Soil Organic Matter Dynamics Along a Vertical Vegetation Gradient in the Gongga Mountain on the Tibetan Plateau

Received 22 Oct. 2004 Accepted 20 Jan. 2005Supported by the Knowledge Innovation Project of the Chinese Academy of Sciences (KZCX3-SW-339-04) and the National NaturalScience Foundation of China (40331006).*Author for correspondence. Tel (Fax): +86 (0)10 6488 9697; E-mail: <[email protected]>.

Journal of Integrative Plant BiologyFormerly Acta Botanica Sinica 2005, 47 (4): 411−420

http://www.blackwell-synergy.comhttp://www.chineseplantscience.com

Soil Organic Matter Dynamics Along a Vertical Vegetation Gradientin the Gongga Mountain on the Tibetan Plateau

Lin WANG1, 2, Hua OUYANG1*, Cai-Ping ZHOU1, Feng ZHANG1, 2, Ming-Hua SONG1 and Yu-Qiang TIAN1, 2

(1. Institute of Geographical Sciences and Natural Resources Research, the Chinese Academy of Sciences, Beijing 100101, China;2. Graduate School of the Chinese Academy of Sciences, Beijing 100039, China)

Abstract: Our knowledge about soil organic matter (SOM) dynamics is limited although this is an importantissue in the study of responses of ecosystems to global climate changes. Twelve sampling plots were set upevery 200 m from 1 700 to 3 900 m along the vertical vegetation gradient along the east slope of GonggaMountain. Samples were taken from all 12 plots for SOM content measurement, although only 5 of the 12plots were subjected to radiocarbon measurements. A radiocarbon isotope method and a time-dependentmodel were used to quantify the SOM dynamics and SOM turnover rates along the vertical vegetationgradient. The results showed that the SOM turnover rate decreased and turnover time increased with soildepth for all vegetation types. The litter layer turnover rates presented a clear trend along the gradient. Thelitter layer turnover rates decreased with an increase in elevation, except that the litter layer turnover rate ofmixed forest was higher than that of evergreen forest. Climatic factors, such as temperature and precipitation,were the main factors influencing the surface soil carbon dynamics. The turnover rates of the subsoil(including the A, B, and C horizons in the soil profiles) along the vertical gradient had no clear trends. TheSOM of subalpine shrub and meadow turned over more slowly than that of the forest types in almost all soilhorizons. The characteristic of short roots distributing in the upper part of the soil profile leads to differentSOM dynamics of shrub and meadow compared with the forest types. Coniferous and mixed forests weresusceptible to carbon loss from the young carbon pool, but their long and big roots resulted in high ∆14Cvalues of the deep soil profiles and increased the input of young carbon to the deep soil. In evergreen forest,the carbon cumulative ability from the B horizon to the C horizon was weak. The different vegetation types,together with their different modes of nutrient and carbon intake, may be the mechanism conditioning thesubsoil organic matter dynamics.Key words: carbon cycle; radiocarbon isotope; Tibetan Plateau; turnover rate; turnover time.

The impact of global climate changes on terrestrialcarbon storage and its feedback has attracted consid-erable interest in recent years (Sarmiento et al. 1995;Houghton et al. 1998; Chen et al. 2002a) because ofthe close relationship between climate change and ter-restrial ecosystems. Soil is the largest pool of terres-trial organic carbon in the biosphere (Jobbagy and Jack-son 2000). The amount of carbon stored in soils isapproximately twice the amount of carbon in the

atmosphere and is approximately 3-fold that in livingplants (Post et al. 1982; Schlesinger 1990; Wang andAmundson 1999). The contents and composition ofsoil organic matter (SOM) are important soil charac-teristics and any change in the size of soil carbon poolcould alter the atmospheric CO2 concentration and af-fect the global carbon balance (Jenkinson et al. 1991;Trumbore et al. 1996). However, there has been con-siderable uncertainty regarding SOM dynamics

Journal of Integrative Plant Biology (Formerly Acta Botanica Sinica) Vol. 47 No. 4 2005412

(Schlesinger 1990; Oades 1994; Schimel 1995); thatis, what the SOM dynamics along spatial and temporalscales are and how to quantify the turnover rate ofSOM remain unclear. This restricts the understandingof global carbon cycling and the response of terrestrialecosystems to global climate change (Trumbore 1993;Torn et al. 1997).

