In New Zealand, to avoid the risk of urban sewage bringing N and P into Lake Rotorua, Rotorua city sprays treated sewage direct into Whakarewarewa Forest as irrigation and to evaluate capacity of the forest ecosystem of assimilating the nutrients brought in with the irrigation, a long term field experiment on sewage irrigation of forest land has been established and carried out. To understand effects of long term sewage irrigation on content and form of soil P, and on the environment, soil samples were collected from the system for analysis. Results show that soil TP（total phosphorus), Olsen P, M3P (Mehlich-3 P) all increased significantly (p <0.05) in the top soil (0~10 cm) in all the effluent treated plots as compared with the control plots and some even in the 20~40 cm or deeper soil layer. Most of the P brought in with the irrigation accumulated in the top 40 cm soil. Fractionation of soil P changed due to long term effluent irrigation. NaOH P_o (organic P extracted by NaOH) was replaced by NaOH P_i (inorganic P extracted by NaOH) in dominancy. Significant change in WSP (water soluble P) was observed in top soil. To set measurement value 60 mg kg^-1 Olsen P (measured value) and 150 mg kg^-1 for M3P as environment thresholds enables prediction of the risk of P leaching, and WPS may be a better indicator of environmental crisis of P leaching and applicable to more soils.