| Forests are the largest carbon pools in terrestrial ecosystems.The annual carbon sequestration of forests accounts for about two-thirds of the total carbon sequestration of terrestrial ecosystems,and of course forests are the largest and most stable carbon sink of terrestrial ecosystems.China’s total forest area is more than 1.8×10 8 hectares,which is the main carbon pool and sink in the national ecosystem.Both the carbon pools and sinks of forest ecosystems are very large,even small changes are enough to have a significant impact on global climate change.As a result,the process and mechanism of carbon cycle in forest ecosystems have attracted more and more attention nowadays when the impact of global change is becoming more and more obvious.How to increase the carbon sequestration capacity of forest ecosystem has become a very important issue in forestry and ecological engineering research.In the field of carbon sequestration in forest ecosystem,natural stable isotope method has the characteristics of saving manpower,less damage and higher accuracy compared with other methods.The experimental results can more truly reflect the objective laws and processes,and it is an effective method to solve some difficult problems of carbon sequestration in forest ecosystem.However,although some studies has been made in the natural stable isotope method recent years,there are still some problems for the application of carbon sequestration in forest ecosystems.The aims of this study is to solve some key problems of forest ecosystem carbon sequestration process(including soil carbon sequestration and forest biomass carbon sequestration)using natural stable isotope method.In this study,the autotrophic respiration and heterotrophic respiration driven by rhizosphere priming effect were evaluated by using the 13C natural abundance method in a Robinia pseudoacacia plantation.In order to investigate the response of soil respiration to soil temperature and moisture,we partitioned soil respiration into three components(heterotrophic respiration,root respiration and rhizomicrobial respiration)using 13C natural abundance during the growing season in a Robinia pseudoacacia plantation in north China.To evaluate and improve the ecological benefit of forestation programs,we collected soil samples from three plantations in northeast China and used 13C natural abundance to measure the amount of SOC sequestrated since 1998.The DNA combined with stable isotope method were used to estimate variations in microbial communities,newly sequestrated soil organic carbon,and δ15N driven by Pinus tabuliformis and Quercus variabilis roots.Acer catalpifolium Rehd were selected as the research object,and were carried out field investigation and sampling research.The results showed that priming effect existed in deep soil of the plantation.The proportions of autotrophic respiration and heterotrophic respiration deriving from priming effect to total soil respiration varied with soil depth.Rhizomicrobial respiration(RMR)accounted for about 15%of total soil respiration,and the rate of priming decomposition of soil organic matter(PSOM)was only about 5%of total soil respiration.RMR was significantly positively correlated with PSOM.Heterotrophic respiration derived by priming effect was too weak to have a positive impact on atmospheric CO2.Root respiration and soil microbial respiration had a significantly positive relationship with soil temperature.Heterotrophic respiration was positively correlated with soil moisture,while rhizomicrobial respiration significantly decreased with a reduction in soil moisture.Our findings suggest that the responses of plant roots and soil microorganisms to soil temperature and moisture were different.According to the prediction of the root-microbial model developed in this study,average soil respiration will increase by 12 mg C m-2 h-1 when soil temperature increases by 2℃ in the plantation.By modelling the relationship of a root-microbial system during the growing season in a plantation in north China,the temperature and moisture sensitivities of soil respiration can be characterized.Newly sequestrated SOC decreased with the increasing soil depth in the Larix gmelinii plantation,but increased with soil depth in the Populus trichocarpa plantation.In contrast,newly sequestrated SOC in the Armeniaca sibirica plantation peaked at a soil depth of 30-40 cm.With increasing distance from the tree trunk,average newly sequestrated SOC gradually decreased in the L.gmelinii plantation,did not vary in the P.trichocarpa plantation,and exhibited a gentle unimodal curve in the A.sibirica plantation.Overall,the average density of newly sequestrated SOC in the L.gmelinii,P.trichocarpa,and A.sibirica plantations at a soil depth of 0-50 cm was 0.60,0.70,and 0.61 g cm-3,respectively.Determination of δ15N and the C/N ratio provided evidence to support the hypothesis that newly sequestrated SOC was primarily regulated by the root-microbial system.Soil depth and tree species should be taken into account when estimating soil carbon sequestration in future studies.The results showed that microbial communities and newly sequestrated soil organic carbon contents changed with different tree species,environments,and successive stages.The fungal unweighted and weighted UniFrac distances could better show the different microbial species structures and differences in successive stages.Newly sequestrated soil organic carbon was positively correlated with the bacterial order Rhizobiales(in Pinus tabuliformis forests),the fungal order Russulales(in Pinus tabuliformis forests),and δ15N.Consequently,the bacterial order Rhizobiales acted as a keystone for Pinus tabuliformis root-driven carbon sequestration,and the fungal order Russulales acted as a keystone for Quercus variabilis root-driven carbon sequestration.The 815N of soil organic matter could be an important indicator to estimate root-driven carbon sequestration.Natural stable carbon isotope method can effectively reveal some undetectable rules of carbon sequestration during tree growth.According to the "two-pool model",the fractionation values between leaves and stems(ΔL-S),and between leaves and pods(ΔL-S)can reflect the utilization of young and old carbon pools by trees.The results showed that the fractionation values between leaves and stems(ΔL-S)mainly changed with the increase of tree volume,while the fractionation value between leaves and pods(ΔL-S)mainly changed with the increase of tree height.Combined with the results of field investigation,this study concludes that whether trees use photosynthates mainly for carbon sequestration depends on two aspects:on the one hand,whether individual biomass of trees accumulates to a certain extent;on the other hand,whether trees are in a relatively unfavorable environment.Moreover,tree height also affects the material allocation strategy of trees.Therefore,this study considers that unfavorable habitats for tree growth,especially those caused by human destruction,are harmful factors that restrict carbon sequestration of tree biomass.Therefore,the cultivation and management of forests should follow the different growth stages of trees in forestry engineering.This study shows that the natural stable isotope method is an important way for study carbon sequestration in forest ecosystem.Finally,this paper prospects this field of study.This study will provide methods,references and theoretical basis for future research. |
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