Spatial variability of grain cadmium and soil characteristics in a durum wheat field

The accumulation of Cd in edible crops is undesirable because of potential risks to human health. Agronomic efforts to lower Cd in foods will be more effective if we understand the extent and causes of spatial variability of Cd in crops in the field. The primary objective of this study was to characterize the spatial variability of grain Cd and eight soil characteristics in a field of durum wheat [Triticum turgidum L. subsp. durum (Desf.) Husn.] in northeastern North Dakota. A rectangular area of about 0.5 ha was selected to provide a range in several soil characteristics that are reported to influence Cd uptake by plants. These included chelate-extractable Cd, total soil Cd (Cd,), pH, cation-exchange capacity (CEC), organic C (OC), and soluble Cl, S, and Na. Location data and paired samples of soil and durum grain were collected at 124 sites. Semivariograms showed that the variations of grain Cd and most soil properties were strongly spatially dependent, with range distances that varied from about 30 to 55 m. A spherical model was fitted successfully to variograms for all characteristics. Maps of grain Cd and soil characteristics were generated by interpolating among measured values by block kriging. Visual and statistical comparisons of maps showed that grain Cd was distributed similarly to measures of soil salinity, especially to the logarithm of soil Cl. Locations with somewhat poorly drained soils,,which contained elevated Cl, produced grain with much higher Cd. Our results suggest that knowledge of the spatial distribution of soil characteristics, especially salinity, should be helpful in developing or applying agronomic practices to reduce Cd in durum grain.