Surface soil liming reduces cadmium uptake in cacao seedlings but subsurface uptake is enhanced

Abstract

Cadmium concentrations in cacao (Theobroma cacao L.) beans from South America often exceed trade limits. Liming soil is advocated as a remediation option, but amendments cannot be incorporated into the entire root zone without harming the trees. An experiment was set up to identify how Cd uptake varies within the root zone when surface and subsurface soil layers are either limed or not. The experiment used 22-cm-height pots with top and bottom layers using surface and subsurface soil samples from a cacao field. The potted soils were either surface limed or not or fully limed and layers spiked with stable 108 Cd isotope in various combinations to trace the plant Cd provenance. The root distribution was neither affected by liming nor by soil source; 70% of the root biomass was present in the top layer. Plants grown on the fully limed surface soil had 1.7 times lower Cd concentrations in leaves than the unlimed treatments, whereas this concentration was 1.2 times lower when only the top layer was limed (surface soil used in both layers). The isotope dilution data showed that surface soil liming enhanced Cd uptake from the unlimed bottom layer compared with the unlimed soil, suggesting compensating mechanisms. The pots containing surface soil over subsurface soil also showed that compensating effect but, due to lower phytoavailable Cd in the subsurface soil, surface liming still effectively reduced foliar Cd. We conclude that liming might be a feasible mitigation strategy, but its effectiveness is limited when Cd phytoavailability remains untreated in the subsurface layer.

Mitigating the level of cadmium in cacao products: Reviewing the transfer of cadmium from soil to chocolate bar

“…In 30 established plantations, lime incorporation to deeper soil horizons is not possible without damaging the root system, which may compromise the efficacy of the amendment. Indeed, Argüello et al (2020) showed in a pot experiment with cacao seedlings that partially liming the rooted soil (lime in top compartment only as would be the case in field liming), enhanced Cd uptake from the non-limed bottom compartment.…”

“…However, this effect was only found in soils spiked to Cd concentrations well above environmentally relevant soil Cd concentrations. Argüello et al (2020) found that Cd uptake in cacao from deeper soil layers was enhanced by surface liming and the authors related this to increased activity of deeper roots to cope with micronutrient deficiency in the topsoil (i.e. Zn) caused by surface liming.…”

Mitigation of Cadmium Uptake in Cocoa: Efficacy of Soil Application Methods of Lime and Biochar

“…The application of lime and biochar to Cd contaminated soils have been shown to be effective in mitigating Cd uptake into the cacao tree and into the beans (Ramtahal et al 2018;. However, a number of studies have recommended that the method of application needs to be improved in order to improve the effectiveness of the soil ameliorants under eld conditions (Argüello et al 2020;Ramtahal et al 2018Ramtahal et al , 2019. The effectiveness of the various methods of application were explored in this study based on (a) the time taken for the amendment to take effect (b) the magnitude of reduction in leaf Cd and (c) the duration of effectiveness compared to the control.…”

Cadmium distribution in soils, soil litter and cacao beans: a case study from Colombia

“…Moreover, a very recent paper (Liu et al 2021) shows the benefits of dolomite phosphate rock (DPR), humic acid-activated dolomite phosphate rock (ADPR) and biochar (BC) in immobilizing Cd in subtropical soils. However, regarding the application in cocoa crops during the past years, although the application of liming material such as dolomite it has been extensively studied as an option to stabilize Cd in the soil, neutralize acidic soil and enhance metal stabilization in soil by reducing the extractable Cd concentration (Hamid et al 2018;Ramtahal et al 2019;Argüello et al 2020), it has been shown that this technique needs to be reconsidered due to its efficiency in terms of time and cost for cacao in field. Furthermore, the chloride ions released from the application of KCl fertilizers, might form Cd-Cl soluble compounds, increasing Cd bioavailability (Nino-Savala 2019), which could be the case in farms A, B and D, where the application of KCl is regularly used.…”