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 (P-i), Ca-P-i + Al-P-i + Fe-P-i, increased in the treatments with P application at 0-15 cm. Total P-i was greater at 0-15 cm depth than at 30-50 cm depth. Although apparent recovery of fertilizer P (Ca-P-i as superphosphate) was less than 20%, soil Ca-P-i was very low even in the treatments with P application. This meant that unutilized fertilizer P did not remain in the form of Ca-P-i. In contrast to inorganic P, there was no significant difference in total organic P (P-o), Ca-P-o + Al-P-o + Fe-P, among the treatments and soil depths. Regardless of fertilizer treatments, Al-P-i was the predominant form at 0-15 cm depth and Al-P-i concentrations were similar to Fe-P-i concentrations at 30-50 cm depth. On the other hand, Fe-P-o was greater than Al-P-o at 0-15 cm depth. Difference in inorganic P at 0-15 cm depth demonstrated that unutilized fertilizer P was transformed mainly to Al-P-i followed by Fe-P-i. However, wheat seemed to absorb P from Al-P-i and Fe-P-i modestly. (c) 2006 Elsevier B.V. All rights reserved.