Impact of Manure Phosphorus Fractions on Phosphorus Loss from Manured Soils after Incubation

The risk of P loss from manured soils is more related to P fractions than total P concentration in manure. This study examined the impact of manure P fractions on P losses from liquid swine manure‐ (LSM), solid cattle manure‐ (SCM), and monoammonium phosphate‐ (MAP) treated soils. Manure or fertilizer was applied at 50 mg P kg −1 soil, mixed, and incubated at 20°C for 6 wk to simulate the interaction between applied P and soil when P is applied well in advance of a high risk period for runoff. Phosphorus fractions in manure were determined using the modified Hedley fractionation scheme. We used simulated rainfall (75 mm h −1 for 1 h) to quantify P losses in runoff from two soils (sand and clay loam). The proportion of total labile P (total P in water+NaHCO 3 fractions) in manure was significantly greater in LSM (70%) than SCM (44%). Mean dissolved reactive P (DRP) load in runoff over 60 min was greatest from MAP‐treated soil (18.1 mg tray −1 ), followed by LSM‐ (14.0 mg tray −1 ) and SCM‐ (11.0 mg tray −1 ) treated soils, all of which were greater than mean DRP load from the check (5.2 mg tray −1 ). Total labile P (water+NaHCO 3 ) in manure was a more accurate predictor of runoff DRP loads than water extractable P, alone, for these two soils. Therefore, NaHCO 3 extraction of manure P may be a useful tool for managing the risk of manure P runoff losses when manure is applied outside a high risk period for runoff loss.