Research On Soil Nitrogen Mineralization In Eucalyptus Plantation Of Different Generations

In this research, the soil nitrogen availability and mineralization, soil physical and chemical properties, understory vegetation and their correlation in Eucalyptus plantation of different generations (the first, second and third generation) were studied by using close-top tube incubation method in Dongmen Forest Farm of Guangxi province. The purpose of the research was to reveal the variation pattern and influence factors in Eucalyptus plantation of different generations, to provide the scientific basis for sustainable management of Eucalyptus plantation and to provide the basic data for nitrogen cycle mechanism of plantation. The main results were as follows:(1) The soil NH4+-N contents in Eucalyptus plantation of different generations were all in the order of Spring>Summer or Autumn>Winter. The soil NO3--N contents were all in the order of Summer>Autumn>Spring or Winter. The soil available nitrogen contents were all in the order of Spring>Summer>Autumn>Winter. The soil NH4+-N, NO3--N and available nitrogen contents in generation Ⅰ were all higher than those in generation Ⅱ and generation Ⅲ(except the soil NO3--N contents in Summer of generation!).The NH4+-N contents was the main existing form of soil available nitrogen, which respectively accounted for78.83%～87.22%in generation Ⅰ,73.28%～90.20%in generation Ⅱ and72.05%～88.00%in generation.(2) The soil net NH4+-contents were all highest Ⅲin Spring or Winter and lowest in Summer. The soil net NO3--N contents were all highest in Summer and lowest in Spring or Autumn. The soil net NH4+-N+NO3--N contents were all highest in Spring or Winter and lowest in Summer or Autumn. The annual net NH4+-N, net NO3--N and net NH4+-N+NO3--N contents of Eucalyptus plantation were all in the order of generation Ⅰ> generation Ⅱ> generation Ⅲ.(3) The soil natural moisture contents in Eucalyptus plantation were all highest in Summer or Autumn and lowest in Winter. The soil pH were both in the order of Summer>Spring>Winter>Autumn in generation Ⅰ and generation Ⅱ and in the order of Winter>Summer>Spring>Autumn in generation Ⅲ. The organic matter contents were in the order of Winter>Summer>Autumn>Spring in generation Ⅰ and both in the order of Summer>Autumn>Spring>Winter in generation Ⅱand generation Ⅲ. The total nitrogen contents were all highest in Winter and lowest in Spring. The C/N ratio was in the order of Autumn>Spring>Summer>Winter in generation Ⅰ and were both in the order of Spring>Summer>Autumn>Winter in generation Ⅱand generation Ⅲ. Most of the soil physical and chenmical properties in generation Ⅰ were higher than those in generation Ⅱ and generation Ⅲ.(4) The positive correlation was extremely significant between soil net NO3--N contents and soil natural moisture contents, and was significant between soil net NH4+-N+NO3--N contents and soil natural moisture contents. The positive correlations were all extremely significant between soil NO3--N contents, soil net NO3--N contents, soil net NH4+-N+NO3--N contents and soil pH, and was significant between soil available nitrogen contents and soil pH. The positive correlation was significant between soil net NO3--N contents and soil organic matter contents. The negative correlation was significant between soil net NH4--N contents and soil total nitrogen contents. The positive correlation was extremely significant between soil net NO3--N contents and soil total nitrogen contents. The positive correlations were both extremely significant between soil net NH4+-N contents, soil net NH4+-N+NO3--N contents and soil C/N ratio. The negative correlation was significant between soil net NH4+-N contents and soil C/N ratio.(5) The population density and cover of understory shrub layer in Eucalyptus plantation of different generations were both in the order of generation Ⅰ>generation Ⅱ> generation Ⅲ. The aboveground biomass was in the order of generation Ⅰ>generation Ⅲ> generation Ⅱ. The population density and cover of understory herb layer were both in the order of generation Ⅰ <generation Ⅱ<generation Ⅲ. The aboveground biomass was in the order of generation Ⅰ>generation Ⅲ> generation Ⅱ.(6)There were significant or extremely significant positive correlations between soil nitrogen mineralization of Eucalyptus plantation and population density and cover of understory shrub layer, but not between nitrogen mineralization and aboveground biomass. There were significant or extremely significant positive correlations between soil nitrogen mineralization and population density and cover of understory herb layer, but not between nitrogen mineralization and aboveground biomass.