Biao Wang·Changcheng Mu·Huicui Lu·Na Li·Yan Zhang·Li Ma

Abstract Wetlands play an important role in the global carbon cycle,but there are still considerable uncertainties in the estimation of wetland carbon storage and a dispute on whether wetlands are carbon sources or carbon sinks.Xiaoxing’anling are one of several concentrated distribution areas of forested wetland in China,but the carbon storage and carbon sink/source of forested wetlands in this area is unclear.We measured the ecosystem carbon storage (vegetation and soil),annual net carbon sequestration of vegetation and annual carbon emissions of soil heterotrophic respiration of five typical forested wetland types(alder swamp,white birch swamp,larch swamp,larch fen,and larch bog) distributed along a moisture gradient in this area in order to reveal the spatial variations of their carbon storage and quantitatively evaluate their position as carbon sink or source according to the net carbon balance of the ecosystems.The results show that the larch fen had high carbon storage (448.8 t ha?1) (6.8% higher than the larch bog and 10.5–30.1% significantly higher than other three wetlands (P <0.05),the white birch swamp and larch bog were medium carbon storage ecosystems (406.3 and 420.1 t ha?1)(12.4–21.8% significantly higher than the other two types(P <0.0 5),while the alder swamp and larch swamp were low in carbon storage (345.0 and 361.5 t ha?1,respectively).The carbon pools of the five wetlands were dominated by their soil carbon pools (88.5–94.5%),and the vegetation carbon pool was secondary (5.5–11.5%).At the same time,their ecosystem net carbon balances were positive(0.1–0.6 t ha?1 a?1) because the annual net carbon sequestration of vegetation (4.0–4.5 t ha?1 a?1) were higher than the annual carbon emissions of soil heterotrophic respiration(CO2 and CH4) (3.8–4.4 t ha?1 a?1) in four wetlands,(the alder swamp being the exception),so all four were carbon sinks while only the alder swamp was a source of carbon emissions (? 2.1 t ha?1 a?1) due to a degraded tree layer.Our results demonstrate that these forested wetlands were generally carbon sinks in the Xiaoxing’anling,and there was obvious spatial variation in carbon storage of ecosystems along the moisture gradient.

Keywords Temperate forested wetlands·Ecosystem carbon storage·Carbon sink or source·Xiaoxing’anling of China

Introduction

Wetlands occupy only 4–8% of the global land surface,but they actually store about 14–30% of terrestrial soil carbon.This makes wetlands a major part of the global carbon budget (Whiting and Chanton 1993;Bridgham et al.2006;Mitsch and Gosselink 2007).Wetlands are unique ecosystems having a great influence on carbon dynamics and carbon cycling in nature (Zhang et al.2002).Wetlands sequester and store an estimated 535 Pg carbon,equivalent to nearly 72% of the total atmospheric carbon (Mitsch and Gosselink 2007),meanwhile,they are also considered as the largest natural sources of methane (CH4),emitting 20–26% of the global CH4into the atmosphere (Mitsch and Gosselink 2007;Bridgham et al.2013).Therefore,wetlands play an important role in global cycling of these atmospheric gases (Whiting and Chanton 2001;Kayranli et al.2010;Mitsch et al.2013).

At present,there are two controversial problems in the study of wetland carbon cycling.One is that there are still many uncertainties in the estimation of global wetland carbon storage (Gorham 1991;Bai et al.2005).For example,climatic changes and the anthropogenic disturbances have reduced the areas of wetlands (Gorham 1991;Noble et al.2000;Neher et al.2003),and about half of the world’s wetlands have been converted to other land uses (such as reclamation of farmland,drainage for afforestation) (Dugan 1993).This has led to a net carbon emission by global wetlands (Armentano and Menges 1986;Maltby and Immirzi 1993),and will inevitably affect the accuracy of total storage estimation.At the same time,the variability of carbon storage in different climate zones and by different types of wetlands is considerable (Roulet 2000;Bockheim et al.2003;Bai et al.2005;Alongi et al.2007).This will also affect an objective and accurate estimation of carbon storage in wetlands worldwide.The best way to resolve this problem could be to determine the carbon storage of various wetlands and their area of distribution.

