Assessing the potential of enhanced silicate weathering in a tropical soil to remove atmospheric CO2 and improve soil fertility : a modeling study

Details

Supervisors

Delmelle, Pierre

Faculty

Faculté des bioingénieurs

Degree label

Master [120] : bioingénieur en sciences et technologies de l'environnement, à finalité spécialisée

Abstract

Enhanced silicate weathering in croplands is increasingly suggested as a NET that could remove significant quantities of CO2 from the atmosphere and induce ancillary benefits to soil fertility and crop growth. However, many uncertainties remain as most existing ESW studies rely on the extrapolation of laboratory weathering rates to calculate CO2 removal and barely evaluate ESW impact on soil properties. Moreover, different application modes and silicate materials are discussed but no studies precisely assess their effect under similar conditions. Given the scarcity of reactive transport modelling studies on ESW with basalt and olivine, we use the reactive transport model CrunchFlow to investigate the effect of adding both crushed silicates to an Oxisol to enhance chemical weathering and atmospheric CDR. We tested two application rates (5000 and 150000 g m-2) either added to the soil surface or mixed within the top 20 cm of the soil profile. We show here that repeated addition of basalt leads to higher annual CO2 removal than olivine. However, the basalt annual CDR gradually decreases with time, whereas the olivine CDR is predicted to remain stable. We estimate annual CDR in the range of 2172-4188 and 4231-6951 g CO2 m−2 yr−1, after 30 annual applications of olivine and basalt respectively, subjected to 100 years of weathering. If deployed on tropical croplands, ESW may thus sequester about 3.3-10.4% of the current annual anthropogenic CO2 emissions. Our results demonstrate that both CDR efficacy and soil fertility improvement are maximized when the material is incorporated into the top soil layer. We found that basalt application may lead to higher co-benefits for soil fertility as significant concentrations of K+, Ca2+ and Mg2+ are released into the soil solution. As ESW with olivine and basalt induces a significant increase in the soil solution pH, these nutrients will be readily available to crop plants. Moreover, repeated basalt application induces an increase in CEC, thereby increasing soil chemical fertility. In contrast, a Ca2+ deficit in plants may arise after olivine amendment due to Mg2+ continuous release. Enhanced silicate weathering studies based on an RTM would benefit from a comparison with data acquired through a long-term in situ ESW experiment so as to validate the silicate weathering rates inferred theoretically and to better assess the actual co-benefits of ESW for soil fertility. Nevertheless, RTM offers a unique tool for exploring ESW in soils under various environmental conditions, soil properties and silicate application modalities.