The radioactive isotope of carbon (14C) is a usefultool for deciphering the dynamics of SOM in differentecosystems (Balesdent 1987; Scharpenseel et al. 1989;Trumbore et al. 1989). Atmospheric 14CO2 can be in-corporated into plants by photosynthesis and into soilsthrough decomposition processes. The degree to whichthe 14C/12C ratio in SOM differs from the vascular plantmatter is often used to calculate a mean residence time(MRT; Trumbore 1993). Using this method, SOM hasbeen separated effectively into a series of carbon poolswith the turnover times ranging from 10 yr to millennia(Harrison et al. 1993; Gaudinski et al. 2000; Neff etal. 2002). However, the carbon dates should be cor-rected after the 1950s because the soil surface is proneto contamination by the nuclear bomb 14C as a resultof thermonuclear bomb testing in the 1950s. This ap-proximately doubled the amount of 14C in the atmo-spheric CO2. Tracing this bomb 14C through SOM res-ervoirs over the past 50 yr provides a means of deter-mining the size and turnover time of fast-cycling SOMpools (Balesdent 1987; Townsend et al. 1995;Trumbore et al. 1996).

The Tibetan Plateau, the third pole of the earth, isone of the regions in the world that has suffered theleast human disturbance. Great uplift of the plateau sincethe Late Cenozoic has strongly affected the physicalenvironment of the plateau itself and its neighboringregions (Pei et al. 2004). Meanwhile, the plateau is alsoa sensitive trigger of climate change for the Asian mon-soon region, which is one of the most important criti-

cal regions for global climate change (Zheng and Zhu2000). Abundant vegetation types on the Tibetan Pla-teau varied from tropic to frigid and their vertical dis-tributions were much narrower than their horizontaldistributions, which would lead to strong sensitivitiesto global climate change.

The objectives of the present study were to: (i) quan-tify the below-ground SOM dynamics along the verti-cal vegetation gradient on the Gongga Mountain on theTibetan Plateau using 14C measurements; and (ii) ana-lyze the relationship between SOM dynamics and envi-ronmental factors. We hypothesized that: (i) the turn-over rate of SOM would decrease along the verticalvegetation gradient; and (ii) the turnover time of SOMwould decrease from lower to upper parts in the soilprofiles.

1 Materials and Methods

1.1 Site descriptionGongga Mountain (29.30–30.30° N, 101.50–

102.25° E), located on the eastern side of the TibetanPlateau (Fig. 1), is the main peak of the DaxueshanMountains of the East Hengduan Mountain Range andits highest peak is 7 556 m above sea level. The climateis warm and humid, strongly impacted by the south-west monsoon. The study area is along the east slopeof Gongga Mountain with an elevation varying fromapproximately 1 000 to 3 900 m over a distance of 30km. The mean annual temperature declines from 11 to–1 °C along the elevation gradient (Table 1). The meanannual rainfall increases with elevation, from approxi-mately 1 300 mm/yr at lower elevations to over 3 000mm/yr at higher elevations (Gao and Peng 1993; Table1). The vertical gradient of Gongga Mountain is a typi-cal representative of the southeast Tibetan Plateau.

