| Research Abstract |
The microbial biomass (biomass) plays a very important role in soil fertility both as a source and sink for mineral nutrients, such as nitrogen (N), phosphorus (P) and sulfur (S). The nutrients held in the biomass may become available to plants during biomass turnover. There have been very few studies on the role of roots exudates in biomass formation or the role of nutrients held in the biomass for plant nutrition. Soil incubation experiments and soil-plant roots system experiments were conducted 1) to investigate the utilization of root-derived ^<14>C-labelled compound by biomass, 2) to estimate nutrient requirements (N, P, S) of biomass formation and the utilization of nutrients held in the biomass by plant-roots, and 3) to study the effects of biomass formation on the movement of P in soil.
1. About 0.5% of ^<14>C-photosynthate in plants was incorporated into the biomass. This shows that ^<14>C-organic compound released by the roots is utilized by the biomass as C-substrate for biosynthesis and energy supply (Supple. of Soil Sce ＆ Plant Nutri., 46, 32,20000).
2. The critical N, P and S concentration in biomass (defined as that required to achieve 80% of the maximum synthesis of biomass C) was estimated to be 85, 62 and 11mg g^<-1> biomass respectively. It was clarified by using ^<15>N tracer that N held in the biomass could be utilized by plants under the N concentration above 20mgN kg^<-1> soil. Turnover time of biomass P was more than twice as fast as that of biomass C and it was estimated to be about 37 days for glucose and 42 days for ryegrass (Soil Biol. ＆ Biochem., 32, 845-852, 2000, Biol. Fertil. Soils, 32, 310-317, 2000, Soil Biol. ＆ Biochem (in press))
3. The distribution of P in soil-plant roots system was examined by the improved rhizobox system composed of several compartments. It seemed that the movement of P in soil was stimulated by the biomass formation.
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