Soil carbon cycling in a Japanese cedar (Cryptomeria japonica) plantation

Fores;~dogy Management ELSEVIER

Forest Ecologyand Management 72 (1995) 185-197

Soil carbon cycling in a Japanesecedar (Cryptomeria japonica) plantation KaneyukiNakane’ Department of Environmental

Studies, Faculty of Integrated Arts and Sciences, Hiroshima Japan

University, HigashiHiroshima

724,

Accepted 11 August 1994

Abstract Soil carbon cycling was measured synthetically and quantitatively throughout a year in a Japanese ceder plantation, Mt. Amida, Hiroshima Prefecture, west Japan, and analyzed by a compartment model. The results observed and analyzed were compared with those obtained by the same methods in natural secondary (Japanese red pine) and climax (evergreen oak) forests developed in the same warm-temperate zone, west Japan. There was no clear seasonal trend in litterfall rate and accumulation of & layer, but soil respiration increased in summer and decreased in winter with the change in soil surface temperature. The relative decomposition rate of the A, layer (0.187 year-‘) observed in the cedar plantation was one-third of that in an evergreen oak forest and rather less than in a Japanese red pine forest. Transfer of humus from the A0 layer to mineral soil was faster than in the pine forest, and thus the loss rate of the & layer (0.262 year-t) was somewhat larger than that in the pine forest. However, the relative decomposition rate of humus in mineral soil (0.0063 year-’ ) was only one-third of those in the pine and evergreen oak forests. This suggests that soil carbon cycling was extremely slow in the cedar plantation. This is probably due to the great resistance of cedar litter to decomposition and to less broadleaf litterfall than in the pine forest, because the soil environmental conditions were no worse than in other two forests. Based on these results, a discussion is presented on the management of cedar plantations, how to maintain their productivity under conditions with slow material cycling and how to enhance this cycling. Keywords: Cedar plantation; Compartment model; Cycling; Soil carbon;Warm-temperate forest

1. Introduction Japanese cedar (Cryptomeria japonica D. Don) plantation constitute the most common type of artificial forestin Japan.Sincethe 195Os, cedar, Japanesecypress (Chamaecyparis obtsusa Sieb. et Zucc.) and larch (Larix leptolepis Gord.) plantation hasbeenencouragedand thus they occupyabout 40% of its total forest areato’ Fax. +81-824-24-0758.

day, mainly replacing the natural broadleaved forests.There has,however,beenno attempt to study synthetically and quantitatively the cycling of soil carbon in Japaneseand other cedar plantations or forests.Only fragmentarydata on soil carboncycling in Japanesecedarplantations havebeenreportedsuch asthe accountof litterfall, accumulation of the & layer (Andow, 1970), litter decomposition (Tsutsumi et al., 1961) and soil respiration rates (Simon0 et al., 1989))in contrastwith the more numerousstud-

0378-l 127/95/$09.50 0 1995 ElsevierScienceB.V. All rights reserved SSDZO378-1127(94)03465-6

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ies for the seconddominant artificial forest, cypress plantation (Kawahara, 1971; Kawahara and Saito, 1974; Kawahara, 1975; Iwatsubo, 1976; Saito, 1981;Tsutsumi et al., 1983, 1985; Simon0 et al., 1989). Soil carbon cycling is an important component of the decomposition processin forest ecosystemsand has wide ramifications for how ecosystems function, including consequencesfor forest productivity. High forest productivity can be supported by environments with rapid cycling of materials. The carbon cycle is a useful standard for other material cycles in terrestrial ecosystem because carbon occupies approximately 50% of organic matter. Thus, an attempt was made to observemajor flows and reservoirsof soil carbon synthetically and simultaneously throughout a year in a 35year-old cedar plantation and analyzedthem by a compartment model. To clarify the characteristic of soil carbon cycling in the cedar plantation, these data and results of the analysis were compared with those obtained in a natural secondary (40-year-oldJapanesered pine) forest on Mt. Tokao in Fuchu, Hiroshima Prefecture (Nakane et al., 1984) and climax (evergreen oak) forest on Mt. Kasuga in Nara City (Nakane, 1975), both in the same warm-temperate zonein west Japan. 2. Study site

The study area was a Japanesecedar plantation on northeast slope of Mt. Amida in Yukicho, which is adjacent to Hiroshima City, west Japan (34”25’N, 132”30’E),asshowninFig. 1. This region has a warm-temperate, monsoon climate. The averageof annual mean air temperatureand annualprecipitation in Yuki during the last decadewere 13.3“C and 1655 mm, respectively. The monthly mean temperatureshoweda maximum in August and a minimum in January or February. Precipitation was lower in winter and higher in summer except for August. Rocks are sedimentary, the soil is loam, and strongly weathered.The altitude and slope inclination at the researchplot range from 440 to 450 m and

