Influence of feedlot manure on reactive phosphorus in rainfall runoff during the transition from continuous to legacy phases

Little research has compared land application of stockpiled (SM) or composted (CM) beef feedlot manure with straw (ST) or wood-chip (WD) bedding on loss of reactive phosphorus (RP) in runoff. We conducted a 6 yr (2013-2018) rainfall simulation-runoff study and utilized surface (0-5 cm) soil collected from a long-term (since 1998) field experiment on a clay loam soil in southern Alberta, Canada. The treatments consisted of SM or CM with ST or WD bedding applied at 13, 39, and 77 Mg.ha(-1) (dry weight), as well as an unamended control and mineral fertilizer treatment. Surface soil was collected from all treatments after 15-17 (C15, C16, C17; 2013-2015) continual annual applications, and then 1-3 yr (L1-L3, 2016-2018) into the legacy phase after manure applications were first discontinued in 2015. The soil was packed into runoff trays, and flow-weighted mean concentrations (FWMCs) and mass loads of RP5 (<5 mu m filter) in runoff water were determined during rainfall simulations. Our findings generally supported our null hypothesis of similar RP5 losses for manure type (CM = SM) and bedding (ST = WD) for most years. Successively higher application rates increased RP5 loss by 32%-121%. Termination of long-term applications dramatically reduced FWMCs by 58%-77% and mass loss by 56%-65% from the C17 to L3 years. This suggests an accumulation of soil P during continuous phase and depletion during legacy phase; therefore, lower application rates or termination of applications may reduce RP5 loss in runoff.