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 Lowland soil and Brown Lowland soil) and two farmers' fields (Gray Lowland soil and Wet Andosol) were collected before flooding. Measurements of the contents of carbon (C) and nitrogen (N) in the soil microbial biomass 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 dichromate 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 than those in Gray Lowland soils and Brown Lowland soils. E(C) values measured 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 show appreciable differences among the soil types. There was a close linear 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 were 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, respectively. These findings indicate that the chloroform fumigation-extraction method is suitable for estimating the changes of microbial biomass C, N, and C/N ratio affected by the soil type and organic matter application in paddy soil before flooding.