REGULAR ARTICLE

Humification processes of needle litters on forest floorsin Japanese cedar (Cryptomeria japonica) and Hinoki cypress(Chamaecyparis obtusa) plantations in Japan

Kenji Ono & Syuntaro Hiradate & Sayaka Morita &

Kenji Ohse & Keizo Hirai

Received: 30 October 2009 /Accepted: 13 April 2010 /Published online: 6 May 2010# Springer Science+Business Media B.V. 2010

Abstract We quantitatively clarified the early humi-fication processes on Japanese cedar and Hinokicypress forest floors by using a litterbag experimentand the solid-state 13C CPMAS NMR technique.There was no significant effect on litter mass lossduring early humification between both coniferouslitters regardless of the shape of their needles. Carboncomposition in both litters showed similar trendsduring early humification. A/O-A as a humificationindex was low, around 0.6, in both litters throughoutthe experiment period although 60% of litter masswas lost. Coniferous litter incubated for 3 years mightnot be well-humified and would be susceptible tophysical fragmentation. Carbon mass loss rates inconifers were in the following order: O-alkyl >aliphatic > aromatic > carbonyl carbons, differingwith hardwoods. Conifers had concomitantly higherand lower mass loss rates of aliphatic and aromaticcarbons than hardwoods. Soil organic carbon (SOC)

accumulated in topsoil for conifers had relatively highand low contents of aliphatic and aromatic carbonsthan that for hardwood. These compositional differ-ences of SOC among forests could be caused by thehigh and low supply rates of aliphatic and aromaticcarbons from litter to topsoil. Consequently, initiallitter nature and humification processes can affect thecompositional qualities of SOC accumulated in soil.

Keywords Solid-state 13C cross polarization magicangle spinning nuclear magnetic resonance(CPMAS NMR) . Litterbag experiment . Earlyhumification processes . Coniferous plantations .

Carbon compositions of humified litter

Introduction

Organic matter fixed by photosynthesis in planttissues is deposited in soils and on soil surfaces,decomposed and utilized by soil organisms, andconverted into humic substances. This humificationprocess is an important biochemical processes for theformation of soil organic matter (SOM) and hasreceived much attention for a long time.

Solid-state 13C cross polarization magic anglespinning magnetic resonance (CPMAS NMR) spec-troscopy is one of the special applications that haveperformed an important role in the characterization ofthe chemical compositional changes in humifiedforest organic materials unlike traditional proximate

Plant Soil (2011) 338:171–181DOI 10.1007/s11104-010-0397-z

Responsible Editor: Rich Conant.

K. Ono (*) :K. HiraiTohoku Research Center,Forestry and Forest Products Research Institute,92-25, Nabeyashiki, Shimokuriyagawa,Morioka, Iwate 020-0123, Japane-mail: don@ffpri.affrc.go.jp

S. Hiradate : S. Morita :K. OhseNational Institute for Agro-Environmental Sciences,3-1-3 Kan-nondai,Tsukuba, Ibaraki 305-8604, Japan

wet-chemical analyses (Fründ and Lüdeman 1991;Kögel-Knabner 1997). These kind of study consis-tently demonstrate that the most decomposablecomponent is O-alkyl carbon while aliphatic carbondecomposes slowly and the changes in aromatic andcarbonyl carbons are less consistent during humifica-tion (Baldock and Preston 1995; Osono et al. 2008).They attribute the rapid decrement of O-alkyl carbonto the preferential biodegradation of plant-derivedpolysaccharides by decomposer organisms (Zech andKögel-Knabner 1994) and the slow decrement ofaliphatic carbon to the preferential resynthesis byfungal mycelium (Hopkins et al. 1997) as well asselective preservation during humification (Theng etal. 1992). Some useful indices to evaluate the degreeof humification and hydrophobicity in plant litters andhumic matters have been proposed (i.e., the ratio ofaliphatic to O-alkyl carbon: Baldock et al. 1997, theratio of hydrophobic to hydrophilic carbon: Spacciniet al. 2000, and aromaticity: Hatcher et al. 1981;Almendros et al. 2000).

