| Forests hold a large amount of soil organic carbon(SOC),soil in forest has an important impact on the global carbon(C)balance.Different forest management stratagems provide feasible ways to improve the ecological function of artificial forests.To understand the changes of forest soil C storage and its potential driving factors is important for dealing with climate change.Previous studies have indicated that different management stratagems can change the tree species composition,site conditions and have influence on many environmental factors in the forest.The environmental factors then effect on the soil active organic C and nitrogen(N)components and finally change the storage and stability of soil C.However,this process involves a number of environmental variables and active organic C components.And different forest management stratagems have different impacts on the soil active C pool.How forest soil C components respond to forest management stratagems remains to be further studied.Forest thinning management practices such as thinning play a major role in the process of SOC sequestration,however,the mechanism for SOC variations is still unclear.The objectives of this study were to estimate SOC stock and its active chemical components following a short term density adjustment of stand woods.Three treatments,low intensity thinning(removal of 15%of the trees,three 25×25 m repeated plots),moderate thinning(35%removal),and heavy intensive thinning(50%removal)were compared to control plots with no thinning three years before sampling of soil C composition.A number of C components(i.e.SOC;soil total N,STN;permanganate oxidizable C,POXC;dissolved organic C,DOC;microbial biomass C,MBC)were measured in five soil layers within the Larix principis-rupprechtii plantation throughout the growing seasons of 2015 and 2016.Results indicated both SOC content and its active component,POXC content,were maximized under the moderate density adjustment(p<0.01).Results indicated that STN was affected by tree stem density adjustments in short-term,STN generally increased with decreasing tree stem density,reaching its highest concentration in the MT treatment before decreasing in HT;this pattern was echoed by DON/STN(DON,dissolve organic N),under MT,a lower DON/STN was measured across the seasons;and MBN(microbial biomass N)and the SOC/STN ratios,density treatments had an influence on MBN concentration and inhibited SOC/STN.Density adjustment significantly affected POXC/SOC ratio(p<0.001),with maximum POXC content per unit SOC content also under moderate density adjustment.However,density adjustment hardly had any impact on DOC or MBC,both of which showed more obvious seasonal dynamics than SOC,TN and POXC.summer affected SOC storage by altering the POXC,TN and MBC.Our results suggest that effects of density adjustment on POXC drive variation in SOC.MT tended to accumulate more STN and produce lower DON/STN and generally higher microbial activity,which may be partly ascribed to the higher MBN value,MBN/STN ratio and lower DON/STN;and the water condition(water content,surface runoff and sediment loads)and light and soil temperatures may partly be responsible to the N pool dynamic in the different density treatments.Mixed and pure forests are the usual choices for industrial afforestation,but how these plantations affect the global soil C pool is yet to be understood.We hypothesize that physical,chemical and biological environmental factors together contribute to the C capacity of the forest and that the soil C processes reflected by the dynamics of active C pools may be responsible for differences in C stock between pure and mixed forests.Three 46-year old pure 25mx25m Pinus tabulaeformis forests(PF)and three 47-year old mixed 25m×25m coniferous and broadleaf forests(MF)were selected and sampled in August of 2016.In 2017,the 10cm soil temperatures were measured ever 30 min across the entire vegetation season(from April to October).The important components of the C pool,including the SOC(both soil organic C density and content),STN,DOC,MBC,soil moisture and pH,were also measured across 0-50 cm(five soil layers)at five times across the vegetation growing seasons;150 independent samples were measured for each soil index.Further,all the soils collected in 2017 were cultured for 56 days at 25? to monitor the C loss through CO2 release.The results indicated that the environmental factors of soil temperature,pH and litter thickness were differed in the two type of forest,and finally effected on active C pools.SOC stock was enhanced significantly(p<0.001)20%more C stock in the MF than in the PF.This may be because C was lost through mineralization C in PF and C originated from ground litter.Overall,the interacting environmental factors and C components contributed to the effects of each management type on the soil C pool.Higher soil temperatures were measured across the vegetation seasons in MF,22%higher daily temperature ranges were also observed in the MF plots.There,environmental factors act in conjunction with active C pools(DOC and MC),driving the SOC stock dynamic and finally resulted in better C accumulation in MF forests.Key message A coniferous and broadleaf mixed forest is better able to store C than a Pinus tabulaeformis plantation.The soil temperatures and the active C pool dynamics,especially soil mineralization,play important roles in the C dynamic here.We compared 40-year-old(1GF)and 24-year-old(2GF)plantations of Larix principis-rupprechtii in North China.Specifically,environmental factors(e.g.,soil temperature,moisture,and pH),the active C pools(e.g.,soil organic C,soil total N,dissolved organic C,microbial biomass C),and soil processes(e.g.,C mineralization and microbial activity in different seasons)in five soil layers(0-50 cm,10cm for each soil layer)across the growing seasons in three 25 m x 25 m plots in each age class(1GF and 2GF)were measured.Findings indicated that the soil organic C pool in the 1GF forest(12.43 kg C m-2)was significantly(p<0.05)higher than 2GF forests(9.56 kg C m-2),and that soil temperature in 1GF forests was 9.8?,on average,2.9%(p<0.001)higher than temperature in 2GF forests.The C lost as C dioxide(CO2)as a result of mineralization in the 2GF plots may explain some of the lower soil organic C pool in these younger forests;microorganisms are likely driving this process.Older 1GF Larix forests accumulated more C and had warmer soil temperatures,lower pH than did younger 2GF forests.A likely explanation for this difference is that the younger 2GF forests tended to lose more C through mineralization via CO2.In particular,aspect is an important environmental factor due to variation in insolation.However,the specific driving mechanism affecting the soil C pool is still unclear for many environmental variables,given the complexity of soil C processes involved.We compared an east aspect direction forest plantation(EDF)to a west aspect direction forest plantation(WDF)of Pinus tabulaeformis,sampling each aspect in three 25×25m permanent plots.Site condition information was collected in 2016,and environmental factors,elevation,dominant tree species,and forest age were controlled for.We monitored C and N stocks(kg m-2)throughout 2017.Our results indicated that the east aspect soils were better for C and N retention than west aspect soils.The 10cm soil temperatures measured every 30 min across the whole growing season indicated soil temperatures were slightly elevated in EDF plots,while diurnal temperature variation(DTV)was significantly higher in WDF.Additionally,measurements of soil C components in five layers from 0-10cm to 40-50cm across the seasons suggested that the lower SOC stocks in WDF were driven by the higher DTV compared to EDF,higher DTV correlated with higher microbial biomass C content and lower DOC,microorganisms in WDF consume more DOC and produce more cumulative mineralization C(releasing CO2),causing the loss of C in WDF.Thus,east aspect soils are better for both C and N sequestration than west aspects,suggesting that different management strategies may be appropriate. |
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