Nitrogen Transform and Release in Typical Temperate Forest Ecosystems in Northeastern China
Temperate forest has been playing a key role in the global forest ecosystems,which is one of the most sensitive to the global climate warming.It is necessary to investigate nitrogen turnover and controlling factors for a better understanding of the structure and function of the temperate forest ecosystem.Additionally,these datas could be provided for evaluating the impacts by the climate warming.The research was based on 4 typical temperate forest ecosystems:Korean pine(Pinus koraiensis,HS) and Dahurian larch(Larix gmelinii,LYS) plantation,Mongolian oak forest(Dominated by Quercus Mongolica,MGL) and Hard-wood forest(Dominated by Fraxinus mandshurica,Juglans mandshurica and Phellodendron amurense,YK),which represented secondary forest ecosystems in northeast China.N immobilization and release in different litter compositons(leaf litters,twigs,mixed undecomposition leaf litters and mixed semi-decompositon leaf litters),N immobilization and release in soil microbial biomass,nitrification and N mineralization in soil and N_2O emissions were studied in the temperate forest ecosystems.We selected the Dahurian larch forest ecosystems transplanted from a latitudinal transect in northeast China and measured N_2O fluxes during spring thawing period.The decompositoin of litters was apparently affected by tree species.The decomposition of broad-leaved forest litters was faster than that of conifer forest litters.The decomposition rates of leaves were higher than that of twigs.The decomposition rate of litter was negatively correlated to C/N of litter.The total nitrogen content of leaves,twigs and mixed undecomposed leaves increased with the decomposition of these litters at primary stage of decomposition.The total N increased significantly in primary stage of decomposition.The release of litter N was negatively correlated to initial C/N of litter.And N release from the leaves of Fraxinus mandshurica and Juglans mandshurica with initial low C/N was increased in the primary decomposition stage.Temporal and spatial variability was found in soil microbial biomass nitrogen(N_M) in 4 forest ecosystems.Soil nitrogen was immobilized significantly by soil microbial in later-spring and pre-summer and was released in autumn.The soil N_M showed clearly spatial variability and decreased with soil depth.The soil microbial biomass nitrogens were significantly different among 4 forest soils and broad-leaved forests with microbial higher biomass nitrogen than conifer forests.The change of soil microbial biomass nitrogen at 0-10 cm depth(0-5 cm for MGL) was accord with soil water content and the peak of N_M appeared at the time of the highest soil water during soil thawing periods in spring.The percentages of N_M to total N in YK,MGL,HS and LYS during the growth season were 2.3%,2.4%,2.1%and 1.1%at 0-10 cm (0-5 cm for MGL) depth,respectively.The N_M was positively correlated to soil moisture content,but not to soil temperature.The inorganic nitrogen showed similar seasonal characteristics in four temperate forest ecosystems.Apparent temporal-spatial pattern was observed in nitrification and N mineralization,and the highest net N mineralization and net nitrification rates were found in May-July.Net nitrification and net N mineralization rates decreased with soil depth.Net nitrification and net N mineralization rates at 0-10 cm depth were significantly different among 4 forest ecosystems,and the rank of mean net N mineralization rate was YK(0.60±0.01 mg.kg~-~1.d~(-1))＞HS(0.38±0.03 mg.kg~(-1).d~(-1))＞MGL(0.35;±0.04 mg.kg~(-1).d~(-1))＞LYS(0.24±0.03 mg.kg~(-1).d~(-1)). There were scarcely different in net nitrification and net N mineralization rates among 4 forest soils at 10-20 cm(5-15 cm for MGL).The constitutes of forest ecosystem determined the level of N mineralization.The net N mineralization rates of the broad-leaved forest and pine mixed deciduous forest ecosystem were higher than that of conifer forest ecosystems.There existed clear N mineralizations during spring soil thawing period in 4 forest ecosystems.N mineralizations were positively correlated to soil temperature and N_M,and there was quadratic equation relationship between N mineralization and soil water.Four temperate forest soils were N_2O sources and released 0.07-0.93 kgN_2O-N.hm~(-2) during the growth season.The temperate forest would absorb N_2O from the environment as a ephemeral sinks of N_2O during dry period in the growth season.The differences of N_2O flux in 4 forest ecosystems were not significant.The N_2O flux were significantly positively correlated to soil water content,NH_4~+-N content and N_M.The nitrification is a leading process for N_2O emission during the growth season in the 4 forest ecosystems.A quantity of N_2O was released during soil thawing periods in spring in temperate forest ecosystems(1.49-2.94 kgN_2O-N.hm~(-2).29d~(-1)). The soil temperature and microbial activity were the key factors in controlling N_O2 emissions during soil thawing periods.The results showed that N release of litters was affected significantly by initial C/N,and N immobilization increased with higher initial C/N in the temperate forest.Availibilities of microbial biomass N in broad-leaved forest soils were higher than that of the conifer forest soils.4 forest soils were N_O2 sources in growth and non-growth season obviously.The sensitivity of N_2O release for the four latitudinal Dahurian larch forest soils decreased with higher latitude during the thawing period in spring.