Biochar Addition and the Runoff Quality of Newly Constructed Green Roofs: A Field Study

Abstract

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.