Novel recycling strategy for distillers’ grains waste: prolonged biochar aging promotes cadmium immobilization and lettuce growth by regulating soil nutrient availability and bacterial composition

Converting distillers’ grains (DGs) into biochar (BC) is sustainable option for waste-recycling, but how different aging times and application rates DG-derived BC (DG-BC) influenced lettuce growth and cadmium (Cd) uptake in soil was unclear. This study explored DG-BC rate- and aging time-dependent effect on lettuce growth and Cd uptake, and effect mechanisms from insights of soil nutrient and Cd bioavailability, lettuce metabolic activity and rhizospheric bacterial composition.

Pot experiments involved three DG-BC rates (1%, 2%, 4%) and three aging times (0, 3, 6 months) in Cd-polluted soil. Effects of different DG-BC rates and different aging times on lettuce growth, metabolic activity and Cd uptake, soil nutrient and Cd bioavailability, and rhizospheric bacterial composition were explored.

Rising DG-BC rate and aging time increasingly enhanced lettuce growth, antioxidant activities and soil nutrient availability, while progressively decreased Cd bioavailability and Cd uptake by lettuce. Sphingomonas, RB41 and Nitrospira abundance in rhizosphere soil could be enhanced at higher DG-BC rate, and DG-BC aging in soil further promoted or maintained their enhanced abundance at higher DG-BC rate. Thus, the enhanced abundance of these keystone genera played crucial roles in promoting lettuce growth, soil nutrient availability, and decreasing Cd bioavailability and uptake.

Applying 4% DG-BC with 6 month-aging maximized lettuce growth and minimized Cd uptake by maximally decreasing Cd bioavailability, increasing soil nutrient availability and regulating rhizospheric bacterial composition, thus becoming a suitable DG-BC application strategy to promote lettuce production and mitigate Cd-induced health risk through lettuce consumption.