Set0 Wend Sea Fig. 1. Study site for investigation of soil carbon cycling for a Japanese cedar plantation on Mt. Amida, Yuki-cho, Hiroshima Prefecture. Mt. Kasuga in Nara City, evergreen oak forest; Mt. Takao in Fuchu-cho, Hiroshima Prefecture, Japanese red pine forest.

from 15to 20”) respectively.The plot (20 m x 20 m ) was establishedon the middle of a slope in December1983.The plantation was35 yearsold. The mean diameter at breastheight (DBH.) was 258 cm ( 186-374 cm) a&maximum tree height was 19.4m in the plot. The plantation standhad been disturbed about 5 years previously by the secondthinning, which decreasedthe density of cedartrees in the plot from about 1500to 1075

K. Nakane /Forest Ecology and Management

ha- ’ and excludedmost broadleavedtreescomposingthe undergrowth.The above-and belowground biomasseswere estimated at 107and 25 t C ha-‘, respectively,based on the allometric relationships obtained by Andow et al. ( 1968). The presentstudy was carried out from January 1984to August 1985.

3. Methods

3.1. Soil temperatureand soil moisturecontent The soil temperatureat the surfaceof mineral soil (To) wasobservedtwice a month at the time of soil respiration measurementin the plot. At the sametime, A0 layer and surfacesoil core (5 cm across,5 cm deep) sampleswere collectedat ten points for the estimation of moisture content. The moisture content of A0 layer ( V,) and soil core ( V,) sampleswere calculatedon a dry weight basis. The maximum water holding capacity (MWHC ) of thesesoil corewasalso measured using the pair of core samples in the laboratory. 3.2. LitterjaN Five plastic funnel-type litter traps of 1 mm mesh and with a 1 m2 ‘mouth’ were placed at random 1 m abovethe ground. Litters were collected monthly and sorted into leaf (needle,evergreenand deciduousbroad leaves),fine branch (@< 1 cm) and bark, seeds(cone), and others, and oven-dried to obtained the dry weight and for subsequent measurement of the carbon content. Large branch litter ( 1< $< 10 cm) on the ground was marked with yellow paint at the beginning of the studyand further additionsof large branch litter were collected and weighedafter 1 year.Their wet weight was measuredin the field and dry weight waslater estimatedbasedon subsamplesdried at 85‘C for 1 week.

72 (1995) 185-l 97

187

3.3.Amount of A0 layer and humus in mineral soil The amount of A0 layer was measuredin ten quadrats (50 cm x 50 cm) at 3 month intervals. The wet weight of the A0 layer was measuredin the field, and partial samplesweretaken for dry weight and carboncontent determination. Three profile pits for mineral soil sampling were dug. Soil sampleswerecollectedat eachlayer at 5 cm depth intervals along the profile (O-100 cm depth) by a cylindrical steel sampler ( 10 cm acrossand 5 cm deep). The number of samples for eachlayer was five for five layers (O-25 cm depth), but two for 15layers( 25- 100cm depth). All ( 165= 55x 3pits) sampleswereair-dried and weighed,and excludedduff and roots for carbon contentdetermination,which was repeatedtwice or threetimes for eachsample.The total amount of humus in mineral soil wascalculatedby summing the carboncontent from the surfaceto 100 cm depth. 3.4. Soil respiration Evolution of CO2from the ground surfacewas measuredusing Kirita’s ( 1971) method, which is a modification of the technique of Walter (1952) and Haber (1958). A piece of plastic sponge ( 10 cm acrossand 2 cm thick) wetted with NaOH solution is used to absorbCO2 releasedfrom the ground surfaceinto a spacecoveredby aninverted box placedon the forestfloor. Two setsof Kirita’s apparatus,with and without A0 layer was set on the forest floor, to measure total and mineral soil respirationrates.Five pairs of the apparatuswere placed in the plot in December 1983.A measurementof soil respiration over a 24 h period wasmade twice a month from spring until early winter (April-December), but oncea month in winter (January-March), when the forest floor was covereddeeplyby snow. Another piece of apparatus (metal cylinder with a base,40 cm diameter and 50 cm height) was prepared for measuring only the A0 layer respirationrate.Sevencylinderswereplacedwith their baseat 10cm depth in the forest floor near the plot, asit was expectedthat the temperature

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K. Nakane /Forest Ecology and Management

