Soil Biomass and Turnover Rate of Its Nitrogen in Grassland

OKANO Seigo, NISHIO Michinori, SAWADA Yasuo

doi:10.18946/jssm.29.0_39

Distribution and seasonal changes of microbial biomass and the turnover rate of soil biomass N were investigated to assess the role of soil biomass as the source of available N in grassland ecosystems. 1) Microbial biomass ranged from 72 to 135g dry wt. m^<-2> 20cm^<-1> throughout a year. One third of the microbial biomass was present in the root mat layer(0-3cm in depth). 2) The fumigation-incubation method was suitable for measuring the amount of soil biomass N in the root mat layer(0-2.5cm in depth), but not in the deeper layers. The relationship between the soil biomass C and N and flush N (Fn) in the root mat layer, the amount of mineralized N generated by fumigation-incubation, was expressed by the equations Bc=8.8 Fn and Bn=1.8 Fn, where Bc and Bn are soil biomass C and N estimated from the biovolume determined under the microscope assuming that the C and N contents are 50 and 10% of dry wt., respectively. 3) The amount of ^<15>N in the soil biomass, which was labelled by applying (^<15>NH_4)_2SO_4 and glucose, decreased slowly in a pasture during a period covering 63 to 828 days. Its half-life was found to be 1.25 years, and its turnover rate was estimated to be 0.55 year^<-1>. The total soil biomass N in the standing crop averaged throughout the whole period was 40kg N ha^<-1>. Thus, 22kg N ha^<-1> yr^<-1> was considered to have been released through soil biomass in the root mat layer. This amount was equivalent to 20% of N, which was mineralized from soil and absorbed by herbage.

Soil Biomass and Turnover Rate of Its Nitrogen in Grassland

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