The species composition, richness, structure,productivity, litter layer, and environmental factors were

Table 1 Mean annual temperature and precipitation on Gongga Mountain Elevation (m)

1 700 1 900 2 100 2 300 2 500 2 700 2 900 3 100 3 300 3 500 3 700 3 900Mean annual temperature (°C) 11.8 10.6 9.4 8.2 7.0 5.8 4.6 3.4 2.2 1.0 −0.2 −1.4Annual precipitation (mm) 1 093 1 223 1 353 1 483 1 613 1 743 1 873 2 003 2 133 2 263 2 393 2 523

Lin WANG et al.: Soil Organic Matter Dynamics Along a Vertical Vegetation Gradient in the Gongga Mountain on the TibetanPlateau 413

significantly different among the 5 vegetation types (Luoet al. 1999, 2000, 2003; Shen et al. 2001; Fig. 2). Thesoil textures on Gongga Mountain were generally looseand ragged, much gravel and sandy neonatal matterwere found in the soil, and the degree of weathering ofminerals in the parent materials was weak, all of whichimplied that the soil development of Gongga Mountainwas “young” (Liu 1985). The parent material of soils

varied from hard granite to incompact sandy slope washand to glacial till deposits under different climates, veg-etation types, and rock characters (Liu 1985; Fig. 2).1.2 Soil sampling

Twelve sampling plots were set up every 200 m fromthe secondary forest at the lower elevation(approximately 1 700 m) to the subalpine meadow at ahigher elevation (approximately 3 900 m) in July 2003.Three soil profiles were excavated at each samplingplot and samples were taken according to the soil ge-netic horizon. After being air-dried and removal of thestones and plant residuals, samples were screenedthrough a 0.1 mm sieve for measuring SOM contentusing the sulfuric acid-potassium dichromate method.

Only samples of five of the 12 sampling plots wereselected for measurement of the SOM 14C radioactiv-ity because of financial restrictions. The five plots se-lected were representative of the five vegetation typesand their visible characteristics are given in Table 2. All5 soils had clear development layers (Table 2). Samples

Fig. 1. Location of Gongga Mountain.

Fig. 2. Distribution of the vegetation and soils on GonggaMountain.


for radioactivity measurement were taken from downto up in the profile to avoid disturbance.1.3 14C radioactivity of SOM

A sample of every horizon in the soil profile wastaken for radioactivity measurements. After picking outvisible roots and stones, soil samples were dispersed indistilled water and washed through a 1 mm sieve toremove rootlets and coarse sand (Chen et al. 2002b).Samples were treated with 0.1 mol/L HCl to removecarbonate, rinsed repeatedly with distilled water untilthey were neutral, and were then preserved after dry-ing at 80 °C. Treated samples were put in an airtightvacuous system to be burnt to CO2. The CO2 producedwas absorbed by ammonia water, converted to stron-tium carbonate after the addition of strontium chloride,and was then converted to SrC2 using the Mg method(Qiu et al. 1990). The SrC2 was hydrolysed to C2H2

and C6H6 was then synthesized under catalysis. The14C in the C6H6 was measured using a 1 220 Quantulusultralow level liquid scintillation spectrometer. All treat-ment and measurement processes were performed at

the Inst i tute of Geology, China EarthquakeAdministration. Results are expressed as ∆14C, the de-viation in parts per thousand of the 14C/12C ratio in thesample from that of an absolute standard (oxalic aciddecay corrected to 1950; Stuiver and Polach 1977).

∆14C = −1 × 1 000 (1)

1.4 Turnover rates of SOMThe 14C radioactivity in the atmosphere was approxi-

mately stable before the start of nuclear bomb testing(Torn et al. 1997). The atmospheric 14C content in-creased rapidly because of “bomb 14C” after the early1950s and the 14C content in the atmosphere reachedits highest level during the period 1963–1964(Burchuladze et al. 1989; Levin and Kromer 1997;Levin and Hesshaimer, 2000). After nuclear bomb test-ing was prohibited, the 14C content of the atmospheredecreased annually, but, at present, it is still greaterthan that before nuclear bomb testing started (Fig. 3;

Table 2 Characteristics and sampling depths of the soil profiles for 14C measurements