72 (I 995) 185- 197

would be constantat that level. Water wasadded to the samplelitter (& layer), after it had been dried in air for 1 week,in the cylinder to obtain the following watercontentlevels;0,50, 150,250, 350 and 500%on dry weight basis. The cylinder wasclosedwith a lid and sealedfor 24 h after the spongewetted with NaOH solution had been placedinside it. 3.5. Loss rate ofAOlayer and deadroots Residual (& layer) collected from the forest floor and fresh litter (& layer) from live trees nearthe plot was air-dried for 1 week and about 100-200g wasput into eachnylon litter bag (25 cm x 25 cm ) . One hundred litter bagswere prepared for residual and freshlitter, and placedon the forest floor in the plot in January 1984; 20 bags were collected subsequently at severalmonth intervals to measurethe dry weight loss. Dug up roots ($x5 cm) were dried at 85°C for 1weekand 1000-2000g wasput into a nylon bag (50 cmx50 cm) after separationinto several diameter classes(over 5 cm, 5-3 cm, 3-l cm, lessthan 1 cm). Ten bagswereburied at 1O20 cm soil depth in the plot, and 1yearlater their dry weight was measuredagain. The carboncontent of litter, A0 layer, mineral soil and roots were determined by C-N corder (Yanagimoto, Model MT-500). 4. Results

4.I. Soil temperatureand soil water content Fig. 2 showsthe seasonalchangein soil surface temperature (To) observedin the plot and the values for the two natural forests developed in the same climate zone, the evergreenoak and pine forests. The monthly mean temperature showed a minimum ( 1.5’ C ) in February, increasedin spring and reachedto its maximum (23.6”C) in August; it then decreasedswiftly from autumn to winter. The mean temperature of soil surfaceof the plot during the study year was 12.7”C. Fig. 3 showsthe seasonalchangein the mois-

JFMAMJJASOND

Fig. 2. Seasonal trend in temperature at the surf%ce of mineral soil in the cedar plantation on Mt. Amida, Yuki, Hiroshima Prefecture: solid line, cedar plantation; short dashes, evergreen oak forest in Nara (Nakane, 1975); long andshort dashes, Japanese red pine forest in Fuchu, Hiroshima Prefecture (Nakane et al., 1984).

ture content of the A,, layer and the surf&e layer of mineral soil. The moisture content in A0 layer ( V,, % ) was high ( 140- 180%) in winter owing to the snow cover on the forest floor, but it decreasedto 40-8096in springand summer, except for July, the seasonof heavy rain. The annual mean ( + SD) vale was estimated at 98 +-32O/a. The moisture content in the surfacelayer of mineral soil (V,) maintained the relatively high value (60-9096 af MWHC: 27-41% on a dry weight basis) throughout the year, the annual mean value of which was calculated at 76 If:9% (35+4%onadryweightbasis). 4.2. Litterfall The litterfall recordfor the studiedyearis given in Table 1. There was no dear seasonaltendency in- the leaf, branch and bark litte&& rates.Seedfitter, however, was concentratedin summer, follow-

K. Nakane /Forest Ecology andManagement

72 (1992) 185-197

189

VO

200

rn

JFMAMJJASOND 1984 Fig. 3. Seasonal trend in moisture contents of the Ae layer and surface layer of mineral soil. Clear bars, A0 layer (dry weight basis, 4s); solid bars, surface layer of mineral soil (ratio to the maximum water holding capacity (MWHC), S). The moisture content (W) on a dry weight basis is calculated as: (the ratio (W) to MWHC)/2.2. Table 1 Seasonal change in litterfall Month

January-April May June July August September October November December

(kg ha- ’ ) in a Japanese cedar plantation

Leaves Needle

Broad

859.0 286.4 114.7 31.4 645.0 465.0 29.8 414.4 46.0

10.3 2.0 1.5 1.2 0.9 3.5 10.8 9.4 3.8

in Yuki, Hiroshima Prefecture, west Japan

Branches+ bark (Kg ha-‘)

Seeds

Flowers

Others

Sum

91.8 60.0 17.6 4.6 162.2 80.0 3.4 46.8 0.6

162.0 81.2 38.2 2.0 517.6 255.1 7.6 47.0 3.6

18.2 275.6 43.0 6.0 25.4 12.6 2.4 1.2 0.2

38.3 14.2 14.5 13.8 77.1 35.2 3.2 20.2 8.6

1178.4 719.4 226.2 59.0 1428.2 851.4 57.2 539.0 62.8

Annual

5122 kg ha-’ year-’ Large branch ($> 1 cm) Total

413kgha-‘year-’ 5.535 t ha-’ year-’ (2.66 t C ha-’ year-r)

