Olivine Fertilization for Carbon Dioxide Removal: Field Demonstrations and Insights in Diverse Forest Ecosystems in a Tropical Monsoon Climate

A novel geoengineering technique aims to counteract global warming caused by anthropogenic CO₂ through artificially enhanced silicate weathering, achieved by the dissolution of olivine. Unlike other geoengineering concepts, CDR (carbon dioxide removal) techniques not only mitigate ocean acidification but also efficiently draw down atmospheric CO₂. Models predict that we can sufficiently enhance silicate weathering enough for it to be a useful CDR strategy. The models typically rely on kinetic rate constants derived from benchtop experiments conducted under conditions far from equilibrium. Hence, empirical field demonstrations are crucial to validate the effectiveness of enhanced silicate weathering. Here, we implement additional olivine (Mg₂SiO₄) fertilization in three plots with the same dimensions but various forest forms, grassland, Chinese fir, and mixed woodlands. The olivine doses were equivalent to 200 tons ha^-1 in this study. Combining monthly samplings of runoff chemistry with hourly runoff measurement, this study aims to delineate the chemical weathering flux. Preliminary findings reveal the concentration of Si⁴⁺,Mg²⁺, and Ca²⁺ within runoff at varying soil depths and forest forms. Specifically, in the Chinese fir plot, the concentration of Si⁴⁺ increased from 5.58 to 17.47 mg L^-1 within the initial three months, subsequently diminishing to 5.54 mg L^-1 after one year. Conversely, the grassland exhibited a decline from 4.20 to 2.46 mg L^-1 in the same period. For mixed woodlands, Si⁴⁺ concentration elevated from 4.16 to 10.87 mg L^-1 at three months, followed by a reduction to 5.54 mg/L after one year. The concentrations of Si⁴⁺ within the 30 to 85 cm depth exhibited minimal variation, fluctuating between 5–8 mg L^-1. The initial concentrations of Mg²⁺ for the Chinese fir, grassland and mixed woodlands were 2.70, 0.19, and 1.19 mg L^-1, escalating to 2.90, 1.76, and 2.14 mg L^-1, respectively. Correspondingly, initial Ca²⁺ concentrations were 42.87, 37.13, and 24.07 mg L^-1, increasing to 147.70, 49.40, and 45.58 mg L^-1, subsequently declining to 21.70, 7.04, and 11.78 mg L^-1. The observed trends suggest that nutrient deficiency in experimental plots prompts preferential Mg uptake by plants upon excess olivine addition, resulting in the release of Ca. These insights imply that olivine fertilization necessitates a minimum of three months and persists for at least 30 months in our case. Disparities in concentrations at different depths underscore the predominance of weathering in surface layers. While silicate weathering is more pronounced in forests compared to grasslands, excessive Mg addition may disrupt the equilibrium in plant nutrient uptake.