The other problem is to determine whether wetlands are carbon sinks or carbon sources for atmosphere.Wetlands absorbing and fixing CO2from the atmosphere are considered to be carbon sinks (Krogh et al.2003;Brevik and Homburg 2004;Bedard-Haughn et al.2006;Euliss et al.2006;Alongi et al.2007),at the same time,wetlands also emit methane,approximately 1.45×1011kg CH4–C annually,accounting for 25% of global CH4emissions (Whalen 2005),and are regarded as sources of greenhouse gases(Carroll and Crill 1997;Whiting and Chanton 2001;Whalen 2005).In fact,it is difficult to determine whether wetlands are carbon sinks or sources due to a lack of simultaneous research on the processes of wetland CO2absorption and CH4emission.Therefore,some researchers suggest that foloded wetlands generally absorb CO2from the atmosphere,while at the same time emitting CH4into the atmosphere;the combination of these two processes may determine the total contribution of wetland ecosystems to greenhouse gases (Bedard-Haughn et al.2006).Fortunately,in recent years,a few studies have realized quantitatively the net carbon exchange of ecosystems between wetlands and the atmosphere,but there are still considerable differences in the conclusions.For example,some studies have shown that riparian marshes,freshwater marshes,and tropical forested wetlands are steady carbon sinks (Krauss et al.2016;Nag et al.2017;Zamora et al.2020),whereas others found oligohaline or brackish coastal marshes neither sink or source(Weston et al.2014),and from floodplain wetlands,a carbon source due to drying (Batson et al.2015).However,the C sink/source of temperate forested wetlands remains unclear.

China has about 6.3×107ha of wetland areas,of which 2.5×107ha are natural wetlands,ranking first in Asia and third in the world (Wang et al.2002).Therefore,the wetlands of China play an important role in the global carbon cycle and in greenhouse gas emissions.Northeast China is located in temperate and cold temperate zones,two major climatic zones of global wetlands.In China,this region is one of the main wetlands areas with 1.02×107ha natural wetlands (about 40.4% of the national natural wetland area).The most concentrated forested wetlands of China are found in the Changbai Mountain,the Daxing’anling,and the Xiaoxing’anling,with a total area of 0.45×107ha.The plains are dominated by grass marsh wetlands,with an area of 0.57×107ha (Li et al.2007).At present,research on carbon cycling of the northeast wetlands mainly has focused on greenhouse gas emissions of grassy wetlands of the Sanjiang Plain (Ding et al.2003;Zhang et al.2008) and mountain forested wetlands (Sun et al.2011;Cui et al.2017).However,there is still a lack of research on carbon stocks and carbon sources/sinks in forested wetlands.

The objectives of this study were to:(1) quantify ecosystem carbon storage (vegetation and soil) and reveal their distribution in five typical forested wetlands distributed along a water environment gradient in the northeast temperate zone;and,(2) simultaneously measure the annual net carbon sequestration of vegetation and annual net carbon emissions of soil heterotrophic respiration (CO2and CH4) of the wetlands,and quantitatively evaluate their carbon sink effect according to the net carbon balance of the ecosystem.This will provide vidence for the estimation of wetland carbon storage and understanding of the role of temperate forested wetlands as carbon sinks or carbon sources.

Methods and sampling

Study area

The research area is located at Yongqing Forest Farm(48° 07′ N,128° 38′ E) of the Youhao Forest Bureau near Yichun City in the middle Xiaoxing’anling forest region.This region is 260 and 500 m,a.s.l.with a temperate continental humid monsoon climate with considerable temperature fluctuation.Annual average temperature is about 0.4 °C,and the annual accumulated temperature is between 2000 °C and 2500 °C.The annual average precipitation is 630 mm,70% from rainfall from July to August and winter snowfalls.

