Uptake coefficients for biosolids-amended dryland winter wheat

Biosolids regulations developed in the United States employed risk assessment impacts of trace element additions on plant uptake. The US Environmental Protection Agency adapted the uptake coefficient (ratio of plant concentration to quantity of element added) when developing limitations on selected elemental additions. The nature of the risk assessment requires uptake coefficients to be constants. Our hypothesis was the uptake coefficient for copper, iron, molybdenum, nickel, phosphorus, and zinc for biosolids-amended dryland winter wheat decreases with multiple biosolids applications at the same location. We applied up to 10 applications to two sites (designated North Bennett A and B) in eastern Colorado at rates from 2.24 to 11.2 megagrams per hectare per application from 1993 to 2013. Results indicated that grain concentrations for all six elements followed no discernible trend as the number of biosolids applications increased. The uptake coefficient for these elements compared to the number of biosolids applications followed exponential decay models (R-squared ranged from 0.329 to 0.879). Consequently, uptake coefficient values will likely not provide constants for risk assessment where multiple biosolids applications are made on the same site. We found that the slope between cumulative elemental removal by grain (kilograms per hectare) to the cumulative amount of element added with biosolids (kilograms per hectare) provides a constant over the number of biosolids additions (R-squared ranged from 0.471 to 0.990). As compared to the US Environmental Protection Agency approach, our strategy may provide parameters that allow for improved risk assessment of biosolids-borne elements on plant uptake