Effects of long-term fertilization on organic phosphorus fraction in paddy soil

The changes in the organic P fractions and microbial activity related to P transformation in the plow layer of paddy soil with long-term fertilization were investigated, in relation to the continuous application of chemical fertilizers (NPK), straw-based compost (Compost), and chemical fertilizers and compost (NPK + Compost) for 31 years. Continuous fertilizer application and rice cultivation led to a continuous decrease in ratio of organic P to total P in the plow layer with the lapse of years. In particular, the combined application of chemical fertilizers and compost accelerated the decrease in the organic P fractions, presumably due to the promotion of microbial activity in the plow layer, even though a high amount of organic P was inputted by compost. In this paddy soil, moderately labile organic P (MLOP) was the main organic P compound, which accounted for 70-90% of organic P, and showed the same tendency as that of the total organic P fraction during the test, irrespective of the treatments. Microbial P content and phosphatase activity, which were determined in the fresh soil collected in April 2003, were higher for the combination of chemical fertilizers and compost than for the single application of compost or chemical fertilizers, and showed a strong correlation with the amount of plant available P. In conclusion, the combined application of chemical fertilizers and compost could be an effective method to increase the plant availability of P in soils by promoting microbial activity.