Investigating the potential of enhanced weathering as a carbon dioxide removal technique using a range of materials applied to agricultural soil

Weathering of silicate minerals is a natural method of carbon sequestration; helping to regulate the global C cycle and atmospheric CO2 on geological timescales. The application of silicate minerals to agricultural soils has been suggested as a method to enhance natural silicate weathering rates and increase C drawdown; thereby reducing atmospheric pCO2. The potential for "enhanced weathering" succeeding as a CO2 removal technique is poorly understood due to conflicting estimates for mineral dissolution rate. Laboratory studies estimate dissolution rates to be an order of magnitude faster than field studies and do not consider the effect of mineral addition. Whilst field studies capture the complex array of processes interacting within the soil zone, the experiments are consuming in both time and space, and so by their nature are limited. Further to this, it is difficult to isolate and assess the effect of individual processes occurring during a field study; a problem when trying to apply the findings to different locations. Soil core experiments address the limitations of laboratory and field studies, allow more experiments to run and act as a powerful compliment to field studies.
In this study, soil cores will be taken from an agricultural site and monitored in a controlled laboratory environment. This will allow dissolution rates to be assessed whilst maintaining the complexity of the soil zone. To date, only a few studies have used soil core experiments to investigate the impact of silicate supply on dissolution rates. Soil core flow through experiments by Renforth et al, 2015, suggest that the experimental method described above can provide useful insights into dissolution rate. This study aims to extensively expand this early research using multiple soil cores from the same agricultural site. Different silicate minerals (olivine, wollastonite, K-feldspar, basalt, volcanic ash) will be added to each of the cores. Core experiments will be run in triplicate with a triplicate-control to assess natural variability and the chemical impact of each type of mineral addition. The soil cores will be subjected to realistic UK weather conditions for 6 months by placing them on the roof of the Earth Sciences Department, Oxford. The effect of temperature and rainfall supply on dissolution rate will be assessed by replicating the experiment in a temperature controlled laboratory. Changes in the cation concentration within weekly samples of dripwater will allow comparisons to be made between the dissolution rates of different silicates minerals. The application of enhanced weathering as a C sequestration strategy is further limited by our incomplete understanding of the pathway C follows during the weathering process. To understand the effect of enhanced weathering on C uptake, it is important to identify the extent that C remains in the soil zone, following precipitation as a pedogenic carbonate; or whether the C remains in solution, entering the freshwater and marine ecosystem as bicarbonate ions, and thus increasing ocean alkalinity. To investigate the uptake of C after the application of different silicate minerals to agricultural soil, the carbonate chemistry of the soil core will be examined before and after the experiment; and the bicarbonate concentration of dripwater leaving the soil cores will be measured at regular intervals throughout the 6-month experiment. One concern which could limit the application of enhanced weathering as a large scale CO2 removal technique is the impact increasing silicate supply has on the production of trace metals that are harmful to primary productivity and affect freshwater and ocean chemistry. However, the production of these trace metals and their resultant pathway through the terrestrial, river and marine ecosystem remains uncertain.