| Secondary forest succession is a low-cost,naturally effective strategy for increasing forest cover and biodiversity,as well as restoring soil nutrients and soil functions.Therefore,it is of great significancefor forest growth and carbon sink capacity to have a profound study of changes in soil carbon and nitrogen storage and flux during forest succession and their potential mechanism.As an important ecological functional area and sensitive area of China,the forest ecological services in the Daxing’an Mountains Forest region in the cold temperate zone plays an important role in the entire Northeast and even North China region.In the context of global change,further exploring the changes in forest soil carbon and nitrogen storage and flux during typical forest succession and their responses to relevant environmental factors in this region will help deepen our understanding of the carbon and nitrogen cycle and budget of the forest ecosystem in this region.In this study,we selected a typical forest succession process in the region,from the early stage of succession of the Betula platyphylla forest to the middle stage of succession of the mixed birch-larchforest and the late stage of succession of the Larix gmelinii forest.The storage of soil organic carbon and total nitrogen,the changes of soil greenhouse gas fluxes and their responses to changes in the amount of litter during the succession process were studied in suit,which can provide references for better playing the ecological functions of the forest ecosystem in this region in the future.Based on our study,the forest succession process is accurately quantified through changes in forest age during the succession stage.Thus,we use model method,combining with global data,to analysis the change trend of soil greenhouse gas flux in the future forest ecosystem,which provide a view to better understanding the important role of global forests in carbon sequestration in the future.The specific research results are as follows:(1)The storage of soil organic carbon in the three stages of succession is as follows:late stage of succession(130.50±20.72 Mg ha-1)>intermediate succession stage(97.60±19.38 Mg ha-1)>early stage of succession(77.69 ± 12.50 Mg ha-1),with significant differences in different stages of succession(P<0.05);At the stage of succession,the total nitrogen storage of soil in the late stage of succession(13.90 ±2.60 Mg ha-1)>the intermediate succession stage(8.68 ± 1.58 Mg ha-1)>the early stage of succession(7.46±1.29 Mg ha-1),and the total nitrogen storage of soil in the late stage of succession is significantly greater than that in the early and intermediate stages(P<0.05).More than 58%of soil organic carbon content is concentrated in 0-20 cm deep soil,while more than 50%of soil total nitrogen is stored in 0-40 cm deep soil.(2)The soil CO2,CH4 and N2O fluxes varied significantly in both seasonality and secondary forest succession.Duringthestudyperiod,soil CO2 flux showed obvious singlepeaked curves among three succession stages,while the monthly variation of soil CH4 and N2O fluxes varies at different stages of succession.The highest average soil CO2 flux occurred in the intermediate succession stage,444.72 ± 31.94 mg m-2 h-1,which was significantly higher than that in the late stage of succession(341.81 ± 19.55 mg m-2 h-1)and the early stage of succession(347.12±12.53 mg m-2 h-1).In the three stages,forest soil showed obvious CH4 sink,and the average CH4 uptake rate of soil decreased along the process of forest succession.It is shown as late stage of succession(-35.85±2.28 mg m-2 h-1)>intermediate succession stage(-51.13±6.71 mg m-2 h-1)>early stage of succession(-61.66 ± 4.72 mg m-2 h-1).The average N2O flux of soil is the smallest in the intermediate stage of secondary forest succession,17.59 ± 1.21 μa m-2 h-1,significantly lower than the early stage of succession(20.85±1.21 μg m-2 h-1)and late succession(20.71 ± 0.72 μg m-2 h-1).The seasonal variation of greenhouse gases is mainly affected by soil temperature and water content.The content of soil microbial biomass nitrogen(MBN)and nitrate nitrogen(NO3--N)also has a significant impact on the seasonal dynamics of soil N2O flux.In the process of secondary forest succession,the structural equation model shows that forest succession mainly affects the change of soil CO2 flux through affecting soil temperature and microbial biomass carbon(MBC);During the succession process,soil CH4 flux was mainly affected by soil water content and soil pH;However,soil temperature,NO3--N and MBN are the main factors affecting the change of soil N2O flux during the regulation of succession.