Abstract
The effect of fluctuating soil moisture cycles on soil cadmium (Cd) phytoavailability was investigated in a pot trial with two contrasting soils (Kereone (Allophanic), total Cd 0.79 mg kg−1; and Topehaehae (Gley), total Cd 0.61 mg kg−1) that were either sown with plantain (Plantago lanceolata) or left unseeded. Varying soil moisture contents were established using contrasting irrigation regimes: “flooded” (3 days flooded and then 11 days drained); or “non-flooded” (irrigation to 70% of potted field capacity every 7 days). Overall, there was no significant difference in mean 0.05 M CaCl2 soil extractable Cd concentrations or plant tissue Cd concentrations between flooded and non-flooded irrigation. However, there was a consistent trend for an increase in soil extractable Cd concentrations following irrigation, regardless of the irrigation regime. Mean soil extractable Cd and plant tissue Cd concentrations were significantly greater (approximately 325% and 183%, respectively) for the Topehaehae soil than the Kereone soil, despite the lower soil total Cd concentration of the Topehaehae soil. These results indicate that Cd solubility is sensitive to increases in soil moisture following periods of soil drainage, but insensitive to short-term periods of soil saturation. Plant tissue Cd concentrations in Cd-sensitive forage crops such as plantain are likely to be greater following large rainfall events over summer and autumn. This has the potential to increase animal dietary Cd exposure and rate of liver/kidney Cd accumulation.
Highlights
Cadmium (Cd) is a non-essential element that is readily taken up by plants
The results indicate that the solubility of soil Cd is sensitive to initial increases in soil moisture content following periods of soil drainage, but insensitive to short-term periods of soil saturation
As change in redox potential is expected to influence soil solution pH [29,36,37,38,39], the lack of difference in soil solution pH between irrigation regimes in our study (Table 3) suggests that either (i) both soils buffered potential pH changes brought about by differences in redox processes driven by the different irrigation regimes; or (ii) that the 3-day flooded period was insufficient for generating reducing soil conditions
Summary
Cadmium (Cd) is a non-essential element that is readily taken up by plants. Below phytotoxic concentrations, plants have little ability to regulate Cd uptake and accumulation [1]. Many soil factors are known to influence soil Cd phytoavailability These include total soil Cd concentration, pH, organic matter content, mineralogy (i.e., clay types and abundance), the concentration of other anions and cations in soil solution, and soil redox potential [4,5,6,7,8,9]. Of these factors, soil pH is thought to be the most dominant factor controlling soil Cd phytoavailability, largely due to its overriding influence on the surface charge of variable charge clays and organic matter that dictates the soil’s net negative charge and Cd2+ sorption potential [4,5,8,10,11,12,13]
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