ProfileElevation

Soil typeParent material Main plant Genetic

Depth of Sampling

(m) of soils species horizoneach horizon depth

(cm) (cm)Evergreen broad- 2 100 Mountain yellow Granite Cyclobalanopsis Litter 0 – 4 0 – 4leaved forest brown soil oxyodon A 4 – 9 6 – 8

B 9 – 18 12 – 15C 18 – 40 27 – 29

Mixed broadleaved- 2 700 Mountain dark Slope wash Tsuga chinensis, Litter 0 – 5 0 – 5conifer forest brown soil Betula spp. A 5 – 15 9 – 11

B 15 – 25 19 – 21C 25 – 33 – 36

Dark coniferous forest 3 100 Dark brown forest Slope wash Abies fabri Litter 0 – 4 0 – 4soil A 4 – 10 6 – 8

B 10 – 22 15 – 17C 22 – 33 – 36

Subalpine shrub 3 700 Subalpine podsol Slope wash and Rhododendron Litter 0 – 4 0 – 4glacial till agglutinutum A 5 – 9 6 – 7

B 10 – 23 5 – 18C 23 – 32 – 35

Subalpine meadow 3 900 Subalpine meadow Glacial till Kobresia Litter No data No datasoil cumeata A 0 – 10 4 – 6

B 10 – 22 15 – 17C 22 – 31 – 34

14C12C14C12C

sample

standard


Wang and Amundson 1999). According to the defini-tion of ∆14C (equntion 1), if the ∆14C values of samplesare greater than zero, this indicates that the sampleshave significant amounts of bomb 14C (Trumbore etal. 1996; Trumbore and Harden 1997; Richter et al.1999). If the ∆14C values are negative, this indicatesthat, on average, the carbon has resided in the soil longenough to reflect radioactive decay of cosmogenic 14C(Trumbore et al. 1996).

It is impossible to give 14C ages for samples with∆14C values greater than zero only using the definitionof ∆14C. Thus, in the present study, we used theCherkinsky and Brovkin (1993) model to reconstructthe processes of SOM 14C. It is expressed as:

= (2)

C14t = C14

t−1 – (k+λ) C14t−1 + k C'14

t (3)

where C141955 is the 14C radioactivity in 1955, C14

t is the14C radioactivity in the year of sampling (t >1955),C14

t−1 is the 14C radioactivity in year (t–1), C'14t is the 14C

radioactivity in the atmosphere in year t, C14standard is the

14C radioactivity of the modern carbon standard, λ isthe 14C decay constant (1/8 267 per year), k is theSOM turnover rate, and the SOM turnover time is k–1.The model describes three processes (equation 3) de-termining the SOM 14C contents: (i) 14C incorporation

from the atmosphere; (ii) 14C release by SOM decay;and (iii) spontaneous fission of 14C. The atmosphericradioactivity of every simulated year is obtained frompublished references (Burchuladze et al. 1989; Levinand Kromer 1997; Levin and Hesshaimer 2000). The14C radioactivity of the upper part of each soil profile issusceptible to the 14C of the atmosphere and is consid-ered to be equal to the 14C radioactivity in the atmo-sphere before nuclear bomb testing (i.e. t < 1956). Inaddition, the 14C radioactivity of the atmosphere be-fore testing was stable, so the SOM 14C in year t isclose to that in year (t–1); that is, C14

t = C14t−1. If the

industrial effect (Suess 1955) is neglected, the 14C ra-dioactivity in the atmosphere in 1955 was close to the14C radioactivity of the modern carbon standard(C14

standard) and then equntion 2 can be inferred fromequntion 3 (Chen et al. 2002b).

