Effect of long-term fertilization on phosphorus fractions in different soil layers and their quantitative relationships with soil properties

Investigating the dynamics and distribution of soil phosphorus (P) fractions can provide a basis for enhancing P utilization by crops.  Four treatments from a 29-year long-term experiment in black soil with maize cropping were involved in this study: no fertilizer (CK), inorganic nitrogen and potassium (NK), inorganic nitrogen, phosphorus, and potassium (NPK), and NPK plus manure (NPKM).  We analyzed soil P fractions in different soil layers using a modified Hedley sequential method.  The long-term NPKM treatment significantly increased total P by 0.6–1.6 times in the different soil layers.  The Olsen-P concentration far exceeded the environmental threshold for soil Olsen-P (50.6 mg kg^–1) in the NPKM treatment in the 0–60 cm soil profile.  Moreover, the concentrations and proportion of labile and partially labile inorganic P (Pi) fractions (i.e., NaHCO₃-extracted Pi, NaOH-extracted Pi, and dilute HCl-extracted Pi) to the sum of all P fractions (Pt) in the 0–60 cm soil profile were higher in the NPKM treatment than in the NPK treatment, indicating that manure could promote the transformation of non-labile into more labile forms of P in soil, possibly by manure reducing P fixation by soil particles.  Soil organic matter, Mehlich-3 extractable iron (Fe), and organic-bound aluminum were increased by fertilization, and were the main factors influencing the differences in the P fractions in the 0–20 cm soil layer.  Soil mineral components, i.e., free Fe oxide and CaCO₃, were the main factors influencing the P fractions in the subsoil.  The soil P transformation process varied with soil layer and fertilization.  Application of manure fertilizer can increase the labile (Olsen) P concentrations of the various soil layers, and thus should reduce the mineral P fertilizer requirement for crop growth and reduce potential environmental damage