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K. Nakane /Forest Ecology and Management

Table 2 Monthly mean total (S,), mineral shima Prefecture, west Japan Month

January February March April May June July August

September October November December

(Sn,)

and A0 layer (S,) Total

1.8 1.5 4.2 12.7 16.4 17.8 21.5 23.5 16.7 13.1 8.1 5.8

87.6 77.6 107.6 137.8 228.9 184.5 499.9 432.3 380.4 205.4 101.0 100.4 5.40

ing the relatively high flower litterfall rate in spring. The sum of the litterfall rate was 2.56 t C ha- ’ year- ’ and the total litterfall including big branch (@> 1cm) wasestimatedat 2.66t C ha- ’ year- ‘. 4.3. Accumulation mineral soil

197

rates in the Japanese cedar plantation -__-----

Soil surface temperature (“(3

Annual (t C ha-’ year- ’ )

respiration

72 (1995) I&

ofAO layer and humus in

The annual averageaccumulation of the A,, layer(M0)wasestimatedtobe9.4?1.6tCha-’. The seasonalchangein MO in the cedar plantation wasunclear.The reasonis becausetherewas little seasonalchangein the litterfall rate and the large variation in MO betweenmicro-sites in the plot. The accumulation of carbon in mineral soil (M, 0- 100cm depth) was estimated at 103It_3 t C ha-‘. 4.4. Soil respiration

The monthly mean and annual soil respiration ratesare given in Table 2, where the respiration rate of the A0 layer (S,,) was calculated as the difference betweentotal soil (S,) and mineral soil respiration rates (S,,). All of S,, S,, and

Mineral (mgC02m83.4 72.6 94.4 115.8 180.6 121.1 295.7 239.4 227.5 163.3 79.9 74.8 3.64

in Yuki,-Hiro_..._. .-- _____. -I,, layer

h-‘) 4.1 -5.0 13.2 32.0 48.3 63.1 304.2 192.9 152.9 42.1 21.1 25.6 i.:tJ .- _--

S, increasedswiftly from spring to summer and decreasedfrom autumn to winter, whelkthe respiration rates were low constantly. The annual value of S,, (1.76 t C ha-’ year-‘) occupied 33% of the S, (5.40), which correspondedto about half of the S,, (364). The relationship betweenthe soil surfacetemperature ( r,) and S, or S, is iW&ated in Fig. 4. S, and S,, increasedmore or less exponentially with rising temperatureat the so8 surface. The relation can be expressedapproximtie1.y asfollows sR =sRoewt~TJ SRM =sRMoexp(PTd

(1) (2)

where SR, or S’RM,is SR or SR, at r,==O, cyand j? stand for the coefkients of temperature response. Carbon dioxide evolution from the htter (Ao layer), which is assumedto be equal to the decomposition (mineralizing) of the litter, increasedat any soil surfacetemperature(T, ) with increase of water content of litter (V,) and reachedthe maximum when V0wasabout 300% asshown in Fig. 5. This relation can beapproximated successfully(Nakane et al., 19g4) by

K. Nakane i Forest Ecology and Management

0 Temperature

10 on surface

72 (1995) 185-l 97

20 of mineral

191

30 soil (“C)

Fig. 4. Relationship between the soil surface temperature (To) and total (&, A ) or mineral (S,, A ) soil respiration rates observed in the cedar plantation. Regression curves for & and SRM represent Eqs. ( 1) and (2 ) , respectively, the coefficients of which were calculated by the non-linear least squares method. Solid line, cedar plantation (&,=58.6 mg CO2 m-* h-l), (u=O.O86”C--1, &MO= 52.1,/3=0.070). Short dashes, evergreen oak forest in Nara (Sa,,= 104, (u=O.lOl, SW0=94,fi=0.089). Long and short dashes, Japanese red pine forest in Fuchu, Hiroshima Prefecture (SRO= 147, a=0.072, SRMo= 107, @=0.057).

