Biochar amendment in the green roof substrate affects runoff quality and quantity

- Extensive pre-grown green roofs are a source of nutrient pollution.
- Biochar had a combined impact on runoff quantity and quality over time.
- Biochar amended in the substrate reduced cumulative nutrient leaching.
- Another biochar type increased the leaching of nutrients from green roofs.
- Studies on the properties of biochar are needed to avoid unintended consequences.

The utilisation of ecosystem services has been suggested as one solution to manage urban environmental problems, one of which is the excessive quantity or poor quality stormwater. As roofs contribute significantly to the amount of runoff, vegetated, i.e. green roofs have become an increasingly popular way to manage urban water in densely built areas. However, green roofs may introduce a new source of water pollution evidenced as higher concentrations of nutrients, especially phosphorus, in runoff compared to that in precipitation inputs. In two controlled, replicated experiments, one in the field for 7 months and another in the laboratory for 6 weeks, the amendment of biochar to green roof substrates was studied for its potential to mitigate the leaching of nutrients from newly installed pre-grown green roof Sedum and meadow mats. Nutrient concentrations were an order of magnitude higher in runoff from green roofs than in rain water. In the field experiment, biochar reduced the cumulative leaching of nutrients, even though biochar did not significantly reduce nutrient concentrations. These results can be interpreted as a combined impact of biochar on both the quantity and quality of runoff over time, the quantitative effect being apparently stronger than the qualitative. In the laboratory experiment, one type of biochar reduced nutrient concentrations and load in runoff while another type had an opposite effect. As the properties of biochar can vary considerably, careful studies are necessary before large-scale implementation of biochar amendment in green roofs are considered, to avoid unintended consequences.