A number of negative emission technologies (NETs) have been proposed to actively remove CO 2 from the atmosphere, with enhanced silicate weathering (ESW) as a relatively new NET with considerable climate change mitigation potential. Models calibrated to ESW rates in lab experiments estimate the global potential for inorganic carbon sequestration by ESW at about 0.5-5 Gt CO 2 year −1 , suggesting ESW could be an important component of the future NETs mix. In real soils, however, weathering rates may differ strongly from lab conditions. Research on natural weathering has shown that biota such as plants, microbes, and macro-invertebrates can strongly affect weathering rates, but biotic effects were excluded from most ESW lab assessments. Moreover, ESW may alter soil organic carbon sequestration and greenhouse gas emissions by influencing physicochemical and biological processes, which holds the potential to perpetuate even larger negative emissions. Here, we argue that it is likely that the climate change mitigation effect of ESW will be governed by biological processes, emphasizing the need to put these processes on the agenda of this emerging research field.

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<p>With rising population growth, there is a need for increased food production. With rising temperatures and more frequent droughts due to climate change, it becomes more challenging to keep up with this increased demand for food. Therefore, a change in land use and management is needed in which enhanced silicate weathering (ESW) can play an important role. Weathering of silicate rocks has been regulating the atmospheric CO2 concentrations for over decades, but with the rise in atmospheric CO2, the natural weathering is too slow. Grinding the silicate rocks into a fine powder and spread it over for example agricultural fields will increase the reactive surface area and hence, the amount of CO2 that is stored in soils. The application of silicate minerals to soils can enhance plant growth by multiple processes, for example by counteracting soil acidification and by the release of plant nutrients. In this way, ESW can be used on agricultural fields without competing for land like other carbon capture techniques (e.g. Bio-Energy with carbon capture and storage). This study investigates the use of olivine (a fast-weathering Mg-rich silicate mineral) as a fertilizer in agriculture using a full-factorial mesocosm experiment. Barley and wheat were grown under two different rain regimes (daily rain vs weekly rain) and with application of two different grain sizes of olivine (p<sub>80</sub> = 1020 µm and p<sub>80</sub> = 43.5 µm). Our results showed increased plant growth and biomass with olivine addition, albeit only for fine olivine. However, this was not translated in an increase in yield of wheat and barley. Besides changes in biomass, we found significant differences in plant nutrient concentrations. As expected, Mg concentration increased significantly. However, BSi and Ca concentrations decreased with fine olivine application. Nitrogen in grains was also increased in the fine olivine treatment. In contrast to fine olivine, coarse olivine addition had almost no influence on nutrients. Ca, Mg and Si concentrations in plant samples followed the same trend as in the soil pore water, in contrast to metal concentrations. Olivine addition increased Ni and Cr availability in the soil pore water, but the concentrations of these elements in plant tissue did not increase and were even below detection limit for the majority of samples. While the influence of olivine on metal concentrations in plant samples was not affected by rain treatment, the influence of olivine on nutrients in the plants and plant growth was. Fine olivine addition enhanced the plants resistance to drought as it reduced the decrease in biomass with weekly rain treatment compared to daily rain treatment. This positive effect of olivine addition can be due to the increased weathering rate in combination with enhancement of soil properties like increased soil water retention. In this way, the use of olivine as a fertilizer on agricultural fields can mitigate climate change while it can also contribute to the solution for increased food demand.</p>