~*=y*Aexp(~TO)[l-(l-VO/V;S)*] (3) whereV* is the relative decompositionrate of litter (mgC02m-*h-l) and z& isv,when To=0 and V,= V;. VE stands for the optimum value of V, for litter decomposition, and 1 ( ‘C- ’ ) is the coefficientof temperatureresponse.From Eq. (3)) we can derive the following equation representingthe respiration rate of the & layer (S,, mgC02m-*h-l) inthefield s RA=S*,,exp(rZTo) [ 1- ( 1- V,/Yt;)*] (4) whereSgAO= Y~~M,-,(Nakane et al., 1984). Fig. 6 showsthe reliability of Eq. (4) for application to field data by comparing the observedand calculated values of S,, where the calculatedvalue was derived from T,, V, and annual averagedM,, observedin the field. Bunnel et al. ( 1977) proposed a similar model repre-

sentingsoil microbial respiration rate as a function of temperatureand moisture content. 4.5. Loss rate of litter (A, layer) and dead roots

The weight of both freshand residuallitters in the bagsplacedon the forest floor in the plot decreasedroughly in an exponentialway with time (Fig. 7). The fresh litter, however, lost weight rapidly (0.863 year- ’ ) during the first 3 months and later the same occurredgradually in the residual litter (0.262 year- ’ ) (Fig. 7). The relative loss rate of deadroots (Ed) measuredby the root bagmethod increasedwith the increasein root diameter, e.g. 0.212 year-’ for $< 1 cm, 0.284 year-’ for l-3 cm and 0.381 year-’ for 3-5 cm. eRof the tine roots (# < 1cm) was somewhat lower than that for litter (A, layer). Judgingfrom the value of eRfor the dead

192

K. Nakane / Fores1 Ecology

lh

I1 995) 185- 19 7

21 “C

l

\

.

50 i-

.

.

N 8

72

\

l

50

and Management

17 “C

I 0

Moisture

content

I 8 Tii

i

. /A -A< ./& I, o- 0 ’ 100 200I 300I

/ 4

I

I

400

500

of litter

v()

(“10)

Fig. 5. Carbon dioxide evolution from the litter (A, layer) (vA) in relation to the moisture content ( V,) at each soil surface temperature (To). Curves were obtained by fitting Eq. (3) to the data. Coefficients of Eq. (3) were calculated by the non-linear least squares method: u& = 6.38 mg CO2 kg-’ h-‘,,&O.l40”C-‘, f’;=3001.

I 12

i---l.16

20

(months)

Fig. 7. Loss rate of A,, layer measured on the floor of the cedar plantation by the litter bag method. The gradient of the regression curves was 0.262 year-’ for residual litter (0 ) and 0.863 year- ’ for fresh litter ( 0 ).

fine roots, the accumulation of dead fine roots (AI,) will reachthe asymptotic value within 1520 years,if the root turnover rateis constantduring that period.

5. Discussion 5.1. Environmental conditions .I

2001

/

0.

150 -

100 -

50 -

i’ 0

0

0’ . I

50

I

I

100 150 200 (ing Cot kg-’ h-‘1

I

250

ObservedA~layarrespMiirate

Fig. 6. Correlation between respiration rates of the A0 layer (S,) observed and those calculated in the cedar plantation usingEq. (4) (r=0.81,P
The pattern of seasonalchangein soil surface temperature (To) in the Japanesecedar~pbtation observedin this study was similar to those observed in the climax evergreen oak forest (Nakane, 1975) and in the 40~year-016 &.xnese red pine forest (Nakane et al., 1984). There v&s a little difference in the annual mean value betweenthe cedarplantation ( 12.7’ C ) and that of the others ( 14.1“C for the evergreenoak and 13.1“C for the pine stands). The annual mean moisture content of the &layer ( V,, 106%) and surface layer in the mineral soil (.V,, 76%) in those obthe cedar plantation was higher v,, 63%-j; servedin the evergreenoak ( V,, and the red pine forests (-V,,9W, V,, 42%).

K. Nakane 1 Forest Ecology and Management

5.2. Amounts andflow

rates of soil carbon

The annual litterfall rate (2.66 t C ha-’ year-‘) obtained in the 35-year-oldcedarplantation was lessthan that (4.12 t C ha- ’ year- ’ ) observedin the 60-year-oldcedarplantation by Andow ( 1970). This difference in the litterfall rate may be due to the abovegroundbiomass,in particular the leaf biomass. The annual rate obtained in this study was also lessthan that in the 40-year-oldpine forest ( 3.62t C ha- ’ year- ’ ) or in the evergreenoak forest (4.11) at a climax stage (Nakane, 1975;Nakane et al., 1984)) but similar to that in 38-48 year old (2.1-3.2 t C ha- ’ year- I ) (Saito, 1981) or 30-year-oldJapanese cypress plantations (2.04-3.05) (Tsutsumi et al., 1983). The broadleaf litterfall occupied only 1.5% of the total leaf litterfall in the cedar plantation, while it correspondedto 2535%of the total in the pine forest (Nakane et al., 1984). The extremely small rate of broadleaflitterfall in the former was due to the scarcity of broadleavedtrees in the understoreybecauseof the low light intensity on forest floor beneathits densecanopy (Katagiri et al., 1985) and cutting the undergrowth at the time when thinning was carried out. The amount of A,, layer (9.4 t C ha-‘) in the cedar plantation is about two-thirds that in the pine forest (Nakane et al., 1984), twice as large as that in the evergreenoak forest (Nakane, 1975) and about three times that in the 30-yearold cypressplantation (Tsutsumi et al., 1985). This suggeststhat the turnover of litter on the forest floor in the cedarplantation was the slowest,becauseof the lower, or almost the same,rate of litter supply than in the other forests. The amount of humus in mineral soil ( 103t C ha- ’ ) in the cedarplantation was twice that in needle (pine) forests (Nakane et al., 1984), and also rather larger than the 80-105 t C ha- * reported for a cypressplantation (Tsutsumi et al., 1985) . The soil respiration rates (&, S,, and S,,) observedin the cedarplantation werenearly half thosein the other natural forestsin the sameclimate zone (Nakane, 1975; Yoneda and Kirita, 1978;Nakane et al., 1984). S, (5.40 t C ha-’ year-’ ) obtained in this study was, however,

