Cadmium Concentrations in New Zealand Wheat: Effect of Cultivar Type, Soil Properties, and Crop Management.

Wheat ( L.) grain is a contributing source of dietary Cd in New Zealand, but despite this, there is a dearth of information on Cd concentrations in wheat and the factors that affect uptake. We measured Cd concentrations in 12 wheat cultivars grown in field sites across New Zealand and also assessed the soil, plant, and crop factors that have been reported to affect Cd uptake. We found there was a wide range in grain Cd concentrations (0.004 to 0.205 mg kg fresh weight [FW]). The overall mean concentration (0.066 mg kg FW) was below the maximum limit (ML) of 0.1 mg kg FW. Only 7% of grain samples across seven sites exceeded the ML. There were significant ( < 0.05) differences (2.5-fold) in Cd concentrations in wheat grain between cultivars. No strong significant relationships were found between soil properties and Cd concentrations in grain. Further, management factors affecting grain Cd concentrations were inconsistent. Given the lack of relationships between soils and management practices, the most effective way of limiting Cd uptake in wheat grain at sites where Cd exceeds MLs may be the use of low-Cd-accumulating cultivars. Further studies are required to explore specifically the influence of irrigation and effects of type and rate of fertilizer on Cd uptake in wheat, as well as to better understand the mechanisms of Cd uptake in wheat.

Effects of soil compaction and irrigation on the concentrations of selenium and arsenic in wheat grains.

Arsenic is a known human carcinogen, whereas Se is essential to human health and human Se intake in some European countries has decreased in recent decades. There have been few reports on the potential effect of soil physical conditions on the uptake of Se and As by food crops. Field experiments were conducted to evaluate the effects of soil compaction and irrigation on Se and As concentrations in wheat grains in two seasons. Grain Se concentration varied from 10 to 115 microg kg(-1), whereas the concentration of As in grain was low (<20 microg kg(-1)). Irrigation significantly decreased grain Se concentration by 30-75%, and the effect was likely due to a dilution as a result of increased grain yield, a competition of sulfate added in the irrigation water on Se uptake and increased leaching losses. Soil compaction significantly decreased grain Se concentration in one season. In contrast, soil compaction increased grain As concentration significantly when the crop was irrigated. The contrasting effects of soil compaction on As and Se uptake are explained by the difference in their mobility in soil and the pathway of ion transport to roots. The observed effect on grain Se is significant for human and animal nutrition, because the concentration varied from sufficient to very low levels in response to the alternation of soil physical conditions in the field. Soil physical conditions have to be taken into account when assessing the bioavailability of the contaminant As or the essential nutrient Se.

Assessment of lead availability in contaminated soil using isotope dilution techniques.

Isotope dilution methods using a stable isotope tracer ((207)Pb) were developed for the determination of Pb availability in contaminated soils. The methods included determination of E values (isotopically exchangeable pool), L values (plant labile pool) and isotopic exchange kinetics (IEK). Isotopically exchangeable Pb was monitored at different exchange times based on measurement of the (207)Pb/(208)Pb ratio in soil solution following addition of the tracer. The rate of decrease in the (207)Pb/(208)Pb ratio in solution could be described by using the same IEK equation as used previously with radioisotope tracers. The amounts of isotopically exchangeable Pb in Pb-contaminated soils estimated from long-term IEK parameters were in good agreement with directly determined E values up to 15 days. However, values of some of the fitted IEK parameters cast doubts on the validity of using the IEK approach with (207)Pb, most probably as a result of irreversible fixation of some of the spike by reactive surfaces in the soils. Estimation of isotopically exchangeable Pb using short-term kinetics data was unsuccessful, substantially underestimating E values. Results for the control (uncontaminated) soil were highly variable, most probably as a result of fixation of tracer by the soil and poor analytical precision due to low solution Pb concentrations. A compartmental analysis of the variation in E values with time indicates a good potential for estimating bioavailable Pb in contaminated soils. The amounts of available Pb obtained from summation of the E(1)(min) and E(1 min-24 h) pools (E((available))), accounting for an average of 57.62% of total soil Pb, were significantly correlated with both the L values and with Pb extracted from soil with EDTA.

Atmospheric accessions of heavy metals to some New Zealand pastoral soils.

Total heavy metal deposition was monitored at seven rural sites across New Zealand to determine the rate of atmospheric deposition of heavy metals to soils and to evaluate any regional and temporal variations in metal deposition. Heavy metal deposition was collected monthly in Warren Spring Laboratory type inverted frisbees. The rate of heavy metal deposition followed the order Zn>>Cu>Cr>Pb>Ni>>Cd. There were no obvious trends with regards to either seasonal or spatial deposition for any of the heavy metals measured. With the exception of Zn, heavy metal deposition in New Zealand was generally lower than rates measured in other countries. This reflects the general lack of high temperature industrial processes from urban-based industrial processes that are an important source of heavy metal aerosols in other industrialised regions worldwide. The inverted frisbee technique used provides a reliable indication of the magnitude of deposition of heavy metals from the atmosphere at a reasonable cost and with manageable logistical effort for a national survey.