Abstract: Responses of microbial biomass P (MB-P) to organic C and P in paddy soils with various fertilization practices (inorganic and organic fertilizers) were investigated using 5 long-term field trials in Hunan, China. Results indicated that the amount of soil MB-P and their responses to the applications of different fertilizers differed with soil types. Changes in soil MB-P was insensitive to inorganic fertilizers (NPK) applied in most of the trials. However, the amount of soil MB-P generally increased by application of organic manure and incorporation of rice straw. The relation between MB-P and total P or total organic carbon was indeterminate, although the content of MB-P, to some extent, could respond to the changes of total P or total organic carbon. Meanwhile, the MB-P was more closely related to NaHCO3 extractable P (Olsen-P) or MB-C than to total P or total organic carbon. MB-P amounted to 1.16%-3.19% of total P in the paddy soils, which was similar to the percentage of Olsen-P to total P. When the content of extractable P was less than 8 mg·kg-1, the content of MB-P was more than extractable P no matter where in fertilization or in non-fertilization plots, suggesting that a high content of microbial biomass P could regulate extractable P in paddy soil. The positive correlations between the amount of MB-P and various fractions of inorganic P except O-P indicated it was a possible key that the conversion of Fe-P, Al-P and Ca-P into extractable P was accomplished by soil microorganisms in paddy soils.

Key words: Paddy soil, Microbial biomass P, Organic carbon, Phosphorus