72 (1995) 185- 197

193

larger than that (4.30-4.49) observedin a 16year-old Japanesecedar plantation in Kyoto by Simon0 et al. ( 1989), and similar to that (5.25) in a 30-year-oldcypressplantation (Tsutsumi et al., 1985). As reported also by Kirita and Hozumi ( 1969), the rapid loss at initial stageof litter decomposition observedby litter bag method may be due to mainly the reachingof fraction, which is easily extracted by alcohol benzenesolution, in the needle (Tsutsumi et al., 1961). The relative loss rate of the residual litter ( eA= vA+ IC,, 0.262year- ’ ) wascomparablewith that (0.2470.281 year- ’ ) observedin a cedar plantation in Ashu, Kyoto Prefecture by Tsutsumi et al. ( 1961). The value in this study was somewhat higher than that (0.251 year-‘) in the pine forest, despitethe lower soil temperaturein the cedar plantation. It is suggestedthat the transfer factor (&) of humus from litter ( A0 layer) to mineral soil may be larger in the cedar than in the pine forest, becauseof the very smaller rate of A0 layer respiration (S,= VA&&) and value of ll*& in the former than in the latter. The relative loss rate of dead roots (en) increasedwith increasein root diameter, a similar phenomenon was reported in a pine forest by Kawahara ( 1977), but the reversewas also reported in another pine forest (Nakane et al., 1984). The reasonfor this is unclear. eRof the fine roots ($< 1 cm) was somewhatlower than that for the litter (A,, layer). 5.3. Analysis of soil carbon cycling in a compartment model

A compartment model for the analysis of soil carbon cycling (Nakane and Yamamoto, 1983) is shownin Fig. 8. In this model, pools of carbon in various parts of the soil system were tentatively classified into the following four pools: A0 layer (MO), humus in mineral soil (M) , dead roots (freshroot litter, Mr ) , living fine roots (B,). Arrows in the diagram correspondto the flux betweenpools. Eachflux of carbonis labeledasfollows: litterfall (L), supply of humus from A,, layer to mineral soil (1,) and from deadroots to mineral soil ( lR), root turnover ( LR) , total soil

194

K. Nakane /Forest Ecology and Management

i.e. &+=0.5&. LR can also be assumedto be 0.2B, per year (Dahlman and Kucera, 1965; Shaver and Billings, 1975; Nakane, 1978). vR+ KR (eqUa1 t0 CROf fine dead roots Was Observed,and thus vi& can be derived under the assumptions that M, is nearly constant in the mature forest stage and &= va/&= 11.Xirrespectiveof forest type, as in the caseof A0 layer of broad leaves (Kononova, 1968; Nakane, 1980). Therefore,

TotaJ soil respi. s. Litterfall L

AO layer

resp.

‘I A0 layer MO

Snr

Mirwal soil resp.

Decomposition

M

72 (1995) 185- 197

M,=&/(KR+~R)

Decomposi.

and so Trcsof k&?-M Oead

roots M

=crB,/(l/&+l) Root turnover

Fig. 8. Compartmental model of soil carbon cyclingin a forest ecosystem,proposed by Nakane ( 1980) and Nakane and Yamamoto ( 1983) .

respiration (Sa), which includes A0 layer respiration (S,), root respiration (RR) and mineralizing of deadroots and humus in mineral soil. As shown in Fig. 6, the decomposition and transportation processestaken in and on the mineral soil may be assumedas first order reactions asfollows: s RA= VA&

(5)

1, =lC*Mo

(6)

