Ameliorative Effects of Biochar on Rapeseed (Brassica napus L.) Growth and Heavy Metal Immobilization in Soil Irrigated with Untreated Wastewater

Untreated wastewater carries substantial amount of heavy metals and causes potential ecological risks to the environment, food quality, and soil health. The pot study was established to evaluate the effectiveness of woodchip-derived biochar (BC) on rapeseed biomass, photosynthetic pigments, antioxidant enzyme activities, such as peroxidase (POD), ascorbate peroxidase (APX), polyphenol peroxidase (PPO), catalase enzyme (CAT), and heavy metal-induced phytotoxicity under untreated domestic (DWW) and industrial (IWW) wastewater irrigation. Biochar was applied at three levels (0, 1, and 2%) combined with DWW and IWW treatments. Wastewater analysis indicated higher heavy metal concentrations than the safer limits set by FAO. Results revealed that DWW and IWW treatments without biochar incorporation adversely affected the rapeseed growth performance, photosynthetic pigments, and antioxidative defense system. Compared with DWW and IWW treatments, BC at 2% rate significantly enhanced shoot fresh biomass (41% and 72%), root fresh biomass (34% and 62%), total chlorophyll (79% and 85%), total pigments (77% and 108%), carotenoids (74% and 94%), and lycopene concentration (43% and 61%), respectively. In addition, BC also improved the antioxidant enzymes activities by reducing the heavy metal-induced oxidative stress in rapeseed leaves. Similarly, AB-DTPA extractable Cd was decreased by (44% and 26%), Pb (51% and 54%), Ni (59% and 56%), and Cu (45% and 41%) in soil when BC was applied at 2% application rate along with DWW and IWW treatments and thereby reduced their uptake in shoots and roots of rapeseed. Therefore, BC can be considered an efficient strategy to ameliorate the hazardous effects of untreated DWW and IWW wastewater and to enhance rapeseed biomass, physiological attributes, and antioxidant enzyme activities.