Long-term field evaluation of sewage sludge biochar in green roof substrates reveals hydrological, vegetation, and microbial responses

Sewage sludge biochar (SSB) represents a circular material with the potential to improve the ecological performance of extensive green roofs and valorize nutrient-rich waste streams. Here, we report a 40-month rooftop monitoring experiment under temperate conditions evaluating the effects of SSB incorporation (10% and 20% v/v) on substrate properties, hydrological performance, runoff quality, vegetation development, and microbial diversity. SSB amendment reduced bulk density and increased total porosity and water-holding capacity, resulting in higher substrate moisture availability. These changes led to improved water retention, with annual runoff reduced by 3.3-8.5%, with the highest reduction observed in the SB20 treatment during the driest year (2022), and cumulative seasonal retention reaching 76% during dry years. Despite the nutrient-rich origin of SSB, runoff nutrient concentrations exhibited initial increases and subsequent stabilization, consistent with gradual nutrient release dynamics. Vegetation cover reached to 73% in the control and to 84% (SB20) and 89% (SB10) over the study period, with no significant difference between the two amendment rates. Microbial analyses revealed higher bacterial and fungal richness in SSB treatments, particularly at 20% v/v. Structural equation modelling indicated that variation in microbial richness was associated with changes in runoff chemistry (pH, electric conductivity, total suspended solids) and interannual dynamics, rather than effects explained by a single factor. Overall, this study provides evidence from extended rooftop monitoring, highlighting long-term system responses of extensive green roofs amended with SSB, particularly regarding water balance, substrate physical development, and microbial diversity.