Mitigating nutrient leaching from green roofs with biochar

Abstract

Green roofs may have great potential for managing urban runoff but their effects on the quality of infiltrating water are highly variable. At worst, it can contain high concentrations of nutrient pollutants, especially phosphorus. We made a survey in southern Finland and found much higher concentrations of phosphorus and nitrogen in runoff from existing green roofs than nearby non-vegetated bituminous and tin roofs. However, metal concentrations in runoff generally exhibited an opposite pattern. Media mixtures with aggregates, such as biochar, have been suggested as a means of retaining nutrients in green roof substrate. Thus, we set up a green roof platform experiment in order to study the amendment of biochar in a substrate consisting of mainly recycled crushed brick, by using two vegetation establishment methods: pre-grown mats and planting with seeds and plug plants. We monitored the effect of biochar on runoff during one year across all four seasons, including winter with periods of freezing and thawing. At the beginning of the experiment we found negligible impacts of biochar, but when the systems had matured for less than one year, biochar retained nutrients. The total annual loads of both nutrients were significantly reduced by biochar amendment in both green roof types. Efficient solutions are needed if urban greening, such as green roofs, is to be applied in a way that does not produce ecosystem disservices like nutrient pollution. According to our results, amending green roof substrates with biochar could be a partial solution to the nutrient pollution problem.