Wheat grain yield, phosphorus uptake and soil phosphorus fraction after 23 years of annual fertilizer application to an Andosol

A field experiment was conducted on an Andosol to evaluate wheat (Triticum aestivum L.) yield, P and N uptake and soil P fraction after long-term fertilization (no fertilizer, NPK, NP, NK and PK treatments). Application rates of N, P and K fertilizers were 100, 65 and 83 kg ha−1 year−1 by ammonium sulfate, superphosphate, and potassium chloride, respectively. Phosphorus fertilization was critical for grain yield since the NK treatment did not increase yield compared with no fertilizer treatment. Agronomic efficiency of P was greater than agronomic efficiency of N, although apparent recovery of P and N were 17 and 53%, respectively. Combination application of fertilizer P and N resulted in the greatest grain yield over 23-year cultivation. Interaction impact on grain yield between P and N ranged from 71 to 109%, and was greater than the values for cereals in the earlier works. The N/P ratios of wheat decreased by P application and increased by N application. The N/P ratios in NPK and NP treatments were higher than the values attaining maximum yield for cereal crops reported by other works.Increase in soil available P in the treatments with P application was modest after 23-year fertilization. Total inorganic P (Pi), Ca-Pi + Al-Pi + Fe-Pi, increased in the treatments with P application at 0–15 cm. Total Pi was greater at 0–15 cm depth than at 30–50 cm depth. Although apparent recovery of fertilizer P (Ca-Pi as superphosphate) was less than 20%, soil Ca-Pi was very low even in the treatments with P application. This meant that unutilized fertilizer P did not remain in the form of Ca-Pi. In contrast to inorganic P, there was no significant difference in total organic P (Po), Ca-Po + Al-Po + Fe-Po, among the treatments and soil depths. Regardless of fertilizer treatments, Al-Pi was the predominant form at 0–15 cm depth and Al-Pi concentrations were similar to Fe-Pi concentrations at 30–50 cm depth. On the other hand, Fe-Po was greater than Al-Po at 0–15 cm depth. Difference in inorganic P at 0–15 cm depth demonstrated that unutilized fertilizer P was transformed mainly to Al-Pi followed by Fe-Pi. However, wheat seemed to absorb P from Al-Pi and Fe-Pi modestly.