In-field carbon dioxide removal via weathering of crushed basalt applied to acidic tropical agricultural soil

Enhanced weathering (EW) of silicate rocks such as basalt provides a potential carbon dioxide removal (CDR) technology for combatting climate change. Modelling and mesocosm studies suggest significant CDR via EW but there are few field studies. This study aimed to directly measure in-field CDR via EW of basalt applied to sugarcane on acidic (pH 5.8, 0-0.25 m) Ultisol in tropical northeastern Australia, where weathering potential is high. Coarsely crushed basalt produced as a byproduct of gravel manufacture (<5 mm) was applied annually from 2018 to 2022 at 0 or 50 t ha^-1 a^-1, incorporated into the soil in 2018 but not in subsequent years. Measurements in 2022 show increased soil pH and extractable Mg and Si at 0-0.25 m depth, indicating significant weathering of the basalt, but showed no increase in crop yield. Soil inorganic carbon content and bicarbonate (HCO₃^-) flux to deep drainage (1.25 m depth) were measured to quantify CDR in the 2022-2023 wet season (i.e. one year). Soil inorganic carbon was below detection limits. Mean HCO₃^- flux was 3.15 kmol ha^-1 a^-1 (±0.40) in the basalt-treated plots and 2.56 kmol ha^-1 a^-1 (±0.18) in the untreated plots but the difference (0.59 kmol ha^-1 a^-1 or 0.026 t CO₂ ha^-1 a^-1) was not significant (p = 0.082). Most weathering of the basalt was attributed to acids stronger than carbonic acid. These were, in decreasing order of contribution, surface-bound protons (inherent soil acidity), nitric acid (from nitrification), organic acids, and acids associated with cation uptake by plants. These results indicate in-field CDR via EW of basalt is low where soil and regolith pH is well below the pK_a1 of 6.4 for H₂CO₃. However, increased soil pH, and the consumption of strong acids by weathering will eventually lead to reduced CO₂ emission from soil or evasion from rivers, with continued basalt addition.