The study area has all the typical temperate forested wetlands of Xiaoxing’anling distributed along the moisture gradient;namely,alder swamp (FS1) lies at the lower level of the transition zone from swamp to forest,white birch swamp(FS2) at the middle level,and larch-dominated wetlands at the upper level,where the larch swamp (FS3) is flat,the larch bog (FB) at a low-lying area prone to permanent flooding,and a larch fen (FF) occurs in between.Among them,FS1,FS2 and FS3 haveAlnus sibiricaFisch.ex Turcz.,Betula platyphyllaSuk.,andLarix gmelini(Rupr.) Rupr.as the dominant species,respectively,but they have a similar understory ofCarex schmidtiiMeinsh.,Calamagrostis angustifoliaKom.,andBetula ovalifolia(Rupr.) Tung,albeit with differences in cover.Wetland FB is a unique wetland which only occurs in the mountainous areas of northeast China.L.gmeliniis dominant in the tree layer,Ledum palustrevar.angustumE.A.Busch,andVaccinium uliginosumL.in the shrub layer,andsphagnumspp.dominant on the soil surface with a 90% cover.Wetland FF is considered as the transition between FS and FB,and the shrub and herb species of swamps and bogs are found in FF andL.gmeliniis also dominant in the tree layer.The soil surface is dominated by moss,primarilysphagnumspp.(Lang 1999).More data on environmental characteristics are given in Table 1.

Field sampling

In the five forested wetlands,in the absence of fire,draining,or other anthropogenic disturbances,15 plots with each plot(20 m×30 m) were established in total,each wetland had three replicates,and there was a 5 m buffer strip between each plot.

Diameter at breast height (DBH) of all trees within all plots were measured in April,the beginning of growing season,and in October,the end of growing season,in 2014.The biomass (dry weight) of each tree was estimated using allometric equations which had been established for the study area in 2007 and were applicable to all diameter classes(Table S1) (Zhou et al.2012).They were added together to obtain the tree layer biomass of each plot.The net primary production (NPP) of the canopies can also be calculated by biomass increment over the growing season.To measure the understory and litter biomass,six 5 m×5 m shrub subplots,six 1 m×1 m herbaceous plant quadrats per subplot,and nine 0.5 m×0.5 m litter quadrats were randomly harvested.Fine roots (<2 mm diameter) were sampled from five soil cores randomly collected to a depth of 50 cm using a soil auger.Shrub NPP was obtained by dividing the shrub biomass by its mean age (5 years) (Giese et al.2003;Mu et al.2013a),and the NPP of herbs and fine roots were assessed directly,based on the biomass.All vegetation biomass was measured in August when the biomass maximized during the growing season.At the same time,all vegetation component samples,(roots,stems,branches,leaves of trees and shrubs,roots and leaves of herbaceous plants,litter and fine root)were collected,dried to constant weight at 75 °C and stored at 4 °C until carbon analysis.

Table 1 Parameters of the study forested wetlands in Xiaoxing’anling

Soil samples were collected to a 50 cm depth,which corresponds to the shallowest depth of the active permafrost layer at 50.3 cm (Wang et al.2013).Five points per plot were systematically placed,and 100 cm3cores were taken from each soil layer per 10 cm depth at each point.Soil bulk density,the mass of dry soil per unit of volume,was obtained by drying samples to a constant weight at 105 °C.Subsequently,a second batch of samples (approximately 500 g each) was collected at each sampling point for carbon content analysis.Rocks and roots (>2 mm diameter) were removed and the samples air-dried and stored at 4 °C until analysis.

From April,2014 to May,2015,daily CO2and CH4fulxes were simultaneously measured in 10 day intervals using a static chamber and presented as the 10 day emission levels.The detailed chamber specification and sampling process refer to the previous study (Sun et al.2011).Shrubs,herbs,and tree roots were removed from the chambers to determine the heterotrophic soil respiration as accurately as possible.Gas samples were injected into pre-evacuated aluminum foil bags for storage and transport.During CO2and CH4sampling,environmental factors such as temperature,soil temperature and water table were simultaneously measured and recorded.

Laboratory analysis

The samples were pre-treated by drying,grinding,and sieving according to the standards of previous studies (Mu et al.2013a).The carbon contents of all samples were measured with a Multi N/C 3000 analyzer with a 1500 Solids Module (Analytik Jena AG,Germany).The biomass of trees,shrubs,herbs,and litter were multiplied by their corresponding concentrations of carbon to estimate carbon stored.Soil organic carbon storage (Ct,t ha?1) to a depth of 50 cm was calculated using the following equation:

where,SBDis the soil bulk density(gcm?3),Ccis the soil C concentration (g kg?1),andDsis the soil sampling depth(cm) (Guo and Gifford 2002).

