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

硅酸盐矿物的风化作用是一种天然的固碳方法;有助于在地质时间尺度上调节全球碳循环和大气CO2。已建议将硅酸盐矿物应用于农业土壤，作为提高天然硅酸盐风化速率和增加C下降的方法，从而降低大气pCO 2。由于对矿物溶解速率的估计相互矛盾，人们对“增强风化”作为一种二氧化碳去除技术的潜力了解甚少。实验室研究估计溶解速率比现场研究快一个数量级，并且不考虑矿物添加的影响。虽然实地研究捕捉土壤区域内相互作用的复杂过程，但实验在时间和空间上都很消耗，因此其性质是有限的。除此之外，很难隔离和评估在实地研究期间发生的各个过程的影响;当试图将研究结果应用于不同地点时，这是一个问题。土芯实验解决了实验室和现场研究的局限性，允许更多的实验运行，并作为一个强有力的补充现场研究。在这项研究中，土壤芯将采取从农业网站和监测在一个受控的实验室环境。这将允许在保持土壤区复杂性的同时评估溶解速率。迄今为止，只有少数研究使用土芯实验来调查硅酸盐供应对溶解速率的影响。Renforth等人（2015年）的土芯流动实验表明，上述实验方法可以为溶解速率提供有用的见解。本研究旨在使用来自同一农业场地的多个土芯来广泛扩展这一早期研究。不同的硅酸盐矿物（橄榄石，硅灰石，钾长石，玄武岩，火山灰）将被添加到每个核心。岩心实验将一式三份进行，并设一式三份对照，以评估自然变异性和每种矿物添加物的化学影响。土芯将置于牛津大学地球科学系的屋顶上，在英国现实的天气条件下放置6个月。将通过在温度受控实验室中复制实验来评估温度和降雨供应对溶解速率的影响。在阳离子浓度的变化，每周样品的滴水将允许之间进行比较，不同的硅酸盐矿物的溶解速率。由于我们对风化过程中C的路径不完全了解，因此加强风化作用作为碳封存策略的应用受到进一步限制。要了解增强风化对C吸收的影响，重要的是要确定C在土壤区中的残留程度，降水后作为成壤碳酸盐;或者C是否残留在溶液中，作为碳酸氢根离子进入淡水和海洋生态系统，从而增加海洋碱度。为了研究在农业土壤中施用不同的硅酸盐矿物后C的吸收，将在实验前后检查土芯的碳酸盐化学;在整个6个月的实验中，将定期测量离开土芯的滴水的碳酸氢盐浓度。一个可能限制增强风化作为大规模CO2去除技术的应用的问题是，增加硅酸盐供应对微量金属的产生产生的影响，这些微量金属对初级生产力有害，并影响淡水和海洋化学。然而，这些微量金属的产生及其通过陆地、河流和海洋生态系统的最终途径仍然不确定。

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.