Extensive nutrient leaching has been a major concern in the establishing stage of green roofs. Although an addition of biochar to the green roof substrates has been increasingly recommended, the extent to which this addition can affect the runoff quality is still largely unknown. Using biochars made from maize straws (MSB) and rice husks (RHB), this study investigated the effects of biochar addition rates (0%, 10%, 15%, and 20% biochar, v/v) on the runoff quality of new green roofs over 6 months. Our results show that the addition of biochar significantly affected the runoff quality. With an increasing biochar addition rate (10∼20%), the mean total nitrogen (TN) concentration in the runoff decreased from 103.68 mg L${}^{-1}$ (CK) to 26.21∼52.77 mg L${}^{-1}$ (RHB) and 10.12∼3.97 mg L${}^{-1}$ (MSB), the mean dissolved organic carbon (DOC) concentration decreased from 94.47 mg L${}^{-1}$ (CK) to 101.76∼59.41 mg L${}^{-1}$ (RHB) and 52.45∼26.73 mg L${}^{-1}$ (MSB), and the mean pH increased from 7.15 (CK) to 7.42∼7.50 (RHB) and 7.49∼7.71 (MSB). However, the mean total phosphorus (TP) concentration increased from 0.27 mg L${}^{-1}$ (CK) to 0.22∼0.57 mg L${}^{-1}$ (RHB) and 0.58∼1.07 mg L${}^{-1}$ (MSB). Generally, the N and DOC concentrations were lower in the treatment with added MSB than RHB, but the P concentrations and pH were higher. The N concentration was significantly negatively correlated with the single rainfall and cumulative rainfall in the CK- and RHB-added treatments but not in the MSB-added treatments, suggesting that the addition of MSB affected the process of N leaching from the substrate. Overall, we recommend adding 10% maize straw biochar to the green roof substrate to reduce the initial nutrient leaching from the new green roof and improve the runoff water quality.

The home-field advantage (HFA) effect has been reported to occur in coarse woody debris (CWD) and litter. It is thought that the HFA effect may be due to the specialization of decomposers in their original habitats. However, the relative contribution of microorganisms, particularly fungi and bacteria, to deadwood decomposition is unclear because of differences in their functional at-tributes and carbon requirements, and the microorganisms that drive the HFA effect of deadwood are also unclear. Here, we analysed a dataset of microbial PLFA and substrate properties collected from the soil and CWD of two subtropical trees, Cryptomeria japonica and Platycarya strobilacea, from forests dominated by one or the other of the two species, with both species present in the forests. Our results showed that habitat and tree types all significantly affected CWD respiration rates, the CWD respiration rates were significantly higher in the deciduous broadleaf forests (DBF) than in the coniferous forest (CF) regardless of tree types, but no a large HFA of CWD decomposition found (HFA index was 4.75). Most biomarkers indicated bacteria and fungi were more abundant in the DBF than in the CF, and the concentration of microbial PLFAs was higher in Platycarya strobilacea than in Cryptomeria japonica. In addition, the relative abundance of fungi and soil B/F were remarkably positively correlated with CWD respiration, indicating that fungi may be the primary decomposers of CWD. In conclusion, our work highlights the importance of interactions between the three primary drivers (environment, substrate quality and microbes) on CWD decomposition.