Application of an Innovative Centrifuge-Based Soil Pore Water Sampling Method in Basalt Enhanced Weathering Field Trials.

Basalt Enhanced Weathering as a Carbon Dioxide Removal (CDR) technology accelerates natural weathering, enhancing the CO₂ removal from the atmosphere. The main objective of the ongoing field trials in Scotland and the UK is to combine geochemistry modelling with in-field measurement to most accurately quantify CO₂ sequestration. To measure the weathering signal in the field, we track changes in indicators such as soil inorganic carbon (SIC), soil organic carbon (SOC), exchangeable cations, trace/immobile elements, and soil biomass. Pore water analysis is critical for directly quantifying CO₂ sequestration. Bicarbonate in soil pore water is a  CO₂ removal indicator, as it forms through the reaction of silicate minerals with dissolved CO₂ during the initial weathering process. We analyze pore water for pH, alkalinity, Electrical Conductivity (EC), major cations, and anions. This task can be challenging due to sampling issues, the absence of rainfall, and the time-sensitive nature of alkalinity measurements. Analyses of pore water chemistry rely on the ability to separate water from solids with minimal modification of its chemistry. Rhizon samplers and ceramic lysimeters are commonly used for pore water extraction. They may not be ideal for parameters like pH and alkalinity due to certain limitations, such as degassing of dissolved gases, and biases in molecule diffusion through the membrane. In response, we are testing a centrifuge method for pore water sampling from basalt amended fields. In the initial trial, statistical significance tests were conducted to compare the pH and total alkalinity between control plot and Treatment 126 t/ha in both centrifuge and rhizon samples, revealing a statistically significant difference (p < 0.05) in values within the centrifuge samples. However, no significance was observed in the rhizon samples. We present the results of ongoing tests from different treatments and soil types conducted to investigate whether centrifuge would be a suitable method for pore water sampling and alkalinity measurement for the enhanced weathering field trials.