Abstract

Replacing peat in green roof substrates with sustainable alternatives while maintaining plant performance and ecosystem services remains a critical challenge. We studied biochar-substrate interactions across four commercial green roof formulations (based on the type of organic component) in a greenhouse experiment: pure vermicompost, vermicompost + fen peat, fen peat, and mixed fen/high-moor peat. Substrates were amended with straw biochar, pine bark biochar, or left unamended (5% v/v, n = 4 replicates) and planted with a grass seed mixture mimicking early green roof establishment. Plant growth, nutrient contents (nitrate and phosphate contents), and microbial indicators (microbial biomass carbon (MBC), qCO2, and enzyme activities) were measured 30 days after the experiment began. Straw biochar in vermicompost boosted nitrate (90.8 mg kg−1) and root N (3.1%) compared to the control, while pine bark biochar in mixed peat released phosphate (+375%) and maximized MBC (874 µg g−1). Biochar intensified substrate effects, suppressing CO2 in peat through liming effects (pH from 4.6 to 6.5–7.1) but priming respiration in vermicompost via labile C supply. PCA explained 63% of the variance, with nitrate, plant N, and microbial parameters driving substrate separation. These short-term greenhouse results demonstrate critical biochar-substrate specificity for green roof substrate development, emphasizing formulation-specific matching over universal biochar application.

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