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; 2019). 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 field conditions (Argüello et al. 2020; Ramtahal et al. 2018, 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. The monitoring of pH, the soil Cd phytoavailability, CEC and leaf Cd concentrations allowed for the understanding of the mechanisms through which Cd mitigation was achieved. In addition to the typical challenges experienced with environmental conditions that influences the efficacy of Cd mitigation in cacao-growing soils as elucidated in past studies (Ramtahal et al. 2018, 2019), this study highlighted the influence of application methods to efficiently incorporate the amendment as another consideration affecting the effectiveness.
A number of studies have shown that the local agro-ecology is an important consideration in selecting the appropriate amendment, its rate and timing of application to effectively mitigate Cd (Argüello et al. 2020; Ramtahal et al. 2018, 2019). The soil of the experimental site was strongly acidic (pH < 5.5) with a low CEC and OM content which are key factors known to increase the mobility and phytoavailability of soil Cd for uptake and accumulation by cacao trees (Argüello et al., 2018; Barraza et al. 2017; Chavez et al. 2015; Gramlich et al. 2018, 19; Ramtahal et al. 2016, 19; Zug et al. 2019). Under such conditions, application of (a) alkaline materials to increase soil pH thus promoting metal hydrolysis reactions and/or coprecipitation (Mench et al. 1998) or (b) organic-rich materials to increase the soil capacity for Cd ion complexation through improving the CEC, to mitigate Cd levels in the plant (Beesley and Marmiroli 2011; Laird et al. 2010) have been shown to be effective. Previous studies in cocoa have also shown the effectiveness of alkaline and organic-rich materials such as lime (Argüello et al., 2020; Ramtahal et al., 2019) and biochar (Ramtahal et al. 2019), respectively, to be effective in ameliorating Cd-contaminated cacao-growing soils with similar chemical properties. Thus for this investigation, both of these amendments were selected for use to mitigate Cd at recommended rates (Ramtahal et al. 2019) using three application methods, SA, SI and AA.
Even though the field temperature remained relatively constant (25.5 to 27.5oC) for the duration of the experiment, the rainfall varied with an initial period of low precipitation (February-May) followed by an increase until the month of December and a subsequent decline towards the end of the year-long trial. Such marked seasonal differences in rainfall is typical of tropical climates. Ramtahal et al. (2019) showed that the amendments became effective only in the beginning of the rains and that heavy rains tended leach or wash away the amendments. Regardless of the variability in rainfall over the study period, the soil pH remained fairly constant in the control treatment. However, the leaf Cd concentrations in the control showed a gradual but significant increase as the wet season proceeded, mirroring similar increases in the DTPA-extractable Cd. This indicates that the improved soil moisture conditions during the rainy season resulted in enhanced soil Cd phytoavailability which in turn resulted in higher leaf Cd levels in the control. These findings are in conformity with other studies that also show that soil water content influences the mobility and consequently the phytoavailability of Cd ions (Ramtahal et al. 2019; Stafford et al. 2018).
One of the dominant factors which influences Cd phytoavailability in soils is pH (Adriano 2001; Grant and Sheppard 2008; Tsaldilas et al. 2005). When pH increases, there is an increase in CEC which results in the strong chelation of Cd to particles of clay and organic matter making it less phytoavailable (Shahid et al. 2016). The application of lime using the SI and SA treatments were able to effect a significant increase in soil pH, which resulted in a reduction in DEC and significantly lowered leaf Cd concentrations. This was also evident from the strong negative correlation between soil pH and soil DEC and the strong positive correlation between soil DEC and leaf Cd concentration. The effectiveness of lime in reducing leaf Cd concentrations through the reduction in the availability of Cd is well documented in the literature both in cocoa (Ramtahal et al. 2018, 2019; Argüello et al. 2020) and in other crops (Chen et al. 2018; Park et al. 2011; Tlustoš et al. 2006; Woldetsadik et al. 2016. The SI was more effective than SA as evidenced by the rapid increase in soil pH to a level not significantly different from 7, within a month of application and the ability to maintain the pH at 7, up to 8 MAA. Consequently, both DEC and leaf Cd levels started declining 1 MAA and reached the lowest levels that was 50% of the control for DEC and 60% of the control for leaf Cd levels, at 5 MAA and 3 MAA, respectively. Although DEC started increasing following 5 MAA (but remained below the level of the control throughout the study) leaf Cd levels remained at 60% of the control for 5 straight months until 8 MAA before it started increasing.
On the other hand, although SA was able to raise the pH to a level not significantly different from 7 within 1 MAA, the levels started declining 3 MAA but remained at a level above 6 throughout the study period. Consequently, both DEC and leaf Cd concentrations started declining 2 MAA and attained their lowest levels 6 MAA which were 70 % and 75% of the control, respectively, compared to 50% and 60% for SI. Following 6 MAA both DEC and Leaf Cd levels started rising for SA and the leaf Cd levels were not significantly different from the control by 8 MAA. Hence the magnitude of reduction and duration of effectiveness of SA was lower than that of SI. This could be attributed to greater losses due to surface run-off in SA compared to SI. Lime and its ability to effectively increase the pH of the soil is diminished with increasing rainfall, acidity of the rain, soil and site management factors (Goulding and Blake 1998).
