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

The concentration of soil Olsen-P is rapidly increasing in many parts of China, where P budget (P input minus P output) is the main factor influencing soil Olsen-P.  Understanding the relationship between soil Olsen-P and P budget is useful in estimating soil Olsen-P content and conducting P management strategies.  To address this, a long-term experiment (1991–2011) was performed on a fluvo-aquic soil in Beijing, China, where seven fertilization treatments were used to study the response of soil Olsen-P to P budget.  The results showed that the relationship between the decrease in soil Olsen-P and P deficit could be simulated by a simple linear model.  In treatments without P fertilization (CK, N, and NK), soil Olsen-P decreased by 2.4, 1.9, and 1.4 mg kg^–1 for every 100 kg ha^–1 of P deficit, respectively.  Under conditions of P addition, the relationship between the increase in soil Olsen-P and P surplus could be divided into two stages.  When P surplus was lower than the range of 729–884 kg ha^–1, soil Olsen-P fluctuated over the course of the experimental period with chemical fertilizers (NP and NPK), and increased by 5.0 and 2.0 mg kg^–1, respectively, when treated with chemical fertilizers combined with manure (NPKM and 1.5NPKM) for every 100 kg ha^–1 of P surplus.  When P surplus was higher than the range of 729–884 kg ha^–1, soil Olsen-P increased by 49.0 and 37.0 mg kg^–1 in NPKM and 1.5NPKM treatments, respectively, for every 100 kg ha^–1 P surplus.  The relationship between the increase in soil Olsen-P and P surplus could be simulated by two-segment linear models.  The cumulative P budget at the turning point was defined as the “storage threshold” of a fluvo-aquic soil in Beijing, and the storage thresholds under NPKM and 1.5NPKM were 729 and 884 kg ha^–1 P for more adsorption sites.  According to the critical soil P values (CPVs) and the relationship between soil Olsen-P and P budget, the quantity of P fertilizers for winter wheat could be increased and that of summer maize could be decreased based on the results of treatments in chemical fertilization.  Additionally, when chemical fertilizers are combined with manures (NPKM and 1.5NPKM), it could take approximately 9–11 years for soil Olsen-P to decrease to the critical soil P values of crops grown in the absence of P fertilizer.