Investigating terrestrial biogeochemical responses to enhanced rock weathering for carbon capture in a whole-watershed experiment

Enhanced weathering (e.g., spreading crushed silicate rock on agricultural fields) is a promising approach for carbon sequestration. As rainwater weathers the crushed rock, atmospheric CO2 is converted to bicarbonate, which is transported through soils to streams and rivers that drain to the ocean, where the bicarbonate is stored. Other products of the weathering reaction, including nutrients and trace metals, may be exported to streams, retained within soils, or taken up by plants. While results of laboratory experiments and theoretical assessments of enhanced rock weathering are encouraging from the standpoint of CO2 removal, observations that illuminate associated ecosystems responses to weatherable-rock additions are scarce. Our objective is to help address this deficiency through execution of a watershed-scale experiment on enhanced rock weathering. In June 2023, crushed basalt was applied at a rate of 20 tons per hectare to hayfields and pasture of a small, agricultural watershed in northern Vermont, USA.  Prior to the basalt application, 64 sampling plots distributed over eight transects were established for soil and vegetation sampling. Each transect was oriented parallel to the hillslope gradient and traversed pasture and hayfield. Four transects received the basalt amendment, and the other four served as basalt-free controls. Soil samples have been collected from all sampling plots and are currently being analyzed for exchangeable nutrients (B, Mg, P, S, K, Ca, Mn, Fe, Ni, Cu, Zn, Mo), trace metals (Cu, Cd, Cr, Pb), pH, bulk density, soil organic carbon (particulate and mineral associated), soil inorganic carbon, and microbial biomass. Vegetation from the hayfield plots has been collected four times over the growing season and is being analyzed for nutrient content (B, Mg, P, S, K, Ca, Mn, Fe, Ni, Cu, Zn, and Mo) and to estimate above-ground net primary productivity. We find that the basalt treatment has had a statistically significant and positive effect on above-ground primary production within the hayfield. These results, together with observations on changes in vegetation nutrient content and trace-metal concentrations in soil and vegetation, will be described and interpreted in our presentation. These findings should improve understanding of the agricultural benefits of enhanced rock weathering and the role of this carbon-capture approach in modifying soil chemistry.