Long-Term Effects of Tillage and Manure Applications on Soil Phosphorus Fractions

Application of manure on the basis of crop nitrogen (N) need increases the level of soil phosphorus (P), which is concern for deterioration of surface water quality. Soil samples were collected from a long-term field study to investigate the impact of crop N need–based manure application on soil P fractions and P adsorption and release kinetics. The field experiment was initiated in 1990. The soil was moderately well-drained Kennebec (fine silty, mixed, mesic Cumulic Hapludolls). No-tillage (NT) and conventional-tillage (CT) treatments were established in main plots, and subplots had five N treatments, including a control, and annual application of 84 or 168 kg N ha^?1 applied as ammonium nitrate (NH₄NO₃) or beef (Bos taurus) manure. Manure at the high N application rate had significantly greater Bray 1 P under NT than under CT at 0- to 5-cm soil depth. Nitrogen fertilizer treatments were not significantly different than the control for Bray 1 P. Continuous application of manure at the high N rate significantly increased all Hedley P fractions; however, the major increase was observed in high bioavailable P pools iron oxide (FeO) P and sodium bicarbonate (NaHCO₃) P_i] and hydrochloric acid (HCl) P fractions. Soil organic P (P_o) pools, including both labile (NaHCO₃-P_o) and resistant sodium hydroxide (NaOH) P_o], were increased by application of N from any source, suggesting biomass production and return of residue to soil surface was the responsible factor. Continuous application of manure based on N need also significantly increased FeO-P, NaHCO₃-P_i, and HCl-P fractions at lower soil depths (5–15 and 15–30 cm). Results from the P-adsorption study suggest that ability of soil to adsorb additional P was decreased by manure application and that EPC₀ was increased. Maximum desorbable P was observed for manure treatments under NT, although the release constant k (h^?1) was significantly less than with fertilizer N treatments.