Although these studies consistently indicate similaroutlines of compositional changes during humifica-tion regardless of plant species, some studies suggestthat the chemical nature of input from which SOMoriginates could influence the chemical compositionof SOM significantly (Krosshavn et al. 1992; Golchinet al. 1995). Litter compositional quality, which isspecies-dependent, controls the supply and accumu-lation processes of organic matter in soils. Namely, itmay be expected that slower rates of litter mass losswill contribute to a greater accumulation of the litterlayer and that higher rates of litter mass loss mightcontribute to a greater supply of organic matter intomineral soil from the litter. Furthermore, differencesof mass loss rates of litter components can directlyaffect the humification degree and the chemicalquality of SOM. To understand the chemical process-es of SOM accumulation, Ono et al. (2009) conducteda litterbag experiment on a forest floor in a typicalnatural hardwood (Fagus crenata and Quercus crisp-ula) forest in Japan and analyzed their humificationprocesses by applying the solid-state 13C CPMASNMR technique. This study was a first report thatenabled to quantitatively simulate carbon accumula-tion process on the floor of hardwood forest based onOlson’s k values (Olson 1963) of various carbonsobtained by field experiment. Also, it indicated thathardwood forest floor might recover after a few years

from the artificial removal of all litters on A horizon.Humification processes in coniferous forest floorshave not been examined although the chemicalprocesses of litter humification are expected to differbetween conifer and hardwood, despite the fact thathalf of the forest areas, which account for ca. 70% ofland areas in Japan (Forest Agency 2008), are coveredwith conifers (Nakamura and Krestov 2005).

In the present study, we aim to quantitatively clarifythe organic carbon dynamics on forest floors inconiferous plantations of Japanese cedar (Cryptomeriajaponica) and Hinoki cypress (Chamaecyparis obtusa).These species are the most prevalent conifers and havebeen widely planted in Japan for timber products. Weapplied the same methods as described in Ono et al.(2009) and clarified humification processes on bothconiferous forest floors. Then, we compared the data ofconiferous forests obtained in the present study withthe published data of a hardwood forest (Ono et al.2009). This approach allowed us to understand thechanges in the organic carbon composition and massesof coniferous litters during early humification, toevaluate the chemical quality of SOM in coniferousforest floors, and to differentiate the SOM accumula-tion processes in soils between coniferous and hard-wood forests.

Materials and methods

Study area

The study was carried out at Tsukuba (TKB) andTengakura (TGR) experimental sites in Ibaraki prefec-ture in the northern Kanto District, Japan. Both sites arelocated in the Tsukuba Mountains massif (36°11′N,140°12′E, 320 m a.s.l. for TKB, 36°19′N, 140°09′E,260 m a.s.l. for TGR, respectively) and classified asbeing in a temperate zone. TKB is a 99-year-oldJapanese cedar (Cryptomeria japonica) plantation andTGR is a 61-year-old Hinoki cypress (Chamaecyparisobtusa) plantation. Mean annual air temperature andmean annual precipitation taken from 1999 to 2008 atKasama automatic weather station (36°23′N, 140°14′E)located near both sites was 13.7°C and 1,400 mm,respectively (Japan Meteorological Agency: http://www.jma.go.jp/jma/indexe.html). Three plots (size:5×5 m) were settled at each of the sites to conduct alitterbag experiment.

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Preparation of samples of humified litters and L, F,and A1 horizons

The litterbag method (Crossley and Hoglund 1962)was applied to examine the humification processes ofneedle litters on coniferous forest floors. Fresh litter-falls of Japanese cedar and Hinoki cypress collectedusing several large litter traps (2 m×2 m) in TKB andTGR, respectively, in November 2002 were dried inan oven at 40°C overnight and used in the litterbagexperiment. In April 2003, 10 g of each litter were putinto a separate litterbag (1 mm-mesh polyethylenebag, litterbag size: 150 mm×200 mm) and placed onforest floor of each study site. Thereafter, threelitterbags at a time were collected every year: 1 yearlater in April 2004, 2 years in April 2005, and 3 yearsin April 2006. Humified litter fragments in thelitterbags were picked out with tweezers to removesoil particular contamination as carefully as possibleand dried in an oven at 40°C for 48 h beforeweighing. Remnant mass of humified litter was usedto calculate remnant carbon masses.

