The potential transfer of Cadmium to food crops where organic matter is applied to soil : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Soil Science, School of Agriculture and Environment, Palmerston North, New Zealand
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2022
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Massey University
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Abstract
Cadmium (Cd) is a non-essential element that can be absorbed by plant roots and accumulate in plant tissues. The presence of Cd in plants potentially poses a serious risk for human health through the food chain. This risk is increasing because of the direct contact between agriculture and urban land use which leads to Cd contamination of agricultural land through disposal of urban and industrial waste. Some widely grown leafy vegetables are a high Cd accumulator, such as Bok Choy (Brassica chinensis. L). The application of organic matter to soil has been considered as a mitigation strategy to reduce Cd availability in the soil due to the ability of this amendment to immobilise Cd in soil solution. In developing countries, where the agriculture sector is mostly run by smallholder farmers, compost application is therefore a low-cost approach to reduce Cd toxicity. However, as organic matter experiences rapid decomposition, particularly in tropical regions, there is a possibility that Cd bound to the soil could potentially be released back to the soil solution. The effect of organic matter decomposition over time on Cd availability and Cd plant uptake has been poorly studied. The research in this thesis aimed to observe the effect of compost application over time in Cd availability in the soil and Cd uptake in Bok Choy as a model crop plant. Such information will establish proper agricultural practices in Cd enriched soils in the Indonesian agriculture systems.
In an incubation experiment (Chapter 3), two soils with contrasting properties (Recent Soil and Allophanic Soil) were spiked with five levels of Cd (0, 1,2,5 and 10 mg Cd/kg) and amended with four rates of compost (0%, 5%, 10% and 15%). Subsamples were incubated at 250C under dark conditions with a moisture content of 70% field capacity for 90 days. Every 30 days a soil sample (20 g) was removed from each experimental unit for soil analysis. The results showed that greater compost application reduced Cd availability in the soil and an increased Cd availability as a function of time. The results varied between the two soils: changes were significant in the Recent Soil and insignificant in the Allophanic Soil. The greatest magnitude of change was observed for the Recent Soil between Day 30 and Day 60.
In a subsequent pot trial experiment, Bok Choy (Brassica chinensis L) was grown in Recent and Allophanic Soil spiked with 10 mg/kg Cd and four levels of compost application (0%, 5%, 10% and 15%). Biomass was destructively sampled from separate pots at Day 30 and Day 60. The results showed that Cd content increased proportional to dry weight biomass. Greater application of compost decreased the magnitude of the Cd content difference between the two sampling times. For 0% compost application, the Cd root content increased with time, ranging from 515% to 850% for the Recent and Allophanic Soil, while for the 15% compost application this range was between 12% to 248%. For shoots, the increase for the 0% compost
treatment was 34.3% in the Recent Soil and 149% in Allophanic Soil. Plant roots experienced a more prominent Cd content increase from Day 30 to Day 60 compared to shoots.
Overall, there was evidence of organic matter decomposition in soil that caused a slight decrease in organic matter percentage with time in both experiments. This led to an increase in Cd availability over time in the soils; a change that was significant in the Recent Soil but insignificant in the Allophanic Soil. Higher compost rates reduced the increase in Cd content over time for Bok Choy (Brassica chinensis. L). compared to no compost application. Based on the results of this thesis, good agricultural practices such as reapplying compost regularly, at a certain application rate, could be followed by farmers to reduce the risk of Cd transfer to the food chain where leafy vegetables are commonly grown in Cd-enriched soils. Further work
is needed to define the optimal application rate and period.