Co-Benefits of Wollastonite Weathering in Agriculture: CO₂ Sequestration and Promoted Plant Growth

Abstract

To lock atmospheric CO2 at anthropogenic timescale, fast weathering silicates can be applied to soil to speed up natural CO2 sequestration via enhanced weathering. Agricultural lands offer large area for silicate application, but expected weathering rates as a function of soil and crop type, and potential impacts on the crops, are not well known. This study investigated the role of plants on enhanced weathering of wollastonite (CaSiO3) in soils. Using rooftop pot experiments with leguminous beans (Phaseolus vulgaris L.) and nonleguminous corn (Zea mays L.), CO2 sequestration was inferred from total inorganic carbon (TIC) accumulation in the soil and thermogravimetric analysis, and mineral weathering rate was inferred from alkalinity of soil porewater. Soil amendment with wollastonite promoted enhanced plant growth: beans showed a 177% greater dry biomass weight and corn showed a 59% greater plant height and a 90% greater dry biomass weight. Wollastonite-amended soil cultivated with beans showed a higher TIC accumulation of 0.606 ± 0.086%, as compared to that with corn (0.124 ± 0.053%). This demonstrates that using wollastonite as a soil amendment, along with legume cultivation, not only buffers the soil against acidification (due to microbial nitrogen fixation) but also sequesters carbon dioxide (12.04 kg of CO2/tonne soil/month, 9 times higher than the soil without wollastonite amendment).

Characterization of Physically Fractionated Wollastonite-Amended Agricultural Soils

“…Tukey's HSD groups for these samples also confirm that they are statistically different from the other fractions of the same soils. In our previous study, Haque et al [24] reported a soil inorganic carbon content of 0.606 wt.% (~2.67 g CO 2 /kg soil) for the agricultural soil sieved through 200 µm, which is much less than the value reported in this study for the 5 wt.% and 10 wt.% wollastonite amended rooftop soil fractions from the pan (<50 µm). This implies that soil fractionation results in isolating soil that contains most of the pedogenic calcium carbonate.…”

“…In the case of coarse minerals, density-based fractionation can potentially be used and will be investigated in future research. The combination of calcimetry and XRD analyses on fractionated agricultural soil, as a result of concentration of mineral phases of interest, also serves to confirm with greater certainty that the results obtained by analyzing the whole soil, as in Haque et al [24], are genuine and not analytical artifacts. If wollastonite and calcite phases are present in detectable quantities in smaller soil fractions, then it will be present in the whole unfractionated soil even when present in quantities near or below detection limits.…”

“…In addition, 5 wt.% RFT sample has 0.9 wt.% (~50% less than that of the control), and the 10 wt.% RFT samples contain 0.6 wt.% (~70% less than that of the control). It is possible that the high amendment ratio used in the rooftop experiments led to a slight decrease in organic carbon content of the soils, which was also reported in Haque et al [24]. It is important to note that such a high amendment ratio is not expected to be utilized in field crops, but rather, in field crops, a similar amount of wollastonite would be amended over several years.…”

“…In our previous work, Haque et al [24] studied the benefits of the wollastonite soil amendment, both in terms of plant growth and inorganic carbon sequestration. In that study, collected soils were analyzed for inorganic carbon content after sieving the soil through a 200-µm mesh, to remove coarse materials that were not important for analysis.…”

Alkaline Mineral Soil Amendment: A Climate Change ‘Stabilization Wedge’?

“…Haque et al [48] studied the co-benefits of wollastonite weathering in agricultural soil, to grow green beans and corn. Wollastonite mineral, containing 55% SiO 2 , 26% CaO, and 9% MgO as major alkaline oxides, was added to sandy loam soil (pH 4.94) to reach a pH of 7.23.…”