Long-term dynamics of phosphorus forms and retention in manure-amended soils

Phosphorus (P) leaching from soils with elevated P levels due to manure applications is increasingly becoming a concern as a source of eutrophication of streams and lakes. This study investigates the relationship between organic and inorganic P in soil pools and equilibrium leachate along a chronosequence of poultry and dairy manure additions in New York state. Resin-extractable P (molybdate-reactive P, RP) and total soil P reached very high levels of 2330 and 7343 mg of P kg(-1), respectively, after more than 25 years of continuous manure applications. After long-term manuring, the ability of these soils to retain additional P was low (Langmuir maximum sorption potential of 5159 g of P kg-1) and equilibrium leachate concentrations of total dissolved P (TDP) were high (5.5-7.6 mg L-1; saturated conditions, 0.15-m lysimeters in closed loop). Total dissolved P concentrations in equilibrium leachate increased linearly (r = 0.737) to a total soil P of 4500 mg kg-1 and increased to a greater extent above 4500-5500 g kg-1 (change point equivalerit to about 1500 mg kg(-1) Mehlich 3-extractable RP). The proportion of dissolved unreactive P (DUP) in equilibrium leachate decreased from 90% of TOP in fields with a short manure application history to 2% of TOP where mainly poultry manure had been applied for > 25 years, while unreactive P (UP) in soil decreased from 44% to 6%. Dissolved RP (DRP) was less mobile than DUP in soils with short duration of manure applications (p < 0.05), while differences between DUP and DRP mobility disappeared with longer duration of manure application and greater total soil P. Organic P forms in NaOH/NaF extracts determined by P-31 NMR did not change with manure history, but sequential fractionation showed that the relative distribution of RP pools in soils changed. Dilute acid Pi increased from 10% to 62% with longer poultry manure additions, suggesting the formation of calcium phosphates as the soil pH increased from 4.1 to 6.0-7.2. The precipitation of P as calcium phosphates appeared to influence leachable P upon high and long-term applications of manure dominated by poultry litter.