OLIVINE AMENDED URBAN COMPOST: USING MUSTARDS TO TRACE MINERAL DISSOLUTION AND POTENTIAL FOR CARBON SEQUESTRATION

CO₂ emissions are driving climate change at an accelerated rate, leading to conversations on geoengineering strategies and future policy focusing on mitigation. One approach that has gained increasing attention is the enhanced sequestering of CO₂ by chemical mineral weathering. Olivine is often a target phase for these experiments given its abundance and ability to fix atmospheric carbon into carbonate phases by the incongruent dissolution reaction: (Mg²⁺ or Fe²⁺)SiO₄ + 4CO₂ + 4H₂O → 2Mg²⁺ (or Fe²⁺) + 4HCO₃^- + H₄SiO₄. In our experiments we explore three key questions: 1) Will olivine amended to compost-rich growth media experience measurable dissolution in a greenhouse experiment during the lifecycle of mustard (Brassica rapa var. Japonica)? 2) Will the mustard plants survive increased Cr concentration as a result of olivine dissolution? 3) Can we measure Fe in mustard leaves grown in olivine amended soil/compost mixtures as a proxy for phytoavailable Fe?

We planted mustard seedlings into 4 different media mixes with 6 replicates of each mix. The control was a 50-50 mixture of fine grained, high Pb compost from The Food Project, a Boston-based farm, and a commercial topsoil mixture. The compost/topsoil mixture was amended with four different volumes of olivine including: control, 1%, 10% and 20% (based on percent volume). Throughout the experiment, we measured the health of the mustard seedlings using a CCM-300 Chlorophyll meter.

After 14 weeks of growth and before the plants bolted, the leaves, stems, roots, and soil were separated and frozen. Samples were dried at 40°C and homogenized using a tungsten carbide mixer mill. Resulting powders were analyzed using p-ED-XRF (Spectro XEPOS-He). The mustard plants thrived and initial results indicate Pb decreased, Fe increased, and Cr was below detection limits across all amendments. Future work will focus on statistical analysis of data to determine if elemental concentrations across experimental parameters can be directly linked to olivine dissolution. Scanning electron microscopy of recovered olivine will be used to examine surfaces for evidence of dissolution. We aim to expand soil amendments in the context of urban agriculture, increasing phytoavailable Fe, and assessing the viability of carbon sequestration.