Effects of basalt application on crop growth and carbon sequestration through enhanced rock weathering

Enhanced Rock Weathering (ERW) is a climate mitigation strategy that accelerates natural rock weathering processes to sequester atmospheric carbon dioxide. Applying crushed basalt to agricultural soils releases base cations (Ca²⁺, Mg²⁺) that form stable carbonates or leach as bicarbonate to the ocean. In addition to carbon sequestration, basalt weathering provides crop nutrients, potentially improving yields and quality. This study evaluated the effects of basalt application on crop growth and soil carbon sequestration through a two-year field experiment, with a focus on elemental dynamics.

A field experiment was conducted from 2023 to 2024 at an experimental field in Hokkaido University (43.07° N, 141.34° E; gray lowland soil), using soybean (Glycine max (L.) Merrill.) in the first year and maize (Zea mays L.) in the second. Five treatments with three replications were established: a control, three basalt application rates (5, 10, and 20 wt.% incorporated into the top 15 cm; BA5, BA10, BA20), and a lime application (0.15 wt.%). While basalt was applied only in 2023, lime was reapplied in 2024 to match the soil pH of BA10. In 2023, soil and plant samples were collected at the flowering (R2), seed development (R6) and R8 stages. In 2024, soil and plant samples were collected at the vegetative (V9), reproductive (R1), and harvest (R6) stages. We measured plant dry weight, elemental concentrations, and grain yield. Soil analyses included pH, exchangeable cations, available silicon (Si), and mineralogical composition via X-ray powder diffraction (XRPD). Total carbon budgets were calculated by integrating plant and soil data.

Soil pH increased similarly in both basalt and lime treatments. Basalt application significantly increased soil exchangeable magnesium (Mg) and sodium (Na) concentrations throughout the entire cultivation period. Additionally, available Si concentrations significantly increased in 2024. In contrast, exchangeable calcium (Ca) concentration showed no significant change with basalt application, increasing only in the limed plots. This likely reflects the high initial exchangeable calcium concentration in the original soil. XRPD showed a decrease in Ca-plagioclase in 2024 compared to pre-cultivation soil in 2023, with the greatest decrease observed in BA20. This reduction occurred primarily in planted plots, suggesting that crop roots may enhance basalt weathering. While basalt application showed no growth-promoting effects on soybean, it significantly increased maize plant height at V9, leaf dry weight at R1, and stem cross-sectional area at R6. In soybean, shoot manganese (Mn) and nickel (Ni) concentrations decreased in both basalt and lime treatments. In maize, shoot Mn concentration decreased in the lime treatment, while shoot Mg concentration increased significantly and shoot Si concentration showed an increasing trend in basalt treatments. Soil exchangeable Mg concentration was positively correlated with shoot dry weight.

Overall, basalt application had no negative effects on crop growth and can be beneficial depending on the crop species. The growth-promoting effects arise not only from pH neutralization but also from the supply of essential elements such as Mg and Si released through weathering. Mineralogical evidence indicates that basalt weathering progressed over two years, suggesting potential carbon sequestration through ERW in agricultural soils.