The present study mainly concerned with biochemi-cal changes in litter materials throughout the early SOMformation processes so the term ‘early humifica-tion’ was introduced to avoid the confusion ofwhole processes of ‘litter decomposition’. Broadlydefined, the term ‘litter decomposition’ includesphysical, chemical and biological mechanisms thattransform plant litters into carbon dioxide andSOM. This broad definition includes physicalfragmentation by wet-dry, shrink-swell, hot-cold, ani-mal, raindrop, and wind as well as biochemical changesby leaching, water transport, oxidation, condensation,and digestion (Berg and McClaugherty 2003).

Soil survey and sampling

A soil survey was carried out near the study plotaccording to Guidelines for soil description (FAO2006). Before making soil pits, the representativenessof surveyed soil profiles regarding the pedologicaland topographical conditions for each site wereconfirmed by multi-point exploratory survey usingreadily hand probe according to Richard et al. (1999).The parent material is crystalline schist in TKB andgranite in TGR (FFPRI 1998a, b). Also, we confirmedthat the thick Quaternary volcanic ash has beenwidely deposited in both sites. Volcanic ash soils are

quite common in Japan and some unique character-istics: thick black soil layer with rich-humus, very lowbulk density, high water retentivity, large reactivealuminum pools. These reactive aluminum poolsoften cause high phosphate retention and humusilluviation in volcanic ash soil due to phosphatesorption and Al-humus complex formation (Kyumaet al. 1993). Thus, it is considered to be important toclarify the mechanism of the humification of organicmatters in volcanic ash soils. The soil was classifiedas a Dystic Cambisol for TKB and a Humic Andosolfor TGR (FAO system; IUSS, ISRIC, FAO 2006).The detailed features of the soil profile were describedin Table 1. The soil horizon samples were taken fromforest floor (L and F horizons) and the top mineralhorizon (A1 horizon) using 100-ml cylinder cores.Fine roots contaminating the collected soil sampleswere removed with tweezers and sieves as carefullyas possible before chemical analyses.

Sample preparation

The samples of humified litters, L, F, and A1 horizonswere air-dried and then sufficiently ground to passthrough a 200-mesh (75-μm mesh) sieve using a high-speed vibrating sample mill (TI-200; CMT Co. Ltd.,Tokyo, Japan). The powdered samples were suppliedfor the analyses of carbon and nitrogen contents andsolid-state 13C CPMAS NMR spectroscopy. For thesolid-state 13C CPMAS NMR measurement, all thesamples were treated with 46% hydrofluoric acid (HF)to remove inorganic minerals as described in Ono et al.(2009).

Measurement of carbon, nitrogen contentsand solid-state 13C CPMAS NMR spectrum

Carbon and nitrogen contents in the samples weredetermined by a dry combustion method (CHNanalyzer 2400 II, Perkin-Elmer, Massachusetts, USA).

The solid-state 13C CPMAS NMR spectra of theHF-treated powder samples were recorded with an FTNMR system (Alpha 300, JEOL, Tokyo, Japan). Theanalytical condition in the present study was the sameas described in Hiradate et al. (2006). A powdersample was transferred into a KEL-F spinning tube(6 mm ϕ, JEOL, Tokyo, Japan), and signals of 13Cwere recorded at 75.45 MHz with magic anglespinning of 6 kHz, at a contact time of 1 ms, and a

Plant Soil (2011) 338:171–181 173

pulse interval of 3 s. A broadening factor of 100 Hzwas employed in the Fourier transform procedure.Chemical shifts were quoted with respect to tetrame-thylsilane but were determined by referring to anexternal sample of adamantane (29.50 ppm). Accord-ing to Skene et al. (1996, 1997), a NMR spectrumwas divided into four chemical shift regions repre-sentative of the major types of carbons present inthese samples: 0–45 ppm (aliphatic carbons), 45–110 ppm (O-alkyl carbons), 110–160 ppm (aromaticcarbons), and 160–190 ppm (carbonyl carbons). Thecontent of each type of carbon was determined basedon the integration of the spectral regions.

