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Soil Biology & Biochemistry 42 (2010) 986e997

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Soil Biology & Biochemistry

journal homepage: www.elsevier .com/locate/soi lbio

Carbon dioxide emissions of soils under pure and mixed stands of beechand spruce, affected by decomposing foliage litter mixtures

Torsten W. Berger a,*, Erich Inselsbacher a, Sophie Zechmeister-Boltenstern b

aDepartment of Forest- and Soil Sciences, Institute of Forest Ecology, University of Natural Resources and Applied Live Sciences (BOKU), Peter Jordan-Straße 82, 1190 Vienna, Austriab Federal Research and Training Centre for Forests, Natural Hazards and Landscape, Department of Forest Ecology and Soil, Seckendorff-Gudent-Weg 8, 1131 Vienna, Austria

a r t i c l e i n f o

Article history:Received 9 October 2009Received in revised form2 January 2010Accepted 18 January 2010Available online 11 March 2010

Keywords:CO2 effluxFagus sylvaticaLitter decompositionMixed species effectsNutrient cyclingPicea abies

* Corresponding author. Tel.: þ43 1 47654 4107; faE-mail address: torsten.berger@boku.ac.at (T.W. B

0038-0717/$ e see front matter � 2010 Elsevier Ltd.doi:10.1016/j.soilbio.2010.02.020

a b s t r a c t

Soil respiration is the largest terrestrial source of CO2 to the atmosphere. In forests, roughly half of thesoil respiration is autotrophic (mainly root respiration) while the remainder is heterotrophic, originatingfrom decomposition of soil organic matter. Decomposition is an important process for cycling of nutri-ents in forest ecosystems. Hence, tree species induced changes may have a great impact on atmosphericCO2 concentrations. Since studies on the combined effects of beechespruce mixtures are very rare, wefirstly measured CO2 emission rates in three adjacent stands of pure spruce (Picea abies), mixed spru-ceebeech and pure beech (Fagus sylvatica) on three base-rich sites (Flysch) and three base-poor sites(Molasse; yielding a total of 18 stands) during two summer periods using the closed chamber method.CO2 emissions were higher on the well-aerated sandy soils on Molasse than on the clayey soils on Flysch,characterized by frequent water logging. Mean CO2 effluxes increased from spruce (41) over the mixed(55) to the beech (59) stands on Molasse, while tree species effects were lower on Flysch (30e35,mixed > beech ¼ spruce; all data in mg CO2eC m�2 h�1). Secondly, we studied decomposition afterfourfold litter manipulations at the 6 mixed species stands: the Oi e and Oe horizons were removed andreplaced by additions of beech e, spruce e and mixed litter of the adjacent pure stands of knownchemical quality and one zero addition (blank) in open rings (20 cm inner diameter), which were coveredwith meshes to exclude fresh litter fall. Mass loss within two years amounted to 61e68% on Flysch and36e44% on Molasse, indicating non-additive mixed species effects (mixed litter showed highest massloss). However, base cation release showed a linear response, increasing from the spruce e over themixed e to the beech litter. The differences in N release (immobilization) resulted in a characteristicconverging trend in C/N ratios for all litter compositions on both bedrocks during decomposition. In thesummers 2006 and 2007 we measured CO2 efflux from these manipulated areas (a closed chamber fitsexactly over such a ring) as field indicator of the microbial activity. Net fluxes (subtracting the so-calledblank values) are considered an indicator of litter induced changes only and increased on both bedrocksfrom the spruce e over the mixed e to the beech litter. According to these measurements, decomposinglitter contributed between 22e32% (Flysch) and 11e28% (Molasse) to total soil respiration, strengtheningits role within the global carbon cycle.

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1. Introduction

Fluxof carbon fromsoils to the atmosphere occurs primarily in theform of CO2 and is the result of soil respiration. Soil respiration is thelargest terrestrial source of CO2 to the atmosphere and currentlyrepresents an annual flux of one magnitude larger than that fromanthropogenic fossil fuel combustion (Raich et al., 2002). Hence,small changes of soil respiration rates could have a large effect on theconcentration of CO2 in the atmosphere (Schlesinger and Andrews,

x: þ43 1 47654 4129.erger).