The gas samples were analyzed within a week after collecting using a gas chromatograph (GC,Agilent HP5890DII) equipped with a flame iodization detector.The fluxes of CO2and CH4were calculated using a timeseries gradient of gas concentrations during the sampling.The flux measurements were rejected when the linear regression with anR2<0.90 occurred.Gas fluxes were further concerted into annual CO2–C and CH4–C emissions,ecosystem annual carbon emissions were obtained by adding up the values.The ecosystem net carbon balance can also be calculated by comparing annual carbon sequestration and annual carbon emissions.

Statistical analysis

SPSS 16.0 software was used for data analyses.Variance(ANOVA) and least-significant-difference tests were performed for all comparisons,withP<0.05 as the significance level.Excel software was used for date plotting.

Results

Vegetation carbon storage

There were significant differences in vegetation carbon storage among the five forest wetlands (Table 2),and varied from 18.9 to 47.8 t ha?1.The white birch swamp (FS2) and larch fen (FF) had the highest carbon storage (41.3–152.9%significantly higher than the other three wetlands).The larch swamp (FS3) and larch bog (FB) had medium carbon storage(63.2–74.9% significantly higher than in the alder swamp FS1).The FS1 had the lowest carbon storage.

According to further analysis,carbon storage of the canopies of FS2 and FF were 53.1–158.4% higher than that of FB and FS1 canopies (Table 3).The carbon storage of the shrub layers in FB and FF were 389.8–863.0% higher than in the other three wetlands.For the herb layer in FS2,the carbon storage was 71.4–361.5% higher than that of FS3,FF and FB,and the litter layer carbon storage of FS2 was 117.2–193.5% higher than those of FS1,FS3 and FB.Therefore,the carbon storage of the canopy,shrub,herb and litter layers in the white birch swamp (FS2) were high among thefive forest wetlands,while the carbon storage of the tree and shrub layers in the larch fen (FF) were relatively high.

Table 2 Vegetation carbon storage (t ha?1) of each component of five forested wetlands in Xiaoxing’anling

Table 3 Carbon storage (t ha?1)of each component of canopy layers of five forested wetlands in Xiaoxing’anling

Soil organic carbon storage

There were also significant differences in soil organic carbon (SOC) storage among the five wetlands (Table 4).SOC storage varied from 326.04 to 400.98 t ha?1.The larch fen(FF) and larch bog (FB) had higher SOC storage 18.5–23.0%higher than those of the alder swamp (FS1) and the larch swamp (FS3).

Further analysis shown that the SOC storage of FF were the highest in the 10–40 cm soil layers (9.0–48.7%higher than the other four wetlands),and SOC of FB were 4.2–36.3% higher than the wetlands except for FF.FB SOC was highest in the 40–50 cm soil layer (25.6–59.1% higher than the other wetlands).Therefore,FF had the highest SOC in the middle soil layer,while FB had the highest in the lower soil layer.

Total ecosystem carbon storage

There were significant differences in the carbon storage of the five wetland ecosystems,which varied between 345.0 and 448.8 t ha?1(Fig.1).The carbon storage of the larch fen (FF) was 10.5–30.1% higher than that of the other three wetlands except for the larch bog (FB),and that of FB and the white birch swamp (FS2) were 12.4–21.8% higher than those of the alder swamp (FS1) and the larch swamp (FS3).Therefore,these wetlands in Xiaoxing’anling could be divided into three types,namely,high carbon storage (FF),medium carbon storage (FB and FS2) and low carbon storage (FS1 and FS3).

Fig.1 Total ecosystem carbon storage for each wetland: a total carbon amount by ecosystem and component,b relative C apportion at each wetland;upper case letters indicate significant differences(P <0.05) among wetlands;error bars are standard deviation

In addition,the apportioned ratio of ecosystem carbon pools of the five wetlands were similar,i.e.,they all were dominated by the soil carbon pool (88.5–94.5%);the vegetation carbon pool was secondary at 5.5–11.5%.

