Carbon dioxide sequestration model of a vertical greenery system

A dynamic model simulating carbon dioxide (CO2) sequestration by plants embedded in a vertical greenery system (VGS) was developed. The aim of the model was to demonstrate the advantages, in terms of CO2 sequestration, obtained by installation of large vegetated wall surfaces in urban areas. The ability of various plants, tested separately or in a mixed-species set, to accumulate carbon in different compartments and moments of their life (plants, compost, soil and microbial biomass in soil) was quantified. The model simulates a series of processes, from planting and growth of herbaceous plants in the VGS to the end of their lives, when compost is produced from green residues and is added to agricultural soil. The amount of CO2 removed from the atmosphere is the portion finally stocked in the soil in the form of microbial biomass. The plants considered are commonly used in VGS because they are perennial and suitable for hydroponic culture. Typical Mediterranean climate was assumed for the VGS model. The herbaceous species selected were the perennial grass Zoysia matrella, the succulent Sedum spurium, Mediterranean shrubs Salvia nemorosa and Rosmarinus officinalis and flowering ornamental plants Geranium sanguineum, Carex brunnea and Facia japonica. As estimated by the model, the mean yearly CO2 flow absorbed by plant biomass of these species (expressed in carbon dioxide equivalents, CO2eq) was 0.44-3.18 kg CO2eq m(-2) of vertical garden. Green residues from plant replacement and trimming/pruning were treated to produce compost and added to agricultural soil. We estimated that a 98 m(2) VGS can capture an average carbon dioxide flow of 13.41-97.03 kg CO2eq per year. The model predicts increasing long-lasting carbon dioxide accumulation in the VGS over time, indicating a feasible solution for climate change mitigation in urban areas. The model can be adapted to different plant species and different geographical, climatic and biological conditions.