Pyrogenic carbon improves the physiological performance of a C₃ species planted on a green roof

Plants utilizing C₃ physiology have a more difficult time establishing in rooftop environments than plants with more heat and drought adapted constitutions, such as species that employ crassulacean acid metabolism (CAM). CAM species are much less susceptible to limitations of shallow, infertile soil-less media under abiotic and biotic stress. It is thought that soil amendments might improve rooftop media in a way that allows for C₃ species to prosper in rooftop environments. While compost is typically added to media to achieve this goal, we hypothesized that the addition of an anthropogenic pyrogenic carbon (PyC) supplement, instead, would enable better organic and mineral sorption and water retention, resulting in improved physiological performance of C₃ species. To test this, we grew a C₃ legume species, wild indigo (Baptisia tinctoria L R.Br. ex), in control compost-amended media and media amended by PyC on a rooftop in Massachusetts, USA. We found PyC-amended media had greater mean organic and mineral nutrient sorption. We also found 16% greater soil water holding capacity (GWL/ψ_g) than control media. In addition, wild indigo photosynthetic intrinsic water use efficiency (iWUE) was significantly increased by 19% when grown in PyC-amended as compared to control media. We conclude that amending green roof media with PyC provides greater benefits than compost amendments for colonization of a C₃ legume, wild indigo. Our results gathered over seven years suggest that PyC from converted waste stream cardboard could be used to improve the rooftop performance of other leguminous species, including agricultural crops.