Biochar-amended substrate improves nutrient retention in green roof plots

Abstract

Green roofs can act as pollutant sources due to the nutrients nitrogen (N) and phosphorus (P) leaching from the engineered soil-like substrate. Designing substrate to reduce this effect, while continuing to provide nutrients for plants, is essential to minimize this ecosystem disservice. Biochar is a water-retaining soil additive with the potential to increase stormwater retention and bind nutrients, thus could reduce loss of nutrients in runoff and simultaneously improve plant performance. Over two growing seasons, our study evaluated plant cover, nutrient retention and water retention in green roof experimental plots after the addition of biochar to the substrate. Replicated plots of green roof substrate amended with different amounts of biochar were established, both vegetated (Sedum mixture) and unvegetated. After initial establishment, plant cover was highest in the intermediate (5% w/w) biochar treatment, and lowest in the high (10% w/w) biochar treatment. Biochar addition did not significantly affect water retention, but improved runoff water quality by decreasing phosphorus, organic carbon and organic nitrogen export, all of which were high in runoff from the standard green roof substrate. Biochar was found to be a minor source of nitrate, but this effect was counteracted by plant presence, with plants greatly reducing N runoff losses. Overall, these results strengthen the case for biochar as a potentially useful amendment for green roofs.