Net primary production and annual net carbon sequestration

There were also significant differences in net primary production (NPP) and annual net carbon sequestration (ANCS)among the five wetlands (Table 5).The NPP ranged from 4.9 to 9.7 t ha?1a?1.The white birch swamp (FS2),larch swamp (FS3),larch fen (FF) and larch bog (FB) had higher NPP (79.7–101.2% signifciantly higher than that in the alder swamp FS1).The FS1 had lower NPP.Further analysis showed that the NPP of the trees and fine roots of FS2,FS3,FF and FB were 94.1–117.2% and 108.2–165.3% higher than that of FS1,NPP of the shrub layers of FF and FB were 400.0–891.7% higher than that of FS1,FS2,and FS3,and the NPP of the herb layer of FS2 was 52.7–389.7% higher the other four wetlands.

Table 4 Soil bulk density,SOC content,and SOC storage at different depths of five forested wetlands in Xiaoxing’anling

The annual net carbon sequestration (ANCS) ranged from 2.3 to 4.5 t ha?1a?1.The ANCS in FS2,FS3,FF and FB were 78.0–104.5% higher than in the alder swamp (FS1),but there were no significant differences in ANCS among the other wetlands.The ANCS of the trees and fine roots of these wetlands were 88.7–111.9% or 104.5–159.1% higher than that of the FS1.The annual net carbon sequestration of the shrub layers of FF and FB were 380.0–940.0% higher than FS1,FS2,and FS3.The ANCS of the herb layer of FS2 was 46.3–361.5% higher than the other four wetlands.

Annual soil carbon emissions

The annual mean flux of CO2emissions of soil hetero-trophic respiration and annual CO2–C emissions were 157.4–179.6mg m?2h?1and3.7–4.2 tha?1a?1respectively,(Table 6).There were no significant differences in both among the five wetlands.However,there were significant differences in the annual mean flux of CH4emissions from soil (up to 7.8 mg m?2h?1)and annual CH4– C emissions (up to 0.5tha?1a?1).Annual mean flux of CH4emissions of FS1 was significantly higher than that of the other wetlands by 3 to 775 times,while that of FS2 was 20 to 191 times higher than the other three wetlands,although it was not significant.Therefore,the alder swamp (FS1) was a major source of CH4,the white birch swamp (FS2) a medium source,and FS3,FF and FB were weak sources of CH4.

Table 5 Net primary production and annual net sequestration of each component of five forested wetlands

Table 6 Fluxes of CO2 and CH4 emissions and annual carbon emission in five forested wetlands in Xiaoxing’anling

Although there were no significant differences in annual carbon emissions (3.8–4.4 t ha?1a?1) of the five wetlands,and CO2– C emissions were dominant (88.4–100.0%),CH4–C emissions were secondary (up to 11.6%).However,CH4– C emissions of FS1 and FS2 account for a relatively high proportion of total carbon emissions (3.3–11.6%),while the proportion of CH4– C emissions of FS3,FF and FB to total carbon emissions were relatively low (up to 0.2%).

Carbon sink/source

There were significant differences in carbon sink and carbon source of the five wetlands.The net carbon balance of the five ecosystems s varied from?2.1 to 0.6 t ha?1a?1(Fig.2),of which only alder swamp (FS1) was a source of carbon emissions (?2.1 t ha?1a?1) because the annual net carbon sequestration by its vegetation was lower than the annual carbon emissions of its soil heterotrophic respiration.The net carbon balance of the other four ecosystems werepositive and they were carbon sinks (0.1–0.6 t ha?1a?1).There were no significant differences in amount of carbon sink among the other four ecosystems.Therefore,the forest wetlands along the moisture gradient acted as carbon sinks except for the alder swamp (FS1).

