Effects of basalt and biochar addition on base cations and trace metals in plants and soil in an urban field trial

Enhanced weathering (EW) and biochar amendment are proposed carbon dioxide removal (CDR) techniques with potential co-benefits for soil health and plant productivity. However, knowledge gaps remain regarding their impacts on soil carbon dynamics and heavy metal mobility. This study investigates the effects of basalt and biochar amendments on soil base cation dynamics, biomass yield and trace metal uptake in clover ( Trifolium pratense ) and mustard ( Brassica juncea ) field plots. Basalt addition did not increase soil inorganic carbon, Tessier-extractable base cations, or dissolved inorganic carbon (DIC) in leachates, indicating no detectable inorganic C sequestration during this experiment. Weathering of the relatively coarse, albite-rich basalt may have been limited in the alkaline soil and/or the weathering products were retained in soil pools not accessible to the extraction scheme. Basalt increased Ni and Cr in the reducible soil fraction but did not elevate plant metal concentrations and even tended to reduce Ni and Zn in leachates. Biochar increased plant biomass, mustard phosphorus and reduced plant uptake of several trace metals in both mustard and clover plants, while basalt did not affect any of the 33 assessed elements in aboveground plant biomass. Co-application of basalt and biochar did not lead to observable rock weathering while also no synergistic gains in biomass or reductions in plant heavy metals were observed after co-amendment in this alkaline soil. Overall, our findings suggest that in this alkaline urban soil, basalt weathering was limited and provided few immediate co-benefits, whereas biochar showed clearer advantages for plant growth and metal immobilization.