Matching plant-available nitrogen from biosolids with dryland wheat needs

Biosolids are stabilized solids from municipal wastewater treatment that meet federal standards for land application. Good estimates of biosolids N availability are needed to develop sustainable biosolids management programs. We conducted this study to (i) determine the availability and fate of biosolids N applied to a dryland soft-white winter wheat (Triticum aestivum L.)-fallow rotation, (ii) determine if N availability predictions for biosolids are applicable under dryland conditions, and (10) make practical recommendations for biosolids management. We applied dewatered (21-30% solids) biosolids (4.3-5.5% total N; 257-853 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 ammonia [AA]) and unfertilized treatments were established at each site for comparison. We measured yield and N uptake of grain and straw at harvest, and determined soil profile nitrate N (plus ammonium N, 0-12 in. 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(fallow) was dependable and consistent over the three sites, despite differences in environment among the sites. ANR(fallow) averaged 29%, similar 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 leaf N all indicate that N was sufficient at the lowest biosolids rates used, and that higher levels of biosolids did not benefit the crop. Drawbacks to higher rates include risks of reduced crop yield (from moisture stress) and quality (from increased protein), and increased risk of nitrate movement below the root zone. Storage of nitrate in the soil profile does not appear to be a reliable strategy for supplying N for a second crop. Lower biosolids rates seem to reduce economic risks to the farmer and reduce leaching risk Evaluation of low biosolids rates (10-300 lb total N/acre) and second crop response will be valuable in refining biosolids application recommendations.