Towards Sustainable Chocolate: Two Years of Enhanced Weathering in Tropical Cacao Agri-Ecosystems

Theobroma cacao is a cash crop that is both economically important and environmentally intensive, presenting a major sustainability challenge. In Brazil, cacao production is largely carried out by smallholder farmers on highly weathered, nutrient-poor, acidic soils (Oxisols and Ultisols). These soils have low fertility due to acidic pH, phosphorus (P), and potassium (K) levels, but high aluminium (Al) content. Enhanced rock weathering (EW) could be a pathway to access carbon financing while potentially having a ‘liming’ effect, which increases nutrient availability by raising the soil pH. However, evidence-based empirical data demonstrating the utility of EW-cacao on carbon capture and the impact of rock-dust spreading on agronomic productivity and forest ecosystem health are currently lacking.

Here we report on a field trial spanning three years that has been designed to evaluate EW deployment on cacao under field conditions. Within Brazil, specifically Bahia, cacao is often grown in a multi-strata, forest-like environment, known locally as the ‘Cabruca’ system. This system has cultural and environmental significance, often cited for its preservation of endemic trees (Cassano et al., 2008). This field trial investigates EW on both traditional and commercial agroforestry systems to determine potential synergies and additive effects. Carbon removal rates (CDR), soil fertility and agronomic co-benefits have been assessed.

Here, we present the outcome of this trial after two years of EW (basalt) application and a parallel trial assessing the potential for a novel iron chelating biotechnology to accelerate EW and CDR rates in a field setting after 9 months. We show that carbon removal can be monitored through the magnetic extraction of weathered basalt grains and their subsequent analysis. We also discuss the implications of using an iron chelator to disrupt the rock surface passivating layer for cation loss, particularly calcium. In addition, we report on important agronomic indicators, including tree height, canopy size, pest and disease incidence rate, litterfall rates and yield after two years of basalt application.