To uniformly evaluate the degree of humificationfor the samples of humified litters, L, F, and A1

horizons, the ratio of aliphatic to O-alkyl carbons (A/O-A) were calculated as:

A=O�A ¼ relative area of aliphatic carbon=relative area of O�alkyl carbon:

Calculation of the specific decomposition constants(k values) of respective carbon componentsin humified litters

Remnant masses of respective carbon componentswere calculated by the multiplication between each ofthe carbon proportions and remnant masses ofhumified litters at each humification stage. Thespecific decomposition constants (k values; Olson1963) and the regression coefficients of respectivecarbon components were calculated by approximationof remnant carbon mass with the following exponen-tial function of variables t using the method ofnonlinear least squares:

Wt ¼ W0 � e�k�t

where k is the decomposition constant, W0 is theinitial carbon mass at time zero and Wt is the remnantmass at time t.

Statistical analysis

Carbon, nitrogen contents, the ratio of carbon to nitrogencontents (C/N), and respective carbon compositions inhumified litters were analyzed by two-way ANOVAwiththree field replicates to determine the effects of litterspecies and incubation time. Data of remnant mass lossof humified litters and respective carbon componentswere also analyzed by two-way ANOVAwith three fieldreplicates to determine the effects of litter species andincubation periods. These mass loss data were logtransformed to satisfy the assumption of normal distri-bution prior to analysis. Mean, standard deviation, andcoefficient of variation (CV) were calculated for everysample set. All statistical analysis was performed withJMP software (SAS Institute 2005).

Results

Changes in remnant masses, total carbon and nitrogenconcentrations of humified litters of Japanese cedarand Hinoki cypress during the litterbag experiment

About 65% of litter mass for Japanese cedar and Hinokicypress were lost after 3 years of incubation (Table 2).The mass loss rate did not differ significantly amonglitter species (Table 3) although the mass loss rate of1-year incubated cypress litter was 11% faster than thatof cedar litters (Table 2). Concentration of nitrogen inboth litters increased concomitantly with time, resultingin the decrease of the C/N value from ca. 55 to 25 after

Table 1 Description of horizon, soil color, texture, nitrogen and carbon concentrations, and bulk density of fine soil (BD) in the soilprofile of Tsukuba (TKB) and Tengakura (TGR) experimental sites

Studying site Horizon Thickness Soil color Texture N conc. C conc. C/Ncm g kg−1 g kg−1

TKB L 6–1 14.6 525.6 35.9

(Tsukuba) F 1–0 11.9 327.7 27.6

A1 0–8 brownish black Clay Loam 6.1 106.3 17.3

TGR L 6–4 10.4 554.9 53.4

(Tengakura) F 4–0 17.6 486.7 27.6

A1 0–15 brownish black Clay Loam 6.6 116.7 17.6

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3 years of incubation (Table 2). These C/N values ofboth humified litters were almost equal to those of Fhorizon (27.6) in both forests (Table 1).

Solid-state 13C CPMAS NMR spectra and the carboncompositions of humified litters and surface soilhorizons

