Path analyses of grain P, Zn, Cu, Fe, and Ni in a biosolids-amended dryland wheat agroecosystem

Biosolids land application is an effective means of recycling plant nutrients and is the preferred method of biosolids reuse by the US Environmental Protection Agency. One issue concerning biosolids application is the extent of the contribution of biosolids-borne plant nutrients to the overall crop concentration and uptake or removal of these nutrients. We hypothesized that the nutrients associated with the biosolids would have the greatest impact on wheat (Triticum aestivum, L.) grain phosphorus, zinc, copper, iron, and nickel concentrations and uptake at two dryland agroecosystem sites from 1993-2014. We employed path analyses in combination with multiple linear regressions to differentiate the direct and indirect effects of soil ammonium biocarbonate-diethylenetriaminepentaacetic acid, cumulative biosolids nutrient additions, soil pH, and organic carbon. Biosolids rates applied in a wheat-fallow rotation were 0, 2.24, 4.48, 6.72, 8.96, and 11.2 Mg/ha from 1993-2014. Biosolids additions had the greatest positive direct impact on grain iron concentrations and on grain phosphorus, zinc, iron, and nickel uptake (removal). Soil ammonium biocarbonate-diethylenetriaminepentaacetic acid and pH directly and indirectly affected some grain concentrations and cumulative uptake, but no consistent trends were noted. Soil organic carbon did not have any direct or indirect impacts. This approach allowed differentiation between causation and simple correlation effects for the effects of biosolids-borne nutrients on wheat phosphorus, zinc, copper, iron, and nickel concentrations and cumulative uptake