(3)Litter doubling treatment enhanced the emissions of soil CO2 and N2O.while litter removal treatment reduced them.The average soil CO2 flux in the three succession stages increased significantly by 28.05%,23.96%,and 19.60%respectively under the treatment of litter doubling along the succession sequence;However,litter removal reduced the soil CO2 flux during forest succession by 13.99%,13.52%and 12.40%.During the succession,soil N2O flux increased by 36.43%,37.85%and 35.57%respectively under litter doubling treatment;it decreased by 30.88%,14.85%and 16.78%,respectively,in litter removal treatment.For soil CH4,litter doubling decreased soil CH4 uptake by 27.61%,26.61%,and 27.58%respectively;However,litter removal increased CH4 uptake by 14.76%,21.57%and 22.07%in the three stages of succession.The change of soil temperature and humidity caused by litter treatment has a significant impact on the seasonal change of greenhouse gas flux in forest soil.(4)During the spring freeze-thaw period,the average soil CO2 fluxamong three succession stagesranged from 52.87 ± 5.08 mg m-2 h-1 to 73.21 ± 3.86 mg m-2 h-1,showing a decreasing trend with succession.The average soil CH4 flux also showed a decreased trend with succession(-8.52±6.48～6.12 ± 1.37 μg m-2 h-1),while no significant difference was found between early and intermediate succession stages.The lowest soil N2O flux was 13.34 ±2.34 μg m-2 h-1 in intermediate succession stage,which was significantlylower than that in early(17.31 ± 1.96μg m-2 h-1)and late(18.66 ± 1.15 μg m-2 h-1)succession stages.During the autumn freeze-thaw period,the average soil CO2(108.36± 10.35 mg m-2 h-1～134.95±9.88 mg m-2 h-1)and CH4(-44.04 ± 9.41～-19.75 ± 3.66 μg m-2 h-1)fluxes werealso decreased with succession;The average soil N2O flux is 12.15 ± 0.71.g m-2 h-1 in the early succession stage,10.66±0.59μg m-2 h-1 in intermediate succession stage and 14.83±1.68 μg m-2 h-1 in in late succession stage.The comprehensive GWP of accumulated soil greenhouse gases during the freezing and thawing period is 1.48 ± 0.10 t CO2 Eq ha-1(early succession stage),1.31 ± 0.11 t CO2 Eq ha-1(intermediate succession stage)and 1.17±0.11 t CO2 Eq ha-1(late stage of succession).However,the total GWP of accumulated soil greenhouse gases in the autumn freeze-thaw period is significantly higher than that in the spring freeze-thaw period.(5)Through the analysis of variations in soil respiration with forest age in the global soil respiration database(SRDB).The model analysis found that the soil respiration in the forest ecosystem was significantly correlated with the forest age,biomass,theirinteraction,and atmospheric temperature.Soil respiration in the boreal forest,temperate forest and tropical and subtropical forest ecosystems showed an obvious increasing trend with the growth of forest age,especially in 0-100 years scale.And this increase is significantly higher in the tropical and subtropical forests than in the other two biomass.This change was significantly related to soil organic carbon content,total nitrogen content,leaf area index and root biomass during forest growth and development.The components of soil respiration(autotrophic respiration and heterotrophic respiration)also increased with the increase of soil respiration,but with the increase of forest age,the contribution rate of autotrophic respiration to soil respiration gradually increased,while the contribution rate of heterotrophic respiration to soil respiration gradually decreased.In summary,this study reveals the changes in soil carbon and nitrogen storage and flux during secondary forest succession,as well as the main regulatory factors.Considering the accumulation of soil carbon and nitrogen during the succession process,in the future climate change,secondary forest succession in the region can play a mitigating role in regional climate warming. |
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