The solid-state 13C CPMAS NMR spectral changes ofhumified litters of Japanese cedar and Hinoki cypressshowed similar continuous patterns throughout theincubation period (Fig. 1). Carbon compositionsshowed a similar trend in both coniferous litters andwere approximately constant throughout the experi-mental periods (Fig. 2 and Table 3). The proportions ofcarbonyl and aromatic carbons slightly increasedduring 3 years of incubation (Fig. 2 and Table 3).Although the variation of the carbonyl carbon proportionwas large, and coefficients of variation (CV) ranged from12 to 51%, CV values of the remaining three carboncomponents ranged from 3 to 16% (Data are not shown).The ratios of aliphatic and O-alkyl carbons (A/O-A) forboth litters, which is an indicator of humification degree(Baldock et al. 1997), were relatively stable, around0.5–0.6 during the humification period (Fig. 2 andTable 3). The spectra of L and F horizons at both forestsalso showed the same patterns as the humified litters(Fig. 1). The carbon compositions of L and F horizonshad almost the same trends as those of litterbag-experimented samples (Figs. 2 and 3). Therefore, A/O-A values of L and F horizons at both forests remained atlow levels, ca. 0.4 (Fig. 3). However, the carboncompositions and A/O-A values of A1 horizons differed

with those of L and F horizons and litterbag-experimented samples for both forests (Figs. 2 and 3).

O-alkyl carbon occupied the majority of the totalcarbon in humified litters and L and F horizons: itaccounted for 45–60% of total carbon mass (Figs. 2and 3). However, its proportions in A1 horizon werelow, less than 40% at both forests (Fig. 3). On the otherhand, aliphatic carbon was the opposite to that of O-alkyl carbon: aliphatic carbon proportions in humifiedlitters and L and F horizons were stable at low levelranging from 21 to 31% while those in A1 horizonswere much higher at around 37–40% (Figs. 2 and 3).There was no significant difference of the proportionsof aromatic and carbonyl carbons between humifiedlitters and soils of L, F, and A1 horizons (Figs. 2 and 3).

Decomposability of each carbon componentin the humified litters of Japanese cedarand Hinoki cypress

Masses of all carbon components decreased exponen-tially with humification and their decomposition patternswere similar between Japanese cedar and Hinoki cypress(Fig. 4 and Table 3). For each carbon component,Olson’s decomposition constants (k values) of respec-tive carbon components were calculated as indices ofdecomposability by approximating the data of remnantmass of each component with Olson’s exponentialequation (Table 4). According to these criteria, thedecomposability of each carbon had a similar trendbetween Japanese cedar and Hinoki cypress and was inthe following order: O-alkyl > aliphatic > aromatic >carbonyl carbons.

Table 2 Remnant masses, carbon, nitrogen concentrations, and the ratio of carbon to nitrogen of humified litters of Japanese cedarand Hinoki cypress

Studying site Species Incubation period n Remnant Mass (ash-free) C conc. (ash-free) N conc. (ash-free) C/N

yr % of initial weight SD mg g−1 SD mg g−1 SD SD

TKB cedar 0 2 100.0 – 592.7 – 11.1 – 53.2 –

(Tsukuba) 1 3 62.9 5.9 569.2 9.6 19.9 0.1 28.5 0.3

2 3 51.5 5.8 572.8 7.6 21.1 0.9 27.2 0.7

3 3 33.9 3.7 538.5 1.7 22.0 2.3 24.8 2.7

TGR cypress 0 2 100.0 – 609.4 – 9.9 – 57.4 –

(Tengakura) 1 3 51.8 3.5 576.8 1.4 16.5 1.2 34.6 2.4

2 3 45.5 6.5 572.1 8.9 19.7 1.8 29.1 1.4

3 3 37.5 7.3 559.4 6.6 21.9 2.2 25.7 1.4

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Discussion

Difference of the early humification processeson forest floors between Japanese cedar and Hinokicypress plantations

Litter mass loss process in Hinoki cypress needlesstarted earlier than that in Japanese cedar needlesalthough there was no significant effect of litterspecies on the remnant mass changes in the earlyhumification processes (Tables 1 and 2), probably due

to the shapes of both needles being quite different.The shapes of Hinoki cypress needles are flat- andscaly-shaped thus adhere easily to the surface soilhorizon. In contrast, it takes a long time for Japanesecedar litter to adhere to the surface soil horizonbecause its needles radiate in all directions from thebranch stalks.

