Cadmium enrichment in paddy rice grain: understanding the effect of soil oxidation and limiting the risk through soil and foliar treatments.

Abstract

Cadmium (Cd) is a non-essential element and agricultural soil contaminant (Friberg et al., 1974). Cadmium accumulation in paddy rice grain is a human health risk, exacerbated by the fact that grains accumulate more Cd than the nutrients iron (Fe) and zinc (Zn), based on their relative soil concentrations (Chaney et al., 1996), which enhances Cd bioavailability (McLaughlin et al., 1999b). The enrichment of Cd in paddy rice grain occurs during soil oxidation, which accompanies pre-harvest drainage of the flooded paddy (Iimura, 1981b; Inahara et al., 2007) and coincides with the grain-filling phase of rice growth. The objectives of this thesis were to elucidate which soil processes are likely responsible for Cd accumulation and enrichment relative to Fe and Zn in paddy rice grain, and to determine whether soil amendments to paddy fields or foliar sprays to rice plants could be used to limit Cd accumulation and enrichment in paddy rice grain. In Chapter 3 it is demonstrated that differential oxidation of sulfide minerals is not likely the process responsible for Cd enrichment in paddy rice grain. In Chapter 4 it is shown that when sulfide minerals form and then oxidize in a soil environment, this process in fact limits Cd solubility relative to Fe and Zn. This brings to light a new paradigm, that sulfide formation, prior to oxidation, is in fact the best-case scenario in terms of limiting Cd enrichment in paddy rice grain. In Chapter 4 it is demonstrated that under conditions of sulfate-limitation (i.e. worst-case scenario for Cd enrichment), the Cd:Fe and Cd:Zn ratios in soil solution are lower for soil higher in Fe and/or Zn. However, in Chapter 5 it is shown that rice plants grown in soil amended with the same rates of Fe and/or Zn as in Chapter 4, produced grains with higher Cd:Fe and Cd:Zn ratios than controls. When Mn was added to soil, rice grains contained more Zn, and therefore the bioavailability of Cd was lower. When soil was amended with EDTA, rice grains had lower Cd:Fe and Cd:Zn ratios and therefore would pose a lesser risk to human health from Cd, whereas foliar sprays of EDTA or Zn had no effect on relative accumulation of metals in grains. This thesis addresses major knowledge gaps in the area of Cd enrichment in paddy rice. It is suggested that the relative accumulation of Cd, Fe and Zn in paddy rice grain is most likely the net effect of the oxidation of sulfide minerals and concurrent cation competition during pre-harvest drainage. It is also shown that Cd accumulation in paddy rice grain may be greater for rice grown in soils higher in Fe and Zn in the presence of chloride, and that the risk may be lesser for soils high in Mn, or amended with EDTA. Further research is suggested to support these observations under a wider range of conditions.