Removal of carbon dioxide through enhanced weathering of basalt in acidic soils under sugarcane

Enhanced weathering (EW) of silicate rocks like basalt offers a potential negative emissions or carbon dioxide removal (CDR) technology in society?s efforts to combat climate change. Modelling suggests that significant CDR via EW, but this requires field verification. This study aimed to quantify CDR through EW of basalt applied to sugarcane and determine co-benefits. The experimental site had received five basalt applications of 0 or 50 t ha-1y-1 since 2018 and a 0 or 2.5 t ha-1 lime application in 2022, creating a split-plot design for this study. Drainage fluxmeters (DFMs) were installed beneath the root zone to capture leachate during the 2022/23 wet season, and leachate bicarbonate (HCO3-) flux was measured to quantify CDR. Cumulative HCO3- flux was not significantly influenced by basalt (F(1, 12) = 0.713, p = 0.415) or lime (F(1, 12) = 0.505, p = 0.491) or their interaction. All CDR values were small compared to potential HCO3- generation from carbonic acid weathering of the basalt. The discrepancy was attributed to preferential weathering by stronger acids, and consumption of HCO3- in this acidic soil. There was elevated production of nitric and organic acids in the row, where cumulative HCO3- was significantly lower than in the inter-row (p < 0.026). This relationship was consistent with fertiliser application (and associated nitrification), and microbial activity and plant root exudation beneath sugarcane in the row. Results suggest pH of acidic soils may need to be raised to achieve CDR through EW. There was no significant effect of basalt on cane yield, but commercial cane sugar content increased slightly. Although basalt application did not lead to significant CDR in the field, the neutralising effect of basalt in acidic soils, and increased nutrient availability are beneficial.