Fig.2 Net ecosystem carbon balance of five forested wetlands;upper case letters indicate significant differences (P <0.05) among wetlands;error bars are the standard deviation

Discussion

Vegetation carbon storage

There were significant differences in the vegetation carbon storage among forested wetlands,i.e.the larch fen (FF) and white birch swamp (FS2) had the highest carbon storage,the larch swamp (FS3) and larch bog (FB) medium,and the alder swamp (FS1) the lowest.The main reason was that the carbon storage of the canopies (which accounted for 83.2–90.7%).Furthermore,the reason for this was that the habitats of the wetlands distributed along the moisture gradient in the transition zone were different,i.e.,FS1,FS2,FS3 and FF were seasonal flood habitats,three (except for FS1)had fast growing tree species (mean DBH 12.2–12.4 cm at stand ages of 39 to 51 years) due to a lower water table(1.2–26.0,5.4–39.9,and 4.0–38.0 cm below the surface)in the middle and upper habitats of the transition zone,but the carbon storage of trees in FS3was slightly lower than in FS2 and FF due to the relatively low tree density(1007 trees·ha?1).While FS1 had the lowest carbon storage of the tree layer due to a young stand age (only 21 year),although alder is water-tolerant,fast-growing species at the lower habitats of the transition zone (water table 3.5–16.0 cm below the surface).FB was in a low-lying,permanent flooded habitat with a high water table (6.8–25.0 cm below the surface).Larch is a relatively slow-growing species (mean DBH 9.4 cm at stand ages of 39 years) (Table 1),and the carbon storage of the tree layer was lower.In addition,the carbon storage of the herb and litter layers in FS2 were relatively high,and that of the shrub and litter layers in FF were also relatively high,further increasing the amount of carbon stored by FS2 and FF vegetation (Table 2).

Further comparison with other research showed that the vegetation carbon storage (19–48 t ha?1) of these five wetlands was only 31.7–80.0% of the lower limit of carbon storage by temperate conifer and deciduous broad-leaved mixed forests (60–140 t ha?1) (Ni 2001),and 29.7–120.0% of the lower limit of temperate and northern upland forests(40–64 t ha?1) (Blais et al.2005).

Soil organic carbon storage

There were also significant differences in SOC storage among the wetlands.FF and FB had the highest,FS2 medium,and FS1 and FS3 the lowest.The differences were due to their different habitats in the transition zone from swamp to forest.Because FB lay at the permanent flood zone,peat had formed in the upper 50 cm soil layers(Bhatti et al.2006),and its soil carbon content was highest(Table 4) among the wetlands.Because the FF was at the transition between flat and low-lying,the degree of flooding was relatively mild (low water table).Peat formed in the upper 20-cm soil layer and soil carbon content was high.The soil bulk density in the 20–50 cm soil layers was the largest due to eluviation (Sollins and Gregg 2017) and its soil SOC storage was also high.While FS1,FS2 and FS3 lay at the lower,middle and upper habitats of the transition zone,respectively,the degree of flooding gradually became less,the thickness of peat layers became thinner,and soil carbon content also decreased.Overall,the SOC storage of the three types were very similar and low.

In comparison to other studies,the SOC storage(326–401 t ha?1) of these wetlands were 1.4 to 2.8 times higher than that in temperate conifer and deciduous broadleaved mixed forests (105–135 t ha?1) (Ni 2001) and 2.0 to 2.7 times higher than in natural forests of China (109 t ha?1)(Liu et al.2011),indicating the greater soil carbon storage of temperate forested wetlands.

Total ecosystem carbon storage

There were significant differences in ecosystem carbon storage of the five wetlands,namely,FF,the larch fen,had high carbon storage,FB,the larch bog,and FS2,the white birch swamp,had medium carbon storage,and FS1,the alder swamp,and FS3,the larch swamp,had low carbon storage.This was due to the significant differences in the SOC storage (accounting for 88.5–94.5% of the ecosystem carbon storage).FF had the highest SOC storage and vegetation carbon storage.FB had a high SOC storage and a medium vegetation carbon storage,and FS2 had a medium SOC storage and a high vegetation carbon storage,making both medium carbon storage types.FS1 and FS3 had low soil carbon storage and low or medium vegetation carbon storage (Fig.1),making both low carbon storage ecosystems.

