Manure rate reduction improved the yield–nitrate-loss trade-off but retained legacy phosphorus risk in a tile-drained agroecosystem

Introduction Long-term manure application can sustain crop productivity but may accumulate soil nutrients that remain susceptible to loss through subsurface drainage. Methods We evaluated crop productivity and water-quality responses during 2018–2020 following a transition from long-term spring to fall poultry-manure application in continuous corn on tile-drained Mollisols in Iowa, USA. Fall-applied poultry manure supplying nominal rates of 224 kg N ha −1 (PM) or 112 kg N ha −1 (PM0.5) was compared with spring-applied urea–ammonium nitrate supplying 224 kg N ha −1 (UAN). Corn yield, tile drainage, flow-weighted NO 3 − N and PO 4 − P concentrations and loads, yield-scaled nutrient losses, residual soil nutrients, and N-use indices were evaluated across wet, normal, and dry years. Results Corn yield showed significant treatment, year, and treatment × year effects. PM produced 26.9% and 33.2% greater yield than UAN in 2019 and 2020, respectively, whereas PM0.5 did not differ significantly from UAN in any year. Drainage-water responses were evaluated descriptively because retained replication was limited and unbalanced. During the wet year, NO 3 − N loads were 120.55, 24.97, and 40.81 kg N ha −1 under PM, PM0.5, and UAN, respectively. PM0.5 minimized yield-scaled NO 3 − N loss and maximized partial factor productivity of applied N in each year, but it did not consistently minimize yield-scaled PO 4 − P loss. Residual soil NO 3 − N was unaffected by treatment, whereas Mehlich-3 P differed significantly among treatments and depths. In 2020, arithmetic mean Mehlich-3 P concentrations at 0–30 cm were 323.3, 144.3, and 20.0 mg kg −1 under PM, PM0.5, and UAN, respectively. Conclusion Reducing manure inputs improved the descriptive yield–nitrate-loss balance but did not eliminate phosphorus risk associated with long-term manure use. These findings represent contrasting nutrient-management systems rather than an isolated application-timing effect.