Concentration of nitrogen in both litters in-creased concomitantly with time (Table 3). Thisnitrogen concentration increment might be derivedfrom the nitrogen immobilization process by soil

Litter species Incubation periods

d.f. F P d.f. F P

Remnant litter mass 1 0.98 0.33 1 400.3 <0.0001

Carbon conc. 1 5.12 0.04 1 47.8 <0.0001

Nitrogen conc. 1 2.18 0.16 1 55.6 <0.0001

C/N 1 1.52 0.23 1 46.9 <0.0001

Carbonyl C content 1 2.45 0.14 1 6.3 0.02

Aromatic C content 1 3.37 0.09 1 9.2 0.01

O-alkyl C content 1 0.56 0.47 1 3.6 0.07

Aliphatic C content 1 2.01 0.18 1 2.0 0.18

Carbonyl C mass 1 2.24 0.15 1 6.3 0.02

Aromatic C mass 1 1.03 0.33 1 388.5 <0.0001

O-alkyl C mass 1 1.34 0.26 1 189.0 <0.0001

Aliphatic C mass 1 3.04 0.10 1 453.0 <0.0001

A/O-A (humification degree) 1 0.34 0.57 1 0.0 0.88

Table 3 Results of two-wayANOVA to evaluate theinfluences of litter speciesand incubation periods on thechanges in remnant littermasses, carbon and nitrogenconcentrations, the ratios ofcarbon to nitrogen contents(C/N), the contents and theremnant masses of each typeof organic carbon, andhumification degrees inhumified litters

Bold letters indicate signifi-cant differences (P<0.05)

Fig. 1 Solid-state 13CCPMAS NMR spectra ofthe HF-treated samples ofhumified litters, and L, F,A1 horizons at the studyingsites. The Y-axes areadjusted to the same scalefor all spectra

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microorganisms (Hasegawa 2004). Organic carbonproportions of both litters were quite stable through-out the litterbag experiment (Fig. 2 and Table 3)although the masses of both litters decreased morethan 60% of initial litter and these litters were well-decomposed after 3 years (Table 2). The A/O-Avalue, that is an index of the humification degree ofplant litter (Baldock and Preston 1995; Baldock et al.1997), had also been quite stable ranging between0.5 to 0.6 in both litters throughout a 3-yearincubation (Fig. 2). Many previous studies employ-ing solid-state 13C NMR techniques indicated thatthe A/O-A value in humified litters increased withhumification due to the concomitant increase anddecrease of aliphatic and O-alkyl carbons (Baldocket al. 1997; Lorenz et al. 2000; Osono et al. 2008).The results in the present study did not correspond tothe previous study. Thus, the humification degrees of3-year incubated litters for both coniferous litterswould be considered low although no more than40% of initial litter masses remained (Table 2). Itmight be possible that mass losses of Japanese cedarand Hinoki cypress litters without the changes intheir chemical compositions were strongly suscepti-ble to the physical fragmentation by raindrop impact,freeze–thaw, and drenching–drying.

Continuous vertical distributions of soil layerswithout disturbance such as L, F, and A horizons

must be in continuous processes of humification.Thus, the organochemical processes of humificationcan be also understood from the vertical changes oforganic carbon composition in the soil profile. TheA/O-A values for L and F horizons on forest floors ofJapanese cedar and Hinoki cypress were also low,0.5–0.6 (Fig. 3), indicating that litters of L and Fhorizons were not well-humified yet. On the otherhand, the A/O-A values for A1 horizons were high,around 1.0 in both forests (Fig. 3), indicating thatSOM in A1 horizons might be more-humifiedcompared with litters of L and F horizons. This resultmight imply that humification processes in A1

horizons were progressed to further stage of SOMformation interacted with mineral matters of soil,differing with those in litter layers. Compositions ofcarbonyl and aromatic carbons did not changeirrespective of the humification stage (Figs. 2 and 3).

Differences of early humification processesbetween coniferous and hardwood forest floors

Some previous studies pointed out that the vegetationorigin influences the distribution of carbon compo-nents in surface soil horizons, because of thevariations in the chemistry of carbon inputs to soilsand in the nature and the magnitude of decompositionprocesses by decomposer organisms (Krosshavn et al.

