Carbon Density And Storage Of Forest And Grassland Ecosystems In The Loess Plateau Region

With the implementation of ecological engineerings, such as ‘‘Conversion of Farmlandto Forest and Grassland Regeneration’’, natural forest protection and grazing exclusion,natural vegetation resources of the Loess Plateau region are drastically restored, and itsartificial vegetation have been developed quickly. Consequently, carbon sequestrationcapacities of those forest and grassland ecosystems have been improved. However, carbondensity and storage of different forest and grassland ecosystems in the Less Plateau region arepoorly understood. Therefore, we conducted this study to determine and contrast the carbondensity in biomass, soil and ecosystem of typical forest and grassland ecosystems by themethods of field investigation and laboratory analysis. Also, the carbon storage and spatialdistribution of forest ecosystems in Ningxia region and natural grassland ecosystems in theLoess Plateau region were studied. The main results are as follows:1. The ecosystem carbon densities of natural forest sites (Liupanshan Betula platyphyllaforest, Liupanshan Quercus wutaishanica forest, Guyuan Populus davidiana forest,Helanshan Pinus tabuliformis forest, Helanshan Picea crassifolia forest, Luoshan Piceacrassifolia mature forest, Luoshan Picea crassifolia mid-aged forest, Liupanshan Pinusarmandii+Tilia paucicostata forest and Luoshan Pinus tabuliformis+Populus davidianaforest) ranged between228.77Mg C·hm-2and444.77Mg C·hm-2, with the highest in LuoshanPinus tabulaeformis+Populus davidiana forest, while the lowest in Liupanshan Pinusarmandii+Tilia paucicostata forest. The mean ratio of soil carbon density to biomass carbondensity was4.09. Soil carbon densities of natural forest sites ranged between170.15MgC·hm-2and354.29Mg C·hm-2,with the highest and lowest in Luoshan Pinus tabulaeformis+Populus davidiana forest and in Helanshan Picea crassifolia forest, respectively. The meanproportion of50~100cm layer to the total soil carbon density was41.65%. The mean biomasscarbon density of natural forest sites was75.86Mg C·hm-2. Where the arbor layer carbondensity proportions reached66.00%~90.31%, fine root and litter layer carbon densityaccounted for13%, respectively. The proportions of understory shrub and grass wererelatively small.2. The ecosystem carbon densities of plantation sites (Guyuan Populus simonii mid-aged plantation, Wuzhong Populus simonii mature plantation, Wuzhong Ulmus pumila plantation,Pingliang Robinia pseudoacacia plantation、 Liupanshan Pinus tabuliformis plantation,Liupanshan Larix principis-rupprechtii plantation and Liupanshan Picea crassifolia plantation)ranged between132.85Mg C·hm-2and469.86Mg C·hm-2, with the highest in LiupanshanPicea crassifolia plantation, while the lowest in Wuzhong Populus simonii mature plantation.The mean ratio of soil carbon density to biomass carbon density was4.29. The soil carbondensities of plantations ranged between89.31Mg C·hm-2and419.54Mg C·hm-2. The order ofsoil carbon density was similar to that of ecosystem carbon density. For each plantation, themajor part of soil carbon density was at50~100cm layer. The biomass carbon densities ofplantation sites ranged between11.59Mg C·hm-2and155.47Mg C·hm-2, with the highest andlowest in Liupanshan Pinus tabulaeformis plantation and in Wuzhong Ulmus pumilaplantation, respectively. The arbor layer carbon density proportions to biomass density rangedbetween56.48%and93.52%.3. The ecosystem carbon densities of natural grassland sites (grazing grassland,grasslands with grazing exclusion for5years,9years,15years,22years,25years and30years) ranged between212.72Mg C·hm-2and350.42Mg C·hm-2, that of grazing grasslandwas significantly lower than that of each grazing exclusion grassland (P <0.05). In contrast,grassland with grazing exclusion for30years had the highest ecosystem carbon density. Theresults indicated that grazing exclusion could improve the carbon sequestration capacity ofnatural grassland. The soil carbon densities of natural grassland with different grazingexclusion ages ranged between204.90Mg C·hm-2and337.36Mg C·hm-2, and increased withincreasing of grazing exclusion ages. The biomass carbon densities of natural grassland sitesranged between7.82Mg C·hm-2and15.65Mg C·hm-2, with highest in grassland with grazingexclusion for15years, while the lowest in grazing grassland. The mean percentage of rootcarbon density to biomass carbon density was82.56%.4. The ecosystem carbon densities of alfalfa pastures under different topographicconditions ranged between193.26Mg C·hm-2and247.88Mg C·hm-2, with the highest inshady-top slope, while the lowest in sunny-middle slope. The mean ecosystem carbon densityof shady slope was significantly higher than that of sunny slope (P <0.05). The soil carbondensities of alfalfa pastures ranged between193.26Mg C·hm-2and244.09Mg C·hm-2. Themean soil carbon density of shady slope was1.20times of that of sunny slope. The Biomasscarbon densities of alfalfa pastures ranged between2.99Mg C·hm-2and5.03Mg C·hm-2, withthe highest in shady-bottom slope, followed by the shady-top slope. The root carbon density,which was2.57times of the aboveground carbon density, was the major part of biomasscarbon density in alfalfa pastures. 5. The differences in biomass carbon density, soil carbon density ecosystem carbondensity between Natural forests and plantations were insignificant (P>0.05), while naturalgrassland sites had significantly higher biomass carbon density, soil carbon density andecosystem carbon density than alfalfa pastures did (P <0.05). The results demonstrated thatcarbon sequestration capacities of natural forest and plantation ecosystems were similar, whilethat of natural grasslands were higher than that of alfalfa pastures. Comprehensivecomparison showed that forests had significantly higher biomass carbon density andecosystem carbon density than grasslands (P <0.05), however, the difference between the soilcarbon density of forests and that of grasslands was insignificant (t=-0.909, P=0.306).6. The order of ecosystem carbon density among different forest type in Ningxia regionwas: cold-temperate conifer forest> temperate conifer-broadleaf mixed forest> temperateconifer forest> deciduous broadleaf forest. Forest ecosystem carbon storage in Ningxiaregion estimated by methods of mean carbon density and area-volume were26.18Tg C and23.43Tg C (Year2000),42.35Tg C and41.06Tg C (Year2005),51.20Tg C and42.75Tg C(Year2010), resceptively, indicating a continuous increasing along the time series. The forestecosystems in southern part of Ningxia region stored more carbon than the northern part did.The total amount of carbon storage was small, and unevenly distributed.7. Carbon storage in biomass, soil and ecosystem of natural grassland in the LoessPlateau region was0.20Pg C,3.50Pg C and3.70Pg C, respectively. The spatial distributionof natural grassland ecosystem carbon densities was decreased from the southwest andsoutheast parts to the northwest part. Ecosystem carbon densities of different types of naturalgrassland ranged between138.17Mg C·hm-2and354.96Mg C·hm-2, following the order:alpine grassland> meadow grassland> typical grassland> desert grassland (P <0.05). Theproportions of biomass carbon density and soil carbon density were2.06%~6.23%and93.77%~97.94%.This study provides a reliable reference for the accurate estimation of carbon density offorest and grassland ecosystems in similar regions, contributes to the understanding of carbonbalance under global climatic changes. Also, this study offers basic data for the terrestrialecosystem carbon cycle of China, and gives theoretical support for the operation, managementof vegetation and sustainable and healthy development of ecological environment in thisregion.