Further comparison analysis with other research showed that the ecosystem carbon storage (345–420 t ha?1) of these five wetlands was 16.9–172.0% higher than that of temperate conifer and deciduous broad-leaved mixed forests(165–295 t ha?1) (Ni 2001).However,they were close to the lower range of ecosystem carbon storage in the north peatlands (390–1340 t ha?1) (Blais et al.2005);carbon storage of FF,FB and FS2 were 4.2–15.1% above their lower value,while FS1 and FS3 were 73.–11.5% below their lowest value.

Total net primary production and annual net carbon sequestration

Vegetation NPP and ANCS showed a remarkable difference among the wetlands,namely,in four wetlands (FF,FB,FS2 and FS3) both were higher than that of FS1.This is because the NPP and ANCS of tree layers in these four wetlands (tree layers occupied ecosystem NPP and ANCS 67.4–79.9% or 68.1–80.9%) were higher than FS1.In addition,the NPP and ANCS of fnie roots were higher than FS1 (Table 5).The reason for this is that the constituent species of FS1 (Alnus sibiricaFisch.ex Turcz.),is a water-tolerant tree species with a relatively short life.It is currently in a post-recession recovery stage,with stands dominated by small-diameter trees (mean DBH is 7.7 cm) (Table 1),and therefore,both NPP and ANCS are low.

There is no remarkable difference in the NPP and ANCS of the other wetlands because FS2,FS3 and FF are seasonal flooded and the trees grow reasonably well (average DBH is 12.2–12.4 cm).Therefore,their NPP and ANCS are high.FB is permanently flooded and the trees grow relatively slowly(average DBH is 9.4 cm).But because of the density of stand is larger (higher 263–789 ha?1than other three types),the NPP and ANCS of FB are also high.

In this study,the NPP of the wetlands,except for FS1,were 8.86–9.92 t ha?1a?1;this is consistent with the NPP of vegetation in northeast China (6–14 t ha?1a?1) (Mao et al.2012;Mu et al.2013b),and close to the lower limits of NPP of temperate forested wetlands (10–15 t ha?1a?1)(Trettin et al.1995;Campbell et al.2000),which is related to the location of Xiaoxing’anling on the northern border of the temperate zone.The ANCS of these four wetlands(4.04–4.50 t ha?1a?1) were close to the mean of the terrestrial vegetation of China (4.9 t ha?1a?1) (He et al.2005),and to the global average (4.1 t ha?1a?1) (Li and Ji 2001).

Annual carbon emissions

Annual carbon emissions of the five wetlands were not remarkably different,mainly due to the CO2– C emissions accounting for the majority of their annual carbon emissions (88.4–100.0%).But CH4– C emissions from FS1 were significantly higher than those from the other wetlands due to the difference in water table depth and soil temperatures caused by ground cover and litter differences.Because FS1 is at the lower level in the transition zone,seasonal flooding is high (water table from 3.5 to 16.0 cm below the surface).Also,the mean temperatures during the growing season in 15–30 cm soil depths were high,some 1.7–2.8 °C higher than that of FF and FB (Table 7),and moss species on the layer and its residues in surface soil could slow temperatures of the middle and lower soil layers in the growing season(Xiao and Bowker 2020).The higher water table (Bridgham et al.2013;Olefeldt et al.2017) and lower soil temperatures (Grant et al.2015) aided the activity of methane-producing bacteria,which enhanced CH4-C emissions.While the water tables of other four wetlands were relative low(1.2–26.0 cm,5.4–39.9 cm,4.0–38.0 cm,and 6.8–25.0 cm),when the water table changed in a range of 20.0–30.0 cm,there would occur a switch between aerobic and anaerobic activities (Sun et al.2011).This was more advantageous tomethanotrophs activities to consume CH4,reducing CH4–C emissions (Moore and Rouolent 1993;Moore and Dalva 1997).The results of this simultaneous measurement of CO2and CH4emissions indicated that soil heterotrophic respiration emissions from temperate forested wetlands were predominantly CO2–C emissions (88.4–100.0%) except for a few wetlands had a relatively large proportion of CH4–C emissions (11.6%) due to heavy flooding.The amounts of CH4– C emissions from most forested wetlands were relatively small (up to 3.3%) and had an insignificant impact on net carbon emissions of these ecosystems.