(7) If S,, i* and MOare known, we can obtain v, and IC, from Eqs. (5) and (6 ), and if SaM,RR, vi& and Mare measuredor estimated,~1canbe derived from Eq. (7). As mentioned earlier, L, S,, Z, andMOweremeasuredon annualbasisin the plot. Therefore,the annual mean value of VA and KAcan be obtained. However, the annual mean value of p can be also derived according to the following procedures.R,-can be assumedto be about half of total soil respiration rate (S,) in mature forest ecosystems(Wit& and Frank, 1969; I&a, 1978;Nakane et al., 1983;Beheraet al., 1990), s RM=M+RR+VRM~

(9)

Accordingto the proceduresmentionedabove, the annual cycling of soil carbonin the Japanese cedar plantation was obtained and an&yzed.as shown in Fig. 9 and Table 3, respectively, and valueswere compared with thosein the natural forestsin the sameclimate zone. The relative decompositionrate of the & layer (VA,0.187year- ’ ) was rather lower in the cedar plantation than that (0.205 year- *) in the Japanesered pine forest, this being due to the lower soil surfacetemperaturein the former; the value was only one-third of that ( 0.598year- ’ ) in the evergreenoak forest. The relatively low decomposition rate of the needlelitters Mayobe due to its chemical properties, e.g.comparatively high content of lignin and low content of nitrogen (Tsutsumi, 1956; Wit&, 1966; Fog& and Cromack, 1977;Flanaganand Van Cleve, 1983). The value of v’&,=6.38 mg CO* kg-’ h-l (0.0339 tc tc-’ year-‘) in the cedarplantation proved the lower decomposition rate of cedar than pine litter (v&=8.24: 0.0437), because there was little difference between values for other parameters in Eq. (3). Tsutsumi et al. ( 1961) indicated that l&in and crude protein fractions, which were difficult to decompose,in cedarneedlelitter were decreasedbut im quantitatively .with the progressof .dectxn@osition. This may be the one of reasons.why the cedar needlelitter wasdecomposedso . Such a fermented needlelitter, i.e. humus cont&ng

K. Nakane /Forest Ecology and Management Japanese

Japanese cedar plantation

red pine forest

72 (1995) 185-197 Evergreen forest

195 oak

Fig. 9. Annual cycling of soil carbon in the cedar plantation, compared with those obtained in a Japanese red pine forest on Mt. Takoa, Fuchu, Hiroshima (Nakane et al., 1984) and evergreen oak forest on Mt. Kasuga, Nara City (Nakane, 1975), developed in the same warm-temperate zone, west Japan. The box represents the amount (t C ha- ’ ) and the arrow represents flow (t C ha-’ year-‘).

Table 3 The relative rates of carbon flow (year-‘) 9)

Relative decomposition rate of A,, layer Transfer factor of humus from A0 layer to mineral soil Relative decomposition rate of humus in mineral soil

in terms of the constants and coefficients of the compartmental

models (Figs. 8 and

Symbols

Japanese cedar forest

Japanese red pine forest

Evergreen oak forest

VA

0.187

0.205

0.598

K.4

0.075

0.043

0.296

/J

0.0063

0.0183

0.0192

Data sources: Japanese red pine forests, Nakane et al. ( 1984): evergreen oak forest, Nakane ( 1975).

lignin and crude protein fraction in high content is transferredfrom the A,, layer to mineral soil in the cedarplantation. The transfer of humus from the A0 layer to mineral soil (KA), however,was largerin the cedar plantation than in the pine forest, but was one-quarter that in the evergreenoak forest. Eventually, the loss rate (CR)in the cedar plantation wasnearly equal to that in the pine forest, but was only one-third of that in the evergreen oak forest. Although the supply of humus to the mineral soil (K.&Z&+xRi&) in the cedar plantation was the least of the three forests,the accumulation of carbon in mineral soil (M) in the cedar planta-

tion was more than twice that in the pine forest and also larger than in the evergreenoak forest. This phenomenonmight be causedmainly by the lowest relative decomposition rate of humus in mineral soil (p, 0.0063 year-‘) of the cedar plantation, which was only one-third of the valuesin the pine and evergreenoak forests (0.0183 and 0.0192year-’ respectively).The value in the cedar plantation was comparableto that (0.007 year- ’ ) in the cool-temperatebeech/fir forest (Nakane, 1980) and still lower than that in the pine forest, even if the ratio of RR to SR is assumedto be not 0.5 but 0.4 or 0.3 (seeEq. (7) ). The moisture content in the surfacelayer of mineral soil (V,) was much higher in the cedar

