Termination of sewage biosolids application affects wheat yield and other agronomic characteristics

Biosolids land application for beneficial use requires managers to use agronomic rates to avoid nutrient overapplication. Consequently, one question regarding biosolids land application is, "Once application ceases, how long does it take agronomic properties associated with excessive biosolids applications to approach the agronomic characteristics corresponding to an untreated control?" In a summer fallow rotation system receiving 40 dry Mg biosolids ha(-1) rate per cropping (six to nine times larger than the agronomic rate) for five applications (>10 yr), we compared dryland hard red winter wheat (Triticum aestivum L. 'Vona' or' TAM 1071) and 0- to 20-cm soil depth responses to results for an untreated, unfertilized control (0 Mg ha(-1)) for three croppings (>6 yr) following discontinuation. We applied biosolids from the Littleton/Englewood, CO, wastewater treatment plant to an Aridic Paleustoll and an Aridic Argiustoll soil at Sites A and B, respectively. Using a split plot in time design, we found differences, probably due to climatic variations, between the control and the discontinued biosolids treatment for yield and grain N, P, Zn, and Cu concentration at Site A and a significant cropping effect on all plant parameters at both sites. After five applications of biosolids exceeding the agronomic rate by six- to ninefold, the discontinued biosolids treatment produced similar soil NH4HCO3-diethylenetriaminepentaacetic acid (AB-DTPA) extractable P and Zn at both sites, and soil AB-DTPA Cu and electrical conductivity of a saturated soil-paste extract (ECe) at Site A by the third cropping following termination.