Path Analyses of Grain P, Zn, Cu, Fe, and Ni in a Biosolids-Amended Dryland Wheat Agroecosystem.

Land application of biosolids is an effective means of recycling plant nutrients and is the primary method of biosolids reuse endorsed by the USEPA. 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 studied the effects of biosolids application on wheat ( L.) grain P, Zn, Cu, Fe, and Ni concentrations and uptake (removal) at two dryland agroecosystem sites from 1993 to 2014. We hypothesized that biosolids would have the greatest impact on wheat grain and uptake compared with ammonium bicarbonate-diethylenetriaminepentaacetic acid (AB-DTPA)-extractable nutrient levels, soil pH, or soil organic C concentrations. We used path analyses in combination with multiple linear regression to differentiate the direct, indirect, and total effects of cumulative biosolids applications, soil AB-DTPA, soil pH, and organic C. Biosolids rates, applied biennially from 1993 to 2014 at the beginning of a wheat-fallow rotation, were 0, 2.24, 4.48, 6.72, 8.96, and 11.2 Mg ha. None of the parameters had significant direct, indirect, or total effects on grain concentrations. Biosolids applications had the greatest positive direct impact compared with AB-DTPA levels, soil pH, or soil organic C on P, Zn, Fe, and Ni uptake (removal), whereas AB-DTPA had the greatest positive direct impact on Cu uptake. Soil AB-DTPA, pH, and organic C directly affected some grain concentrations and cumulative uptake, but no consistent trends were noted. This pathway approach allowed differentiation between causation and simple correlation for the effects of cumulative biosolids applications on wheat P, Zn, Cu, Fe, and Ni cumulative uptake but did not provide these same results for grain concentrations.