Nitrogen fertilizer equivalency of sewage biosolids applied to dryland winter wheat

Determining the N fertilizer equivalency or mineralization rates associated with continuous application of biosolids are paramount in estimating "agronomic rates" for various crops. We used 6 yr of field-study data from sewage biosolids application to dryland hard-red winter wheat (Triticum aestivum L., 'TAM107') to estimate the N fertilizer equivalency and mineralization rates of Littleton and Englewood (WE) biosolids, We made three LIE biosolids and urea applications to two sites (Weld loam soil) using a dryland winter wheat summer-fallow management system. Biosolids rates were 0, 2.2, 4,4, 6.6, 8.8, and 11 dry Mg ha(-1) while N fertilizer rates were 22, 44, 66, 88, and 110 kg N ha(-1); we replicated all treatments four times. Using N uptake data, we found application of 1 dry Mg of LIE biosolids before each planting delivered an equivalent of about 8.2 kg N fertilizer. By contrast, estimates ranged from 6.2 to 6.7 kg fertilizer N Mg-1 L/E biosolids using the USEPA plant-available N calculations. We also calculated, from our field-study results and USEPA estimations, first-year mineralization rate ranges of 25 to 32%, In a companion greenhouse study, we found that a single application of biosolids provided an N response for only two wheat croppings, These approximations should help managers determine the "agronomic rate" for continuous biosolids additions to dryland winter wheat.