MEASUREMENTS OF MICROBIAL BIOMASS C AND N IN PADDY SOILS BY THE FUMIGATION-EXTRACTION METHOD

Twelve kinds of paddy soils with different applications of fertilizer or organic matter in three experimental fields (Gley soil, Gray Lowlan d soil and Brown Lowland soil) and two farmers' fields (Gray Lowland s oil and Wet Andosol) were collected before flooding. Measurements of t he contents of carbon (C) and nitrogen (N) in the soil microbial bioma ss were carried out by the chloroform fumigation-extraction method, as compared with the adenosine triphosphate (ATP) method. Values of E(C) , K2SO4-extractable organic C after fumigation determined by dichromat e digestion, ranged from 84 to 662 mg C kg(-1) dry soil, corresponding to 0.5-2.1% of soil organic C. E(C) values in Gley soils were lower t han those in Gray Lowland soils and Brown Lowland soils. E(C) values m easured by automated combustion oxidation showed a mean of 18.1% which was larger than that obtained by dichromate digestion. Values of E(N) , K2SO4-extractable total N after fumigation, ranged from 13 to 40 mg N kg(-1) dry soil, corresponding to 0.7-2.0% of total N, and did not s how appreciable differences among the soil types. There was a close li near relationship between the ATP content and E(C) over the whole soil range. However there were different linear relationships between ATP and E(N) in three types of soil. The E(C)/E(N) ratios in Gley soils we re smaller than in other well-drained soils. By continuous application of organic matter, the values of E(C) and E(N) were 1.4-2.3 and 1.7-2 .8 times higher than those in the plot without organic matter, respect ively. These findings indicate that the chloroform fumigation-extracti on method is suitable for estimating the changes of microbial biomass C, N, and C/N ratio affected by the soil type and organic matter appli cation in paddy soil before flooding.