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2000). In forests, roughly half of the soil respiration is autotrophic(mainly root respiration) while the remainder is heterotrophic,originating from decomposition of soil organic matter (Epron et al.,2001; Bhupinderpal-Singh et al., 2003; Anderson et al., 2005).

Measurements of total soil CO2 emissions together with data onlitter inputs (i.e., leaves, wood, coarse and fine roots) over one ormore growing seasons can be used to evaluate soils as sources orsinks of C over shorter periods according to the following equation(Hanson et al., 2000):

Net soil C increment ¼ Litter inputs�ðsoil respiration� root respirationÞð1Þ

T.W. Berger et al. / Soil Biology & Biochemistry 42 (2010) 986e997 987

CO2 lost from root and rhizosphere activity is tied to theconsumption of organic compounds supplied by above grounds ofplants. Hence, the fraction derived from live roots is independent ofsoil C pools. The difference between soil and root respirationaccording to equation (1) is the C evolved from heterotrophicconsumption (decomposition) of soil C. Rapid decomposition isamong the perceived benefits of a broadleaf component, whichresults in faster nutrient cycling and enhanced productivity (Perryet al., 1987; Prescott et al., 2000). Consequently, the target ofincreasing decomposition rates (i.e., mobilizing the forest floor)may be in conflict with the objective of C sequestration (Jandl et al.,2007).

Decomposition is an important process for cycling of nutrientsin forest ecosystems. Decomposition processes are influenced bymacro- and micro-climate, litter quality, activity of decomposingorganisms and soil nutrient status (Vesterdal, 1999). Replacementof beech by spruce is associated with changes in soil acidity, soilstructure and humus form, which are commonly ascribed to therecalcitrance (e.g., high C/N ratios and lignin concentrations) ofspruce (e.g., Ellenberg et al., 1986; Berger et al., 2006). The forma-tion of thick organic layers in monocultures of spruce is associatedwith reduced tree growth and therefore “hampers forest produc-tivity” (Kazda and Pichler, 1998). Hence, knowing howmuch beechmust be admixed to pure spruce stands in order to increase litterdecomposition, will be of practical relevance for forest manage-ment strategies, since conversion of secondary pure spruce standsto mixed species stands is a current issue in Europe (Spiecker et al.,2004).

Mixing leaves from species with differing resource quality andleaf structure changes the chemical environment and physicallyalters the total litter surface where decomposition is occurring(Hector et al., 2000). These alterations can also affect decomposerabundance and activity (Scheu et al., 2003). Thus, chemical andphysical changes in leafmixtures can influence decomposition ratesboth directly (physically) and indirectly (through the decomposercommunity and its activities). Gartner and Cardon (2004) found 30papers that focus directly on decomposition of mixtures of litters,assessing whether decay rates in species mixtures can be predictedfrom known decay rates of the component litters decaying alone.We have to point out that the term decomposition, used in thisstudy, comprises both mass loss (decay rate) and nutrient release(including nutrient transfers among leaves of different species), notnecessarily linked with each other. E.g., this review by Gartner andCardon (2004) revealed that nutrient transfer among leaves ofdifferent species is striking, with 76% of the mixtures showing non-additive dynamics of nutrient concentrations. In accordance to thecomprehensive work of Wardle et al. (1997) these non-additiveeffects of decomposing mixed litter are difficult to generalize.Whether nutrient transfers within the decomposing litters aremediated by physical (e.g., leaching) or biological (e.g., fungi)means, nutrients released from rapidly decaying, higher qualitylitter can stimulate decay in adjacent, more recalcitrant litters(McTiernan et al., 1997; Sariyildiz et al., 2005) or conversely, leaflitter decay can be slowed by release of inhibitory compounds suchas phenolics and tannins (Fyles and Fyles, 1993; Prescott et al.,2000).

We measured soil CO2 emissions in adjacent stands of purespruce, mixed spruceebeech and pure beech on three base-richsites (Flysch) and three base-poor sites (Molasse; yielding a total of18 stands) and studied decomposition of manipulated littermixtures on the 6 mixed species stands over a two-year period: theOi- and Oe horizons were removed and replaced by additions ofbeech-, spruce- and mixed litter of the adjacent pure stands ofknown chemical quality and one zero addition (blank) in openrings, which were covered with meshes to exclude fresh litter fall.