In contrast, the AA method was not able to effect a change in soil pH in comparison to the control and consequently was unable to significantly reduce DEC levels in the soil, which resulted in leaf Cd levels not significantly different from that of the control throughout the study period. That lime using the auger method was unable to effectively elevate soil pH indicates that lime placed in auger created soil depressions along the drip line of the tree was unable to migrate laterally to effect soil pH in the clayey soils of that location. Clayey soils are less permeable and are mostly characterized with low infiltration (Aswathanarayana 2001). It is also plausible that the water accumulating in the depressions facilitated the leaching of the lime below the root zone making it ineffective. Other studies have shown that most of the functional roots of cocoa remain within the top 20 cm within the enriched top soil layer (Hartemink 2005; Toxopeus 2008).
Soil injection helps to prevent treatment loss by placing it below the zone subject to erosion (WPHA 2018). Many studies have shown that in order for lime to be effective as a soil amelioration technique, it should be incorporated as much as possible into the soil profile (Kaminski et al. 2007; Mullins et al. 2011; Natale et al., 2020; Wolkowski and Laboski 2011). Overall, SI performed better where a slurry of lime was injected into the subsurface of the soil at high pressure forcing it throughout the soil, resulting in a greater incorporation into the soil profile with reduced chances for loss by run off. The study shows that the greater effectiveness of SI was a function of its ability incorporate the amendment most effectively into the soil compared to SA or AA.
Although it has been suggested that some biochars can induce a liming effect on acidic soils (Ahmad et al. 2014 Hussain et al. 2017), the biochar used in this study regardless of the application technique had no significant effect on soil pH. This was also evident from the non-significant correlation between soil pH and biochar treatment. The type of material used to manufacture biochar and the process by which it is made determine its ability to neutralize soils (Van Zwieten et al. 2010). However, Biochar, in this study, was able to significantly decrease the phytoavailable soil Cd using both SI and SA treatments but not in the AA treatment.
Biochar has the ability to immobilize metal ions through the formation of complexes owing to its physico-chemical properties of an enhanced surface area and increased CEC (Anawar et al. 2015; Lahori et al. 2017; O’Connor et al. 2018; Paloma and De la Rosa 2020). SI again was more effective than SA, possibly due to the same reasons that were identified for the lime treatments. The effect of biochar on lowering DEC for the SI treatment was around 15–20% compared to the control and was achieved at 3 MAA with the effectiveness of the amendment remaining up to 6 MAA, after which the effect diminished. Consequently, the leaf Cd concentration showed a 22% reduction 4 MAA and increased to 35% reduction by 5 MAA but started to increase again. An average reduction of 30% was maintained up to 10 MAA. The variation in CEC mirrored that of leaf Cd indicating the mechanism by which Cd mitigation is achieved with biochar is through its effect on increasing CEC (Jiang et al. 2012; Komkiene and Baltrenaite 2016; Sun et al. 2020). The effect of biochar on lowering DEC for the SA treatment was more gradual than SI resulting in a much smaller reduction (av. 20%) that occurred 1 month after SI and lasted only for 2 months (6 MAA to 7 MAA), This was also mirrored in the changes experienced with CEC. These results suggest that SI achieved a better incorporation of biochar in the soil than SA, resulting in greater treatment effectiveness. Past studies have shown that biochar needs to be thoroughly incorporated in the soil in order for it to be effective as a technique of soil remediation (Ruysschaert et al. 2016; Guo et al. 2020). The application of biochar using the AA had no significant effect on DEC and leaf Cd levels as was demonstrated for lime application.
Overall the application of the amendments using the SI technique was much more efficient than SA in reducing Cd accumulation in cacao leaves. Neither lime nor biochar application by AA was shown to be effective. Further, the study demonstrated the effectiveness of lime over biochar as a soil amendment in reducing Cd uptake in cacao. Overall, lime application by SI had (a) a faster action in reducing leaf Cd levels within 1 MAA compared to biochar that took up to 3 MAA; (b) achieved a higher magnitude of reduction than biochar (40% compared to 35%) and (c) resulted in the effectiveness lasting longer than biochar (4 months compared to 1 month), before leaf Cd levels started to increase. The study also indicated that lime application even with SI required repeat applications, yearly, as was reported by Ramtahal et al. (2019) to achieve effective mitigation of Cd levels. Although the study investigated the effect of amendments and application methods on leaf Cd, the findings could also be extended to cacao bean Cd, as many studies have reported a positive and significant leaf-bean Cd correlation (Argüello et al. 2019; Barraza et al. 2017; Lewis et al. 2018; Ramtahal et al. 2016). Additionally, it is recommended that studies be carried out to evaluate the cost-effectiveness and practicality of these soil application methods under a variety of conditions as terrain, soil and crop characteristics, availability of equipment and associated labor costs can affect the cost-effectiveness of these treatments