Weathering without realizing inorganic CO2 removal revealed through base cation monitoring

Abstract

Enhanced Weathering using basalt rock dust is a scalable carbon dioxide removal (CDR) technique, but quantifying rock weathering and CDR rates poses a critical challenge. Here, we investigated realized inorganic CO2 removal (defined as the sum of the change in dissolved inorganic C leaching and in neoformed solid inorganic C) and weathering rates by treating mesocosms planted with maize with basalt (0, 10, 30, 50, 75, 100, 150 and 200 t ha−1) and monitoring them for 101 d. We observed no significant realized inorganic CO2 removal, as leaching of dissolved inorganic carbon did not increase and soil carbonate content declined over time. To gain insights into the weathering processes, we traced the fate of base cations in the soil and plants. This analysis showed that most base cations were retained in the topsoil reducible pool, typically associated with iron (hydr)oxides, while increases in the exchangeable pool were about a factor 10 smaller. Soil base cation scavenging exceeded plant scavenging by approximately two orders of magnitude. From the base cations in all pools (soil, soil water and plants), we quantified log weathering rates of −11 mol total alkalinity per m2 basalt per s. The potential inorganic CO2 removal, defined as the maximum inorganic CO2 removal achievable if all weathered base cations, adsorbed by soil pools in this experiment, would leach out of the soil and be fully balanced by carbonate anions, was estimated at 26 kg CO2 t−1 basalt. In conclusion, despite clear weathering of basalt rock, we found no inorganic CO2 removal within the timescale of this experiment. The observed increase of aluminum in association with the reducible soil fraction indicate the formation of secondary minerals. These, along with enhanced base cation exchange, may contribute to long-term soil fertility and promote the stabilization of soil organic matter.