The decomposition study was accompanied by in situ CO2 effluxmeasurements, enabling a distinction between total soil e andlitter CO2 emission. We addressed the impact of tree speciescomposition by asking the following questions:

1) Does tree species composition affect soil respiration?2) Does admixture of beech to spruce increase mass loss and

nutrient release during decomposition?3) How is litter decomposition related to mass loss, nutrient

release and CO2 efflux?4) Do decomposing spruceebeech litter mixtures reveal additive

or non-additive effects?5) Does different respiration between tree species composition

affect net soil C sequestration?

2. Materials and methods

2.1. Study sites

We selected six sites on the two different bedrocks Flysch andMolasse (3 comparable sites on each bedrock). Beech and sprucewere similarly mixed, before one stand at each site was convertedinto the current pure spruce stand. According to Rothe and Binkley(2001) research could take advantage of the spatial scale at whichtrees interact in the absences of replicated-plot experiments.Hence, for this study we selected mono specific beech stands (5e7canopy dominant trees) within themixed species stands. Individualtrees influence soil properties primarily within the radius of thecanopy (e.g., review by Rhoades, 1997). This is in accordance toLovett et al. (2004), whomeasured N cycling characteristics in smallsingle-species plots (2e3 canopy dominant trees) of five dominanttree species within mixed forests of the same species. The currentdesign with 3 tree species compositions (spruce, mixed, beech) persite and 3 site replicates per bedrock (total of 18 stands) enabledtesting mixed species effects. Nutrient fluxes had been monitoredby Berger et al. (2009a) for the same 18 stands and by Berger et al.(2009b) at the extensively studied sites Kreisbach and Frauscher-eck, which are very typical sites on Flysch and Molasse. Detailedsite information is given by these authors and in Table 1 for all 18plots. Later on, throughout this paper, only means and statistics aregiven for the sites on Flysch (Kreisbach, Grünburg, Schlierbach) andon Molasse (Frauschereck, Bradirn 1 and Bradirn 2). Hence, meanstand characteristics are given in Table 1 as well. Standing timbervolume and dominant tree heights are higher on Flysch, despitea somewhat younger stand age. On average, the stands are locatedon N (Flysch) to W (Molasse) facing slopes. Precipitation isdeclining from the western (Molasse) to the eastern (Flysch) partsof Austria.

The study sites on Flysch are spread throughout Lower andUpper Austria at elevations between 480 and 730 m (Table 1). TheFlysch zone is a narrow strip in the foothills of the NorthernLimestone Alps from west to east throughout the country. Flyschconsists mainly of old tertiary and mesozoic sandstones and clayeymarls. Nutrient release from this bedrock is high and consequentlythe prevalent humus forms are mull (beech and mixed stands) tointermediate types between mull and moder (pure spruce stand),indicating quick turnover of the forest litter layer (usually less than3 cm thickness). Soil parameters (Table 2) indicate nutrient richsoils. All soils of these study sites were classified as pseudogley(Scheffer and Schachtschabel, 1998; FAO classification: stagniccambisol), since horizons with a high fraction of finematerial (loamto clay) cause temporary water logging (stagnation zone atapproximately 40e50 cm soil depth). There are hardly any shrubsand the total cover of the herb layer is between 5% (spruce) and 20%

Table 1Forest stand characteristics of adjacent pure and mixed species stands at the experimental sites on Flysch and on Molasse according to a 1997 survey, modified from Bergeret al. (2009a). Mono specific beech stands (5e7 canopy dominant trees) were selected within themixed species stands. Hence, ha-related stand characteristics are the same forthe mixed and the pure beech stand, except for Kreisbach, where the pure beech stand was large enough.

Site Age(years)

Stems(N ha�1)

Timber volume(m3 ha�1)

Basal area(m2 ha�1)

Dominant treeheight (m)

Elevation(m a.s.l.)

