Trichoderma harzianum inoculum vs. biochar for lightweight green roof substrate optimization

Trichoderma harzianum inoculum (THI), with a lower required amendment ratio and product carbon footprint, is promising as an alternative amendment to biochar for green roof (GR) substrate optimization. Our previous studies have shown that biochar does not always benefit lightweight GR substrates, while THI’s potential as an amendment and the impacts of both amendments on the microbial communities of lightweight GR substrates still require further investigation. In the present study, biochar and THI with low and high ratios were added to the lightweight substrate mixture of 40 % peat soil and 60 % vermiculite to install the GR modules. On this basis, the plant landscape, water-holding and nutrient-retaining performance during a three-month field monitoring period were measured, and the microbial communities in the GR modules were analyzed. Results revealed that the high-ratio biochar amendment (20 %) improved the substrate nutrient status but significantly reduced its water-holding performance. In contrast, 1 % THI not only promoted the highest average substrate water content of 33.90 % ± 5.79 % and a relatively low evapotranspiration rate but also contributed to a better plant landscape, which is more applicable for the optimization of GR substrates with low nutrient requirements. Notably, the high and low ratios of biochar and THI amendments separately enriched specific beneficial microbial taxa, including the genera Ellin6055 , Pedomicrobium , Clitopilus , Arthrobacter , JGI_0001001-H03 , and Bauldia . Our findings highlight the differential effects and potentials of biochar and THI for lightweight GR substrate optimization and provide insights into the microbial communities in artificial habitats of GRs. • 1 % THI versus biochar amendment enhances substrate evaporation resistance. • Biochar amendment and its increased ratio improve substrate nutrient status. • 1 % THI can be an alternative to 20 % biochar for lightweight GR substrate optimization. • Beneficial taxa, not microbial diversity, may drive plant landscape enhancement in GR.