Biosolids have the potential to improve degraded soils in grain-fallow rotations. Our objectives were to determine if repeated biosolids applications in wheat ( Triticum aestivum L.) - fallow could supply adequate but not excessive N for grain production and increase soil C without creating a high risk of P loss. A replicated on-farm experiment was established in 1994 in central Washington, comparing anaerobically digested biosolids with anhydrous NH 3 and a zero-N control. Biosolids were applied at 5, 7, and 9 Mg ha -1 every fourth year through 2010 and incorporated 10 cm deep, while anhydrous NH 3 plots received 56 kg ha -1 N every second year. Grain yield and protein were determined. Soil chemical, biological, and bulk density analyses were made in 2012. Medium and high biosolids rates significantly increased grain yield (3.63 vs. 3.13 Mg ha -1) and protein (103 vs. 85 g kg -1) compared with anhydrous NH 3 averaged across all crops. The medium biosolids rate had significantly lower bulk density (1.05 vs. 1.22 g kg -1) and greater total C (0-10-cm depth) (16.9 vs. 9.4 g kg -1), mineralizable N (156 vs. 52 mg kg -1), and extractable P (114 vs. 16 mg kg -1) than anhydrous NH 3. The P index site vulnerability increased from low for anhydrous NH 3 to medium for the biosolids treatments. Soil NO 3-N was nearly always <10 mg N kg -1 soil (0-30-cm depth). Medium and high biosolids rates significantly increased bacteria/fungi ratios, Gram-negative bacteria, and anaerobic bacteria markers compared with anhydrous NH 3. Biosolids can be an agronomically and environmentally sound management practice in wheat-fallow systems.