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plantation (annual mean valueof V,, 74%) than thosein the pine (42%) and in the evergreenoak (63%) forests, while the soil surface temperature (To) in the cedar plantation (annual mean of T,, 12.7”C) was slightly lower than those in the other two forests ( 14.1and 13.1“C). Therefore, the extremely slow decomposition of humus in the cedar plantation may be due less to soil environmental condition than to its chemical quality (Tsutsumi et al., 1961) and poor soil microflora and fauna (Hijii, 1987;Nakamura et al., 1991; Inoue, 1992). The quality of humus originating from needle litter may not be improved by the few broadleaf litterfall; the latter are rich in nitrogen (Tsutsumi, 1956;Tsutsumi et al., 1961;Takeda et al., 1987) and morphologically easy for soil animals to attack (Yamamot0 et al., 1992). 6. Conclusion The slow decompositionof A,, layer in both the cedarand pine forestswasowing to the greatresistanceof the leaf litter to decomposition, becauseof its relatively high content of lignin and low content of nitrogen (Tsutsumi, 1956;Wittkamp, 1966). The low amount of broadleaflitter in the cedarplantation wasa result of the lack of broadleavedtrees in the understorey,this being dueto the shadingof the forest floor by the dense canopy.The pure needlelitter in the cedarplantation delayed the decomposition of organic matter not only in the & layer but also in mineral soil strongly.Total carbon,and perhapsother elementssuch as nitrogen and phosphatecould be accumulatedin the soil; however,most might be unavailable for plants. Such a lower cycling system in the cedar plantation than the natural forestsmay affect the productivity of the stand if the rotation of cedar plantation continued at a short interval, for example an interval of 30-40 years, which is common in the plantation-harvestsystemof Cvptomeria jam&u in Japan.To promote the cycling of nutrients in the cedar plantation it is important that cedartreesbe adequatelythinned.The broadleavedtreescangrow naturally under the canopy of cedar if the light

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intensity on the forest floor is relatively high. Acknowledgments We express our sincere thanks to emeritus ProfessorsF. Takahashi and H. Tsubota of Hiroshima University for their encouragement during the courseof this study. We arealsograteful to Drs. T. Okuda and S. Tanakaof Hiroshima University for their assistancein the field works. This study was supportedpartially by the Grantin-Aid for Scientific Researchfrom the Japanese Ministry of Education, Scienceand Culture, No. 58540415. References Andow, M., 1Y70. Litterfall and decomposition in some evergreen coniferous forests. Jpn. J. Ecol.. 20: 170- I8 1. Andow, T., Hachiya, K., Doi, K., Kataoka, H., Kate, Y. and Sakaguchi, K., 1968. Studies on the system of density control of Sugi (Cryptomeriajtzponica) stand. Bull. Gov. For. Exp. Stn., 209: l-76. (In Japanese with English summary. ) Behera, S.K., Joshi, S.K. and Pati, D.P., 1990. Root contribution to total soil metabolism in a tropical forest soil from Orissa, India. For. Ecol. Manage., 36: 125-134. Bunnel, F.L., Tait, D.E.N., Flanagan, P.W. and van Cleve, K., 1977. Microbial respiration and substrate weight loss. II. A model of the influences of chemical composition. Soil Sci., 9: 33-40. Dahlman, R.C. and Kucera, C.L., 1965. Root productivity and turnover in native prairie. Ecology, 46: 84-89. Flanagan, P.W. and van Cleve, K., 1983. Nutrient cycling in relation to decomposition and organic matter quaiity in taiga ecosystems. Can. J. For. Res., 13: 795-8 17. Fogel, R. and Cromack, Jr., K., 1977. Effect of habitat and substrate quality in Douglas fir needIes decomposition in western Oregon. Can. J. Bot., 55: 1632-1640. Haber, W., 1958. okologische Untersuchung der Bodenatmung, mit einer iibersicht tiber frtthere Bearbeitungen, insbesondere deren Methoden. Flora, 146: 109-l 57. Hijii, N., 1987. Seasonal changes in abundance and spatial distribution of the soil arthropods in a Japanese cedar (Cryptomeria japonica D. Don) plantation, with special reference to Collembola and Acarina. Ecol. Res., 2: 159173. Inoue, N., 1992. Estimation of soil microbes-biomass in natural and artificial Japanese cedar forests in Yoshiwa, Hiroshima Prefecture, Japan. M.Sc. Thesis, Hiroshima University, Hiroshima, 125 pp. (In Japanese.) Iwatsubo, G., 1976. Circulation of plant nutrients in forest ecosystem-On the role of rain water in thecircu!ation. In: T. Kato, S.H. Nakano arid T. Umesao (Editors), For-

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