Abstract
We investigated the contribution of N through microbial biomass (biomass N and its debris N) to plant N uptake in upland soils with different physicochemical properties. Our experiment was carried out in concrete plots using different 5 types of upland soils (Light-colored Andosol, Humic Andosol, Brown Lowland soil, Dark Red soil and Sand-dune Regosol). Nitrogen fifteen-enriched ammonium sulfate was amended to the field soil in order to label microbial biomass N with ^<15>N. At 3 weeks after amendment, upland rice was sowed and cultivated in 15 weeks. Soil and plant samples were taken at every 3 weeks after sowing (WAS), and the ^<15>N abundance of soil microbial biomass N, soil total N and plant N were analyzed. The results we obtained are summarized as follows. 1) The amount of microbial biomass N in the soils was higher in the following order : Dark Red soil>Humic Andosol≒Brown Lowland soil≒Light-colored Andosol>Sand-dune Regosol. The amount of microbial biomass N in all of the soils was almost constant during the cropping period. 2) The amount of plant N uptake was higher in the following order : Brown Lowland soil>Humic Andosol>Dark Red soil>Light-colored Andosol>Sand-dune Regosol. The amount of plant N uptake in all of the soils was highest in the heading stage, suggesting that the amount of plant N uptake is high in the reproductive stage. 3) At 0-3 WAS, the ^<15>N abundance of plant N uptake in all of the soils was close to that of the microbial biomass N. At 6-15 WAS, however, it was close to that of soil total N. Based on this result, we assume that the N taken up by the plant is mainly derived from the N produced through the microbial biomass in the vegetative growth stage (0-6 WAS), and mainly organic N except for the microbial biomass N in the reproductive stage (6-15 WAS). 4) In all the soils except Dark Red soil, the amount of plant N uptake was closely correlated with the amount of microbial biomass N at the vegetative growth stage (R : 0.995^<**>, n= 4), supporting our hypothesis that the N through the micobial biomass is a major source of N for plants at this stage.
