Impact of longterm fertilization on different forms of inorganic phosphorus in aquic brown-soil

In order to enhance the utilization ratio of P fertilizer and to improve supply capacity of soil phosphorus, a 15-year trial of 8 classical modes of fertilization, CK (nonfertilization), M (circulated pig manure fertilization), N (nitrogen fertilization), N+M, NP (nitrogen and phosphorus fertilization), NP+M, NPK (nitrogen, phosphorus and potassium fertilization) and NPK+M, was conducted in aquic brownsoils. Inorganic phosphorus (Pi) content in 0～20 cm soil layer was then investigated. The results show rapid or slow decreases in soil available P_i, (Ca₂-P, Ca₈-P, Al-P and Fe-P) under no chemical phosphorus fertilization, resulting in soil P_i pool deficit. It is, however, the contrary under chemical phosphorus fertilization, especially for NP+M treatment. This is because chemical phosphorus fertilizer not only meet plant growth requirement, but also enriches soil P_i pool. In the shortrun, NP+M treatment has the most favorable conditions for improving supply capacity of soil phosphorus. Though O-P and Ca¹⁰-P are not available to plants, they both increase yearly, part of them are from the transformation of excess P_i(Ca₂-P,Ca₈-P,Al-P and Fe-P). Circulated pig manure slows down reduction of the different forms of P_i, while nitrogen and potassium fertilizers accelerate plant absorption of phosphorus, leading to soil phosphorus pool deficit. The relationship between available phosphorus and Ca₂-P, Ca₈-P, Al-P, Fe-P is significant, providing more indexes for soil phosphorus supply capacity evaluation.