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 fertilizer or organic matter in 6 experimental fields (Gley soil, Gray Lowland soil, and Brown Lowland soil) were collected before flooding. Changes in the contents of microbial biomass carbon (C) and nitrogen (N) measured by the chloroform fumigation-extraction method and adenosine triphosphate (ATP) content in soils were investigated with reference to soil properties, especially available nitrogen and phosphate. Values of Ec, extractable soil organic C after fumigation determined by automated combustion oxidation, were affected by the soil types and manuring practices, ranging from 92 to 545 mg C kg(-1). In contrast, the values of En, extractable total N after fumigation, were markedly affected by manuring practices, ranging from 7.8 to 67.7 mg N kg(-1). The Ec/En ratios also varied with the soil types and manuring practices (range 5-19), and decreased to 5-11 by submerged incubation, suggesting the occurrence of changes in the microbial flora. On the basis of the close linear relationship between Ec and ATP over the whole soil range, the values of biomass C (Bc) and N (Bn) were estimated to correspond 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 Bc, Bn, and ATP. Mineralizable organic nitrogen of moist soils measured by submerged incubation was also closely related to Bn, and a rise in the incubation temperature from 30 to 40 C increased the 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 Bn. These findings suggest that the microbial biomass is affected by the soil types and manuring practices, and can be a suitable index for estimating the changes in the contents of soil organic matter and available nutrients in paddy soils