Table 7 Mean soil temperature (°C) in growing season of forested wetlands in Xiaoxing’anling

Carbon sink or carbon source

This study found that four (FS2,FS3,FF,and FB) of five forested wetlands distributed along a moisture level gradient acted as carbon sinks for atmospheric CO2and their C sink levels were close (0.14–0.6tC ha?1a?1).This is because the annual carbon sequestration of the vegetation were similar and the annual carbon emissions of soil heterotrophic respiration (CO2and CH4) were also close.The annual carbon sequestration of vegetation were higher than the annual carbon emissions of soil heterotrophic respiration.While only FS1,the alder swamp,was shown to be a source of carbon emissions,as it was still in a recovery phase,its annual net carbon sequestration by vegetation was lower than its annual carbon emissions from soil heterotrophic respiration.This carbon sink due to fast-growing,short-lived trees being replaced,has also been observed in temperate forests during stand transition from early to middle succession (Goulden et al.2011;Coursolle et al.2012).

The carbon sink values of forested wetlands in Xiaoxing’anling were nearly consistent with the long-term carbon accumulation rates (0.09–1.29 t ha?1a?1) estimated using sediment cores and14C radiocarbon dating in mountain peatlands of northeast China (Xing et al.2015).They were close to middle (FS2 and FB) and lower (FS3 and FF) levels.This may be related to hydrothermal conditions of Xiaoxing’anling in the north being inferior to those of the Changbailing in the south.In addition,the carbon sink values of forested wetlands were smaller than that of temperate forests in northeast China,which was 1.57 t ha?1a?1by eddy covariance (Liu et al.2021).

Because the measurements of net ecosystem carbon balance were only for one year in our study,the possibilities of larger carbon emissions that turn the carbon sink of wetlands into carbon sources need to be inferred from existing research.Although CH4emissions from wetlands varied significantly from year to year (Zhu et al.2 013;Pugh et al.2017),their impact on the interannual variation of ecosystem carbon fluxes was limited due to the small proportion of CH4– C.As for the interannual variation of net CO2– C balance,Sierra et al.(2009) suggested that it is related to both climate and intrinsic forest dynamics,while other research suggested it was jointly controlled by the length of the net CO2uptake period and the summer peak of net CO2uptake,and that environmental factors showed weak impacts on it(Liu et al.2021).Therefore,we speculate that most forested wetlands may still be carbon sinks at stable forest structure and under relatively constant environmental conditions.

Conclusions

The ecosystem carbon storage of the five forested wetlands distributed along a soil moisture gradient were significantly different in Xiaoxing’anling.Its major carbon storage mechanism is that flooding degree of habitats are different (seasonal flooding and permanent flooding),on the one hand,it affects soil carbon storage (for example,the larch bog and larch fen were higher in carbon sequestration in permanent flooded habitats,and the white birch swamp,alder swamp and larch swamp were lower in carbon storage in seasonal flooded habitats) through the differences in peat accumulation.On the other hand,flooding affects the vegetation carbon storage (the white birch swamp was higher,and larch bog was lower) through the influence tree growth,then causes the ecosystem carbon storage the spatial variation (the larch fen and larch bog showed high carbon storage,the white birch swamp medium carbon storage,and the alder swamp and larch swamp low carbon storage types).

The four types of forested wetlands distributed along a moisture gradient in Xiaoxing’anling were carbon sinks and had similar carbon sink strengths (0.14–0.6 t ha?1a?1),only the alder swamp was a carbon source (?2.1 t ha?1a?1) due to being in post-recession recovery stage.These four forested wetlands had no significant difference in the annual carbon sequestration by vegetation or the annual carbon emissions of soil heterotrophic respiration (CO2and CH4) .Their annual carbon sequestration by vegetation are higher than the annual carbon emissions from soil,thus making them carbon sinks.Our findings indicate that temperate forested wetlands,with the exception of the alder swamp in this study,are carbon sinks,and that the carbon storage of forested wetlands is strongly affected by habitats (seasonal and permanent flooding) and forested wetland types.

AcknowledgementsWe thank the staffs in the Center for Ecological Research of Northeast Forestry University and Yongqing Forest Farm for their support in the field and laboratory.