectively, being 3.1 and 1.2 times higher than that of tree-shrub-grass landscapes. Bioswales in the 0-20 cm soil layer displayed a carbon density of 2.40 kg·m^-2, which was 2.5 times higher than conventional lawns. (3) The main reason for carbon accumulation in sunken green spaces is the increases in plant growth by enhancing organic matter and nutrient accumulation due to larger catchment areas. This study helps improve the understanding of carbon sequestration capabilities and mechanisms within sunken green space ecosystems.

Key words: carbon density, biomass, carbon sequestration, sunken green space, exogenous runoff input