During the simulation, one value of k was selectedfirst, and C14

1955 was calculated using equntion 2. Then,C14

1955 and k were put into equntion 3 and the SOM∆14C radioactivity in the sampling year (2003) wasobtained by iterative calculation. The k value was ad-justed until the calculated value of C14

2003 was close tothe measured value. Putting the final k into equntion 2and combining this with the definition of ∆14C (equation1), we can calculate the turnover rates of the sampleswith ∆14C less zero using the following formula (Chenet al. 2002b):

k = −λ ( +1) (4)

2 Results and Discussion

2.1 The SOM content along the vertical gradientThe SOM content of litter layers increased with

elevation. The SOM content of A horizons also increasedwith elevation, except that there were two peaks at2 700 and 3 500 m (Fig. 4). Wang (1994) had studiedthe forest SOM content at 1 400, 1 900, 2 500, 2 700,and 3 100 m on the Gongga Mountain. The results ofthe present study, namely that the SOM content of lit-ter layers and the A horizon increased with elevation,

Fig. 3. ∆14C values of the atmosphere (data taken fromBurchuladze et al. (1989), Levin and Kromer (1997), andLevin and Hesshaimer (2000)).

C141955

C14standard

kk+λ

1 000∆14C


are identical to those of Wang (1994). The SOM con-tent of the B and C horizons in the present study hadno clear trends according to elevation.

The litter layer and the A horizon were the upperpart of the soil profile; thus, they were prone to beimpacted on by climatic factors, such as temperatureand precipitation. The highest values of SOM for the Ahorizon were at 2 700 and 3 300 m, which indicatedthat the transition zones had different ecological char-acteristics compared with other vegetation types (Wanget al. 2004). The B and C horizons were the deeperpart of the soil profile. The SOM content of thesesamples was impacted less by climate than by soil tex-ture and soil parent materials. Therefore, the B and Chorizons had no clear trends according to elevation.

2.2 Distribution of ∆∆∆∆∆14C values in soil profilesThe distribution pattern of ∆14C values in soil pro-

files was identical for all five vegetation types (Fig. 5);that is, ∆14C values decreased with increasing soil depth.The ∆14C value of the litter layer was lower than thatof the A horizon, despite the litter layer being laid abovethe A horizon. We know that the ∆14C values of theatmosphere decreased after nuclear bomb testing hadbeen banned (Fig. 3), so the ∆14C values of SOM shoulddecrease with decreasing atmospheric ∆14C values, inspite of the time lag, because the soil was derived fromplants that incorporated the 14CO 2 throughphotosynthesis. The ∆14C value of the litter layer wasthe closest to that of the contemporary atmosphere,whereas the ∆14C value of the A horizon was higherthan that of the litter layer because it incorporated more“old” carbon with high ∆14C values than the litter layer.

The ∆14C values of both the litter layer and the Ahorizon were greater than zero and greater than that ofthe atmosphere (∆14C = 85.7‰), which indicated thatsamples of both the litter layer and the A horizon wereimpacted on by the nuclear tests and had a time lagcompared with the atmosphere; plants were not incor-porated into the soil during the same year, but aftermany years.

The ∆14C values of the B and C horizons in all for-est types, except the evergreen forest, were negative,suggesting that, on average, the carbon has resided inthe soil long enough to reflect radioactive decay of cos-mogenic 14C.2.3 ∆∆∆∆∆14C values of every horizon along the ver-tical vegetation gradient

The ∆14C value of the litter layer increased withelevation, whereas that of the A, B, and C horizons hadno clear trends (Fig. 6).

The ∆14C values of litter layers for all vegetationtypes were greater than zero (Fig. 4), which indicatesthat the litter layer was impacted on by the bomb carbon.The ∆14C values of the litter layer increased withelevation, suggesting that the litter was decomposedmore slowly along an increasing elevation gradient. Thelitter layer of the subalpine shrub had the highest ∆14C

Fig. 4. Soil organic matter (SOM) content of every hori-zon according to elevation. A, B, C, SOM content of A, Band C horizon, respectively.

Fig. 5. ∆14C values in the soil profiles for the 5 mainvegetation types.


value and decomposed most slowly because this typeof vegetation retains more “old” carbon. The climateof the subalpine shrub was cooler and more humid(Table 1), which is not helpful for microbialdecomposition, so plant debris were retained in the soilsand the ∆14C value of the soils was high. Climaticfactors, such as temperature and precipitation, wereimportant factors influencing the carbon dynamics ofthe litter layers.

