Turnover Rate of Soil Biomass Nitrogen in the Root Mat Layer of Pasture

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Abstract
A part of the soil biomass N was labelled with 15N by applying (15NH4)2SO4 and glucose to the surface soil of sub-plots (95 × 95 cm) in an experimental pasture which was composed of orchardgrass, tall fescue and several other grasses and maintained for 10 years as a pasture under definite management. The amount and turnover rate of soil biomass 15N were determined by using the fumigation-incubation method for 828 days to assess the role of the soil biomass as a source of available N in the root mat layer. It was found that the fumigation-incubation method could be applied for the measurement of the amount of soil biomass N in the root mat layer (0–2.5 cm in depth) in spite of the presence of roots during the fumigation-incubation procedure, but not for the measurement of this parameter in the deeper layers. Flush N (Fn) in the root mat layer, which represents the amount of mineralized N increased by fumigation, was expressed by formulas Bc = 8.8 Fn and Bn = 1.8 Fn, where Bc is the soil biomass C and Bn is the soil biomass N, both of which were calculated from the biovolume determined under the microscope. After the changes of the level of soil biomass associated with the application of the substrates were no longer present, the soil biomass 15N in the root mat layer decreased slowly. Its half-life was found to be 1.3 years, and the turnover time was assumed to be 1.9 year– 1. The average amount of Bn throughout the whole period was 40 kg N·ha- 1. Thus, a quantity of 21 kg N·ha– 1·year– 1 was considered to have been released through the soil biomass in the root mat layer. These results indicate that the soil biomass plays an important role as the source of available N by turnover at least in the root mat layer of grassland.