“Long effects of enhanced rock weathering: the first two years of The Decade Experiment”

Enhanced rock weathering - the introduction of alkaline minerals into agricultural soil - is a promising atmospheric carbon dioxide removal technology that could scale to billions of tonnes of carbon sequestration annually. However, many questions remain over the speed and magnitude of ERW’s efficacy and the longer term consequences of repeated rock dust applications for sustained carbon removal, soil health benefits, and improved crop yields. A number of studies have tested the effect of alkaline mineral additions under field settings, indicating potential for CDR and improved cropping conditions and soil properties. These studies were performed for one to a few years, however global ERW potential will require applications over the majority of Earth’s croplands for decades. Here we present the results from the initial two years of a ten year trial, The Decade Experiment, involving ERW with metabasalt (CaSiO3, MgSiO3) in cropland soil planted with Zea mays (field corn) in upstate New York. We also present results from agriculture lime additions in the same setting to ascertain the differences between ERW with silicate rocks vs. direct carbonate rock additions. The study was initiated over three acres in 2023 following a systematic step-down model whereby rock dust amendments will be applied annually for up to ten years, with individual rows spanning between zero (control conditions) to ten years of annual rock dust amendments. This entails plots receiving only one year of amendment, followed by two years, traversing all annual increments to ten years of annual rock dust additions for the final plots. With this approach we seek to investigate the effects of single rock dust additions compared to multiples of years of amendments systematically spanning a full decade of continuous measurements from all treatments. 

Carbon dioxide removal was examined by extracting soil pore water with negatively pressurized porous ceramic lysimeters. Lysimeters were installed at two depths, 15 cm and 30 cm, and continuously monitored over the 2023 and 2024 growing season, with 1192 samples collected across all treatments. Measured dissolved inorganic carbon (DIC) and calculated alkalinity of the soil pore water were used to determine carbon removal efficiency, since the initial step in weathering involves silicate minerals reacting with dissolved CO2 to form bicarbonate following Holzer et al. 2023. Bicarbonate concentrations of soil pore waters increased by 10% in the two year amended plots compared to control at the 15 cm sampling depth. Soil pH and cation exchange capacity (CEC) were higher on average in amended soils compared to untreated controls, indicating soil chemical transformation due to ERW. Yield of Zea mays was observed to increase in plots amended with 44.8 t/ha for two years relative to control and the plot only treated for one year. Metabasalt amendments increased yields with repeated application, improved soil quality, and sequestered carbon. These initial two year results will continue to be explored over the next ten years to understand the long-term consequences of ERW, including benefits and risks for carbon removal, soil health and crop yields in The Decade Experiment.