Predicting soil-extractable Zn, P, Fe, and Cu in a biosolids-amended dryland wheat agroecosystem

Biosolids Beneficial Use Programs frequently involve multiple applications at agronomic rates, with plant-nutrient availability changing as elements react with soil constituents over time. Consequently, can regression equations reasonably estimate plant availability of Zn, P, Fe, and Cu, where multiple applications of Littleton and Englewood, Colorado Wastewater Treatment Plant biosolids are applied to a dryland wheat (Triticum aestivum L.)-fallow agroecosystem? Before each growing season, we added Littleton and Englewood biosolids at rates of 0 to 11.2 dry Mg ha?1 to plots arranged in randomized complete blocks with four replications per treatment. Soil samples collected after each wheat harvest were analyzed using an NH4HCO3-diethylenetriaminepentaacetic acid extraction. We completed planar (included the number of applications and elemental additions), linear, quadratic, and exponential-rise-to-a-maximum (as a function of elemental additions only) regression analyses for six applications at two sites. We found that the planar regression models provided superior R2 values and SE of the estimate in almost all cases. These results suggest that lability changes as biosolids-borne Zn, P, Fe, and Cu react with the soil over time. Consequently, predictions of nutrient availability involving multiple biosolids applications to dryland wheat-fallow agroecosystems should account for the number of biosolids additions