In general, the SOM turnover rate decreased withincreasing elevation (Trumbore et al. 1996). The litterlayer ∆14C value of evergreen forests at lower eleva-tions was higher than that of mixed forests at higherelevations in the present study. Some moss appearedin the mixed forest because of the lower temperatureand higher precipitation compared with the evergreenforest (Table 1). While sampling, we removed visiblelarge plants, such as ferns, large leaves, etc. It washard to separate the moss entirely from the witheredbranches and leaves to which it clung. The moss wasgenerally newly come out and had ∆14C values close tothose of the atmosphere in the same year. It may bethe moss that made the ∆14C value of the litter layer ofmixed forest lower than that of the evergreen forest.

The ∆14C values of the A, B, and C horizons had noclear trends according to elevation. The relatively en-riched 14C in the A horizon of mixed forest was partlydue to the fact that the sampling depth here was deeperthan for its counterparts (Table 2; Fig. 6), which led tothe incorporation of “old” carbon. The evergreen for-est had the lowest ∆14C value in the C horizon, which

indicated that it was mainly made of parent materialwith much “old” carbon. This was partly because it isthe foremost part in the uplift course of the mountain.

The ∆14C value of the B horizon in the evergreenforest was much higher than that of the others (Fig.6), whereas the ∆14C value of the C horizon in theevergreen forest was significantly lower than itscounterparts. This indicates that the SOM of the ever-green forest had a different turnover pattern: the SOMturnover rate of the B horizon was quick, but that ofthe C horizon was much slower compared with otherforest types. The SOM imported and accumulated fromthe B horizon was little, although the B horizon had afast cycling rate. Much carbon maybe lost to the at-mosphere in the form of CO2 or was washed to thelower part.

The ∆14C of coniferous and mixed forests in theupper soil horizons (the upper 15 cm depth interval)was lower than that of subalpine shrub and subalpinemeadow, suggesting a preferential loss of C fromyounger carbon pools. However, organic matter in sub-soil horizons (from 10 to 40 cm) of the coniferous andmixed forests was relatively enriched in 14C comparedwith that of shrub and meadow, which suggests anincreased input of carbon in these soil horizons. Thisinterpretation is supported by field observations of rootdistributions in these soils. The roots of the shrub andmeadow are abundant in the shallow horizons and thedensity of roots drops markedly below a depth of 20cm. In the coniferous and mixed forests, there wereroots in the shallow horizons too, but they were basi-cally the roots of understory plants, such as someshrubs, liana, and grass. The roots of coniferous andmixed forests were bigger and longer, and could ex-tend to the deeper soils, which would contribute to thehigh ∆14C value in the deeper horizons. The coniferousand mixed forests were susceptible to carbon loss fromthe younger carbon pool, but their ability to accumu-late carbon from the upper part to the deeper part wasstronger than that of the evergreen forest. The differ-ent vegetation types, with different modes of nutrientand carbon intake, result in different carbon dynamics

Fig. 6. ∆14C values of every horizon according to elevation.A, B, C, ∆14C values of A, B and C horizon.


of the subsoil in the profiles investigated.2.3 Turnover rates and turnover time along thevertical vegetation gradient

Using the Cherkinsky and Brovkin model describedabove, it was found that turnover rates decreased withsoil depth and turnover time increased with soil depth(Table 3). These results are identical to the general re-search and our above hypothesis that the upper part ofthe soil profile cycles more quickly than the lower part.The turnover time of the surface litter layer (the upper5 cm depth) was between 5 and 13 yr, whereas that ofA horizon was 10–30 yr, that of the B horizon was 100– 600 yr, and that of the C horizon was more than1 000 yr.

