MATCHING PLANT-AVAILABLE NITROGEN FROM BIOSOLIDS WITH DRYLAND WHEAT NEEDS

Biosolids are stabilized solids from municipal wastewater treatment th at meet federal standards for land application. Good estimates of bios olids N availability are needed to develop sustainable biosolids manag ement programs. We conducted this study to (i) determine the availabil ity and fate of biosolids N applied to a dryland soft-white winter whe at (Triticum aestivum L.)-fallow rotation, (ii) determine if N availab ility predictions for biosolids are applicable under dryland condition s, and (10) make practical recommendations for biosolids management. W e applied dewatered (21-30% solids) biosolids (4.3-5.5% total N; 257-8 53 lb N/acre) to on-farm test plots at three locations in the 9-12 in. rainfall zone of eastern Washington. Fertilized (anhydrous or aqua am monia [AA]) and unfertilized treatments were established at each site for comparison. We measured yield and N uptake of grain and straw at h arvest, and determined soil profile nitrate N (plus ammonium N, 0-12 i n. depth only) before application, during fallow, and post-harvest. We determined apparent N recovery from the soil at the end of the fallow (ANR(fallow)). Nitrogen release from biosolids as measured by ANR(fal low) was dependable and consistent over the three sites, despite diffe rences in environment among the sites. ANR(fallow) averaged 29%, simil ar to predicted values of 26-31%. The lowest biosolids rates (257-330 lb/acre) supplied more available N than AA. Yield, grain N, and flag l eaf N all indicate that N was sufficient at the lowest biosolids rates used, and that higher levels of biosolids did not benefit the crop. D rawbacks to higher rates include risks of reduced crop yield (from moi sture stress) and quality (from increased protein), and increased risk of nitrate movement below the root zone. Storage of nitrate in the so il profile does not appear to be a reliable strategy for supplying N f or a second crop. Lower biosolids rates seem to reduce economic risks to the farmer and reduce leaching risk Evaluation of low biosolids rat es (10-300 lb total N/acre) and second crop response will be valuable in refining biosolids application recommendations.