Fig. 2 Proportions of car-bon components in the totalcarbon and the humificationdegrees determined bysolid-state 13CPMAS NMRfor humified litters during a3-year incubation. Errorbars are standard deviationsof the mean for the eachcarbon composition

Fig. 3 Proportions of car-bon components in the totalcarbon and the humificationdegrees determined bysolid-state 13C CPMASNMR for the samples of L,F, and A1 horizons. Thesedata for soil samples haveno field replications

Plant Soil (2011) 338:171–181 177

1992; Golchin et al. 1995). To evaluate the influencesof coniferous and hardwood litters, the change inremnant masses of each carbon component during theearly humification was compared using the results ofconiferous forests (present study) and hardwoodforest (Ono et al. 2009). The hardwood forest waslocated 100 km to the north of coniferous experimen-tal site and is classified as being in the same climatezone as coniferous forests (Ono et al. 2009). In thiscomparison, the uncertainty of the hardwood litter 13CNMR analyses were taken as of representative valuesof respective humified litters at simultaneous litterbagcollection because solid-state 13C NMR data in thehardwood forest study had no analytical replicates.

k values of carbon components showed differenttrends between conifers (present study) and hardwoods(Ono et al. 2009). Decomposability of each carbon forconiferous litters were in the following order: O-alkyl >aliphatic > aromatic > carbonyl carbons (Table 4),differing from hardwood litters: O-alkyl > aromatic >aliphatic > carbonyl carbons (Ono et al. 2009). As the

influences of litter species on changes in remnantmasses of each carbon during the early humificationprocesses were analyzed by two-way ANOVA, aromat-ic and aliphatic carbon mass loss rates in the humifiedlitters significantly differed between conifers and hard-woods (Table 5). Namely, the k values for aromaticcarbon in conifers were 0.31–0.34 (Table 4), lower thanthat in hardwoods (0.37–0.40, Ono et al. 2009)probably due to exclusive abundance of chemicallystable guaiacyl lignin in conifers (Nakano et al. 1983;Syafii et al. 1988). Inversely, the k value for aliphaticcarbon in conifers was 0.37–0.43 (Table 4), higher thanthat in hardwoods (0.29, Ono et al. 2009) probablybecause of its quick leaching. These respective lowerand higher mass loss rates of aromatic and aliphaticcarbons of conifers than those of hardwoods mightimply low and high input rates of aromatic andaliphatic carbons into mineral soil from coniferouslitters compared with those of hardwood litters. Resultsof carbon compositions of A1 horizon soils in bothconiferous and hardwood forest floors strongly support

Fig. 4 Remnant mass of each carbon component in litters ofJapanese cedar and Hinoki cypress during 3 years of humification.Remnant mass of each component was calculated by themultiplication between their compositions and remnant litter

masses for each humification stage. Error bars are standarddeviations of the mean for remnant mass of each carboncomponent

Litter species Decomposition constant (k value) (year−1)

(Regression coefficient)

Carbonyl C Aromatic C O-alkyl C Aliphatic C

cedar 0.17 0.31 0.44 0.37

(0.43) (0.90) (0.96) (0.88)

cypress 0.03 0.34 0.46 0.43

(0.12) (0.45) (0.66) (0.92)

Table 4 Specific decompo-sition constants (k) andregression coefficients ofOlson’s exponentialapproximation for eachcarbon in Japanese cedarand Hinoki cypress littersthroughout the 3 year periodof humification

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this suggestion (Fig. 3, Ono et al. 2009). Namely, theproportions of aromatic carbon in coniferous soils (A1

horizon) were 15–16% of total carbon mass (Fig. 3),lower than that in hardwood soils (22%, Ono et al.2009). Aliphatic carbon compositions ranged from 37to 41% in coniferous soils (Fig. 3), higher than that in

hardwood soils (29%, Ono et al. 2009), inversely. Inaddition, a larger content of aliphatic carbons in freshconiferous litters (28–29%, Fig. 2) than that inhardwood litters (20–22%, Ono et al. 2009) also wouldcontribute the high input rate and large accumulation ofaliphatic carbon in A1 horizon soils (Fig. 3). O-alkyland carbonyl carbon mass changes were not signifi-cantly different among the litters (Table 5).

