Abstract: Based on a 20-year experiment of fertilization with organic and chemical fertilizers on a Fluvo-aquic soil under wheat-corn cropping system, this paper studied the relationships between Olsen-P concentration in plough layer and crop yields as well as the accumulation and vertical translocation of Olsen-P in soil profile. The results showed that when the Olsen-P concentration in plough layer maintained at 10-40 mg·kg^-1, the grain yields of wheat and corn were higher, whereas when the concentration of Olsen-P in plough layer was higher than 40 mg·kg^-1, it started to leach, which meant that in light loam Fluvo-aquic soil, the threshold value for P leaching might be 40 mg·kg^-1. In the treatments of chemical fertilization (NPK) and corn straw returning (SNPK) with the P application rate of  77-90 kg·hm^-2, the Olsen-P concentration in plough layer was increased by 0.63-0.72 mg·kg^-1 per 100 kg·hm^-2 of applied P, with an annual increment of 0.49-0.65 mg·kg^-1 and needed 45-60 years for reaching the threshold value for P leaching. In the treatments of chemical fertilization combined with manure application (MNPK, MNPK2, and 1.5MNPK),  the formula of Olsen-P accumulation in 0-20 cm soil layer were Y_MNPK=3.1097x+6.9615 (R²=0.8562), Y_MNPK2=2.4765x+13.563(R²=0.9307), and Y_1.5MNPK=4.506x+6.4464 (R²=0.8862). It might take 8 years to reach the threshold value for Olsen-P leaching when the P application rate in treatment 1.5MNPK was 210 kg·hm^-2, 11 years when the P application rate in treatments MNPK2 and MNPK was 125 and 140 kg·hm^-2. Organic fertilization combined with chemical fertilization increased the Olsen-P accumulation rate being 2.5 times higher than chemical fertilization. Excessive application of organic fertilizer could increase the accumulation and leaching of Olsen-P in soil profile.

Key words: Fluvo-aquic soil, long-term experiment, Olsen-P, accumulation and leaching, organic fertilizer, Amygdalus communis, phenophase, meteorological conditions, correlation analysis, flowering forecast model.