Slope(degrees)

Aspect (degreesthrough Efrom N)

Mean (1971e2000)precipitation(mm)

Coordinates

N E

FLYSCHKreisbachSpruce 53 1012 567 57 27.0 480 11 0.0 850 48�0505000 15�3904600

Mixed 65 976 487 44 27.5 480 11 0.0 850 48�0505000 15�3904900

Beech 65 960 588 47 28.0 480 11 0.0 850 48�0505000 15�3905400

GrünburgSpruce 80 664 717 70 36.0 560 16 337.5 1100 47�5603600 14�1302000

Mixed 86 413 730 46 36.0 600 15 337.5 1100 47�5602100 14�1204300

Beech 86 413 730 46 38.0 600 15 337.5 1100 47�5602100 14�1204400

SchlierbachSpruce 71 574 614 67 35.0 730 16 0.0 1180 47�5602500 14�1101200

Mixed 76 328 679 42 39.5 680 12 0.0 1180 47�5602500 14�1004600

Beech 76 328 679 42 39.0 680 12 0.0 1180 47�5602500 14�1004800

MeanSpruce 68 750 633 65 32.7 590 14 352.5 1043 47�5903700 14�41'2600

Mixed 76 572 632 44 34.3 587 13 352.5 1043 47�59'3200 14�41'0600

Beech 76 567 666 45 35.0 587 13 352.5 1043 47�59'3200 14�41'0900

MOLASSEFrauschereckSpruce 58 1264 432 51 22.0 710 8 292.5 1180 48�0502700 13�1803600

Mixed 89 414 384 42 29.5 700 7 292.5 1180 48�0503300 13�1803900

Beech 89 414 384 42 28.0 690 7 315.0 1180 48�0503500 13�1803600

Bradirn 1Spruce 82 658 415 45 28.0 570 14 247.5 1180 48�0501400 13�1400800

Mixed 77 457 434 45 31.0 610 11 292.5 1180 48�0501700 13�1401400

Beech 77 457 434 45 30.0 610 11 292.5 1180 48�0501800 13�1401400

Bradirn 2Spruce 96 486 442 52 33.0 640 5 247.5 1180 48�0501000 13�1602700

Mixed 108 301 245 25 28.5 640 18 180.0 1180 48�0501000 13�1603400

Beech 108 301 245 25 29.0 640 15 247.5 1180 48�0501100 13�1604200

MeanSpruce 79 803 430 49 27.7 640 9 262.5 1180 48�05'1700 13�16'2400

Mixed 91 391 354 37 29.7 650 12 255.0 1180 48�05'2000 13�16'2900

Beech 91 391 354 37 29.0 647 11 285.0 1180 48�05'2100 13�16'5100

T.W. Berger et al. / Soil Biology & Biochemistry 42 (2010) 986e997988

(beech). The natural forest vegetation of the mixed stands on Flyschis Asperulo odoratae-Fagetum (Mucina et al., 1993).

The study sites on Molasse are located in Upper Austria, ina forested landscape, called Kobernausserwald, at elevationsbetween 570 and 710m (Table 1). Parent material for soil formationare tertiary sediments (so-called “HausruckeKobernausserwald”gravel), which consist mainly of quartz and other siliceous material(granite, gneiss, hornblende schist, pseudotachylite and coloredsandstone). Because of this acidic bedrock with low rates ofnutrient release, the dominant soil types are mainly semi-podzols(Scheffer and Schachtschabel, 1998; intermediate soil type betweencambisol and podzol; FAO classification: dystric cambisol) andpartly podzols. Humus form is acidic moder and the thickness ofthe forest litter layer varies between 5 and 10 cm, indicating slowturnover and accumulation of nutrients. In general, soils onMolasse contain more organic carbon and are more acidic, moresandy and less supplied with nutrients than soils on Flysch(Table 2). There are no shrubs and the total cover of the herb layer is10% (spruce) to 15% (beech). The natural forest vegetation of themixed stands is Luzulo nemorosae-Fagetum (Mucina et al., 1993).

2.2. Soils

The forest floor (O-horizon: Oi þ Oe þ Oa) and mineral soil(0e10 cm) cores were takenwith a core sampler of 70mmdiameter

in summer 2006. There were three distributed replicate samples ateach stand, which were pooled before analysis. The mineral soilwithin each volumetric horizon was separated into the fine soil(sieving, <2 mm) and the coarse fraction (>2 mm). Roots weresorted out by hand.