The effects of meteorological conditions on thehumification processes between coniferous and hard-wood forests would be able to be ignored because ofsmaller mass loss rates of carbon components inconiferous forests with higher air temperature andprecipitation than those in hardwood forest (Table 4,Ono et al. 2009).

The differences of early humification processesbetween coniferous and hardwood forests are sum-marized in Fig. 5. Consequently, the present studyindicated that the differences of litter humificationprocesses and initial litter chemistry between conifer-ous and hardwood forests would strongly affect theSOM qualities of A1 horizon.

Table 5 Results of two-way ANOVA to evaluate the influen-ces of the litters (conifers and hardwoods) and incubationperiods on the changes in the remnant masses of each type oforganic carbon during early humification processes. Data ofhardwood litters were cited from Ono et al. (2009)

Litters Incubation periods

d.f. F P d.f. F P

Carbonyl C mass 1 0.04 0.85 1 59.7 <0.0001

Aromatic C mass 1 5.65 0.02 1 612.1 <0.0001

O-alkyl C mass 1 2.53 0.12 1 1293.1 <0.0001

Aliphatic C mass 1 16.63 0.0002 1 723.5 <0.0001

Bold letters indicate significant differences (P<0.05)

Fig. 5 Summary of the differences of humification processesbetween coniferous and hardwood forests. The circle graphsshow proportions of the carbon components in respectivesamples. The present study suggested that concomitantly lowerand higher mass loss rates of aromatic and aliphatic carbons of

coniferous litters might cause relatively lower and higher inputaccumulation of aromatic and aliphatic carbons into A1 soil ofconiferous forests in comparison with hardwood forest. Conse-quently, initial litter chemistry and litter humification processeswould strongly affect the SOM qualities of A1 horizon

Plant Soil (2011) 338:171–181 179

Conclusion

The present study indicated that litter mass loss processin Hinoki cypress needles started earlier than that inJapanese cedar needles probably due to their different-shaped needles although there was no significant effectof litter species on the remnant mass changes throughoutearly humification. Also, the present study suggestedthat humification processes of coniferous litters withoutthe changes in their chemical compositions might besusceptible to physical fragmentation. Therefore, thelitters of L and F horizons in coniferous plantations hadlow humification degrees and the humification degreesof litterbag samples in coniferous forests were also quitestable throughout the experimental periods. Moreover,the present study clarified that early humificationprocesses of coniferous litters differed with those ofhardwood litters due to their qualitative differences.Thus, the differences of early humification processesand initial litter quality between conifers and hardwoodwould strongly affect the organochemical compositionsof the SOM accumulated in A1 horizons. The findingsof the present study provides useful information thatwill help to quantitatively evaluate and compare thechemical changes occurring during SOM formationfrom plant litters among the vegetation.

Acknowledgements We are grateful to Drs. Shinji Kaneko,Masamichi Takahashi, Yojiro Matsuura, Makoto Araki, andEriko Ito for their valuable advice and comments, and we thankMs. Yumiko Okazaki and Teru Notsukidaira for their help withsample preparation and laboratory analysis. We wish to expressour appreciation to the staff of the Department of Forest SiteEnvironment, Tohoku Research Center, and the Arboretum andNursery Office in the Forestry and Forest Products ResearchInstitute. Their advice and assistance were extremely helpfulduring our fieldwork and experiments. This study wassupported in part by a program of the Japanese Ministry ofthe Environment, entitled “Evaluation, Adaptation and Mitiga-tion of Global Warming in Agriculture, Forestry, and Fisheries:Research and Development (A1120)” and the program of theJapanese Ministry of Education, Culture, Sports, Science, andTechnology for Young Scientists, entitled “Study on theproduction process of humic substances in forest soil by notingrecalcitrant lignin compounds (No. 20780122).”

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