Comparing the turnover rate of evergreen forest withthe results of an investigation of the subtropical mon-soon forest in Dinghushan (Chen et al. 2002b), wefound that the turnover rate of litter in the evergreenforest in the present study was 0.132 0. This rate is

lower than that reported by Chen et al. (2002b) at thesame depth. It is the climate that has led to the differ-ences between the two forest types. For example, themean annual temperature in the present study was ap-proximately 11 °C, whereas that in the study of Chenet al. (2002b) was approximately 21 °C. The turnoverrate of the B horizon was nearly equal in the two studies.The turnover rate of the C horizon in the present studywas lower than that at the same sampling depth in thestudy of Chen et al. (2002b) because our soil profilewas shallower than that in the other study and the soilin the present study was young compared with thesubtropical monsoon forest in south China.

The samples measured in the present study werebased on bulk soil, so the turnover time and turnoverrate were the mean values of a horizon. Thus, the bulksoil may be composed of a mixture of soil with lowturnover rates and soil with a high turnover time.However, it can be clearly seen in Table 3 that the

Table 4 Comparison of evergreen forest turnover rate on Gongga Mountain in the present tudy with that of a subtropicalmonsoon forest in Dinghushan

StudyMean annual Mean annual Turnover rate (/yr)temperature precipitation Litter A B C

(°C) (mm) (0 – 3 cm) (6 – 8 cm) (12 – 15 cm) (27 – 29 cm)Evergreen forest on Gongga Mountain 11 0.000 13 1 353.000 0 0.132 0 0.108 0 0.010 0Subtropical monsoon forest in South China* 21 1 927.000 00 0.401 6 0.125 0 0.007 4 0.001 3*, data from Chen et al. (2002b).

Table 3 Soil organic matter turnover rate and turnover timeForest type

Evergreen broad- Mixed broadleaved- Dark coniferous Subalpine Subalpineleaved forest conifer forest forest shrub meadow

Elevation (m) 2 100 2 700 3 100 3 700 3 900∆14C (‰) Litter 134.5 114.8 148.2 176.1 No data

A 148.7 243.6 171.1 213.9 202.3B 81.0 – 30.5 – 48.5 – 152.0 – 69.4C – 481.5 – 103.0 – 186.0 – 158.9 – 192.0

Turnover rate (/yr) Litter 0.132 0.198 0.107 0.077 –A 0.108 00 0.055 50 0.069 00 0.036 00 0.031 80B 0.010 00 0.003 85 0.002 24 0.000 67 0.001 62C 0.000 13 0.001 05 0.000 53 0.000 64 0.000 51

Turnover time (yr) Litter 7.5 5.1 9.3 13.0 –A 9.3 18.0 14.5 27.8 31.4B 100 259.7 446.4 1 492.5 617.3C 7 692.3 952.4 1 886.8 1 562.5 1 960.8


upper part of the soil profile in the evergreen forestwas dominated by a compartment with a high turnoverrate (i.e. 0.132 for the evergreen forest litter layer) andthat the deeper soil was dominated by a compartmentwith a low turnover rate (i.e. 0.000 13 for the ever-green forest C horizon). The SOM dynamics along thevertical gradient were more complex than we had firstthought (Table 3; Fig. 6). However, if we analyze ourresults, we find that the turnover rates of shrub andmeadow are almost slower than those for forests inevery horizon. This indicates that carbon stays longerin the shrub and meadow soils than in forest soils.Acknowledgement The authors thank Gen-WeiCHENG (Institute of Mountain Hazards andEnvironment, the Chinese Academy of Sciences (CAS))and Ji LUO (Institute of Mountain Hazards andEnvironment, CAS) for their assistance in fieldsampling, and the authors are grateful to Jin-Hui YIN(Institute of Geology, China Earthquake Administration)for his help in the laboratory. Thanks are also extendedto Qing-Qiang CHEN (State Key Laboratory of Estua-rine and Coastal Research, East China NormalUniversity) for his suggestions and assistance duringdata analyses.

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Documents

Soil Organic Matter Dynamics Along a Vertical Vegetation Gradient in the Gongga Mountain on the Tibetan Plateau