Change in forms of phosphorus in a typical Hapli-Udic Isohumosols(black soil) under a 15-year long-term fertilization experiment was studied.Results show that accumulation of soil Al-P and Ca₂-P was positively related to P application rate;application of NK stimulated the plant to absorb Ca₈-P;and the concentration of Occluded-P(Oc-P) was decreased.P application did increase the content of Fe-P in the black soil,but did not the content of Ca₁₀-P.In terms of content in the black soil,the various forms of in-organic P were in the order of Fe-P>Ca₁₀-P>Al-P>Ca₈-P>Ca₂-P>O-P.P fertilizer application increased organic P in the soil,but it did not increase the content of organic matter.Instead,the content of organic matter dropped to a varying degree in different treatments.Application of N 112.5 kg hm^-2 a^-1and P 20 kg hm^-2 a^-1 could keep I-P in balance in the soil.