EFFECTS OF ORGANIC-MATTER APPLICATION ON MICROBIAL BIOMASS AND AVAILABLE NUTRIENTS IN VARIOUS TYPES OF PADDY SOILS

Twenty three kinds of paddy soils with different applications of ferti lizer or organic matter in 6 experimental fields (Gley soil, Gray Lowl and soil, and Brown Lowland soil) were collected before flooding. Chan ges in the contents of microbial biomass carbon (C) and nitrogen (N) m easured by the chloroform fumigation-extraction method and adenosine t riphosphate (ATP) content in soils were investigated with reference to soil properties, especially available nitrogen and phosphate. Values of E(C), extractable soil organic C after fumigation determined by aut omated combustion oxidation, were affected by the soil types and manur ing practices, ranging from 92 to 545 mg C kg(-1). In contrast, the va lues of E(N), extractable total N after fumigation, were markedly affe cted by manuring practices, ranging from 7.8 to 67.7 mg N kg(-1). The E(C)/E(N) ratios also varied with the soil types and manuring practice s (range 5-19), and decreased to 5-11 by submerged incubation, suggest ing the occurrence of changes in the microbial flora. On the basis of the close linear relationship between E(C) and ATP over the whole soil range, the values of biomass C (B-C) and N (B-N) were estimated to co rrespond to 1.1-5.7 and 1.6-6.4% of soil organic matter, respectively. Available phosphate measured by the Truog-method was closely related to B-C, B-N, and ATP. Mineralizable organic nitrogen of moist soils me asured by submerged incubation was also closely related to B-N, and a rise in the incubation temperature from 30 to 40 degrees C increased t he correlation coefficients. In contrast, the increase of the content of mineralizable N by air-drying pretreatment was affected by the soil types and clay contents of soil, and was not closely related to B-N. These findings suggest that the microbial biomass-is affected by the s oil types and manuring practices, and can be a suitable index for esti mating the changes in the contents of soil organic matter and availabl e nutrients in paddy soils.