Soil chemical parameterswere determined by routine proceduresas suggested by Blum et al. (1989) for the standardization of Austriansoil surveys. Samples of forest floor and mineral soil (<2 mm) wereanalyzed for total content of C (LECO SC 444, USA), N (Kjeldahlmethod according to ÖNORM L1082; 2300 Kjeltec Analyzer Unit,Tecator, Sweden), P and S (both after digestion with HNO3/HClO4according to ÖNORM L1085; ICPS, inductive coupled plasma spec-trometry, Optima 3000 XL, Perkin Elmer, USA). Organic carbon (Corg)was calculated as total carbon minus CCaCO3 (Scheibler method:reaction of carbonates with HCl and volumetric determination ofemerging CO2 according to ÖNORM L1084). Calcium,Mg, K, Na, Al, Feand Mnwere measured as total contents after digestion with HNO3/HClO4 in the forest floor and as exchangeable cations (0.1 M BaCl2extract) in the mineral soil by ICPS. Soil acidity was measured as pHwith a glass Ag/AgCl combination electrode with KCl referenceelectrode (10 g soil were mixed with 25 ml of 0.01 M CaCl2 ordeionized H2O, stirred, and the pH was measured next morning30min after stirring again). Elemental stocks were then calculated asthe product of dry (105 �C) fine soil masses (related to area and soildepth) and corresponding element contents.

Table 2Mean soil properties of the forest floor, 0e10 cm mineral soil and top soil (forest floor þ 0e10 cm mineral soil) under the pure and mixed stands of spruce and beech on thebedrocks Flysch andMolasse: total stores of Corg (kg m�2 per horizon) and Cmic, Ntot, P and S (g m�2 per horizon); stores of Ca, Mg, K, Na, Al, Fe and Mn, given as total content inthe forest floor and exchangeable content in the mineral soil (g m�2 per horizon); cation exchange capacity (CEC), sum of base cations (Ca, Mg, K, Na) and sum of acid cations(Al, Fe, Mn, Hþ) in molc m�2 per horizon; base saturation (%); Corg/Ntot ratio; Cmic in percent of Corg.

Site Corg Cmic Ntot P S Ca Mg K Na Al Fe Mn CEC Basecat.

Acidcat.

Basesat.

Corg/Ntot

ratioCmic/Corg

(%)

FLYSCHForest floorSpruce 0.98 11 39 2.8 5.1 17.0 23.0b 30.5b 2.0 118.5b 98.9b 5.7 22.3b 3.6 18.7b 16.3a 25.4 1.2Mixed 0.69 6 29 1.4 3.1 17.6 10.5ab 14.2ab 1.3 57.2ab 43.4ab 2.4 10.9ab 2.2 8.8ab 25.6ab 30.7 1.0Beech 0.39 5 12 0.7 1.3 16.8 4.3a 5.7a 0.4 18.3a 13.3a 1.6 4.2a 1.4 2.8a 32.7b 35.5 1.3

0e10 cmSpruce 2.67b 32 170 24.2 26.0 29.7 2.8 12.9 7.2 58.5 0.0 2.2 9.0 2.4 6.6b 23.7 17.0 1.2aMixed 2.26ab 41 120 22.2 22.1 41.8 3.4 11.8 10.3 51.0 0.0 1.3 8.9 3.1 5.7ab 34.1 19.2 1.8abBeech 2.03a 42 123 24.8 22.3 55.5 4.9 18.9 9.6 33.3 0.0 1.0 7.8 4.1 3.7a 50.2 19.9 2.0b

Top soilSpruce 3.65 44 208 27.0 31.0 46.7 25.8b 43.4 9.3 177.1b 98.9b 7.9 31.3b 6.0 25.3b 19.3 18.4 1.2Mixed 2.94 48 148 23.6 25.2 59.4 14.0ab 26.0 11.6 108.2ab 43.4ab 3.7 19.8ab 5.3 14.5ab 32.1 20.3 1.7Beech 2.42 47 135 25.5 23.6 72.3 9.2a 24.6 10.0 51.6a 13.3a 2.6 12.0a 5.4 6.6a 44.7 20.6 1.9

MOLASSEForest floorSpruce 5.17b 37 206 9.2 26.7 35.7 26.2 34.0 6.5 245.7 239.4 3.2 45.4 5.1 40.3 11.3 25.3b 0.7Mixed 4.70b 40 187 8.8 23.4 36.0 32.8 47.4 6.4 300.0 245.4 3.6 52.7 6.0 46.7 11.7 25.2b 0.9Beech 3.71a 32 172 7.8 20.5 33.7 29.4 42.4 5.0 235.1 187.7 3.3 41.8 5.4 36.4 13.2 21.6a 0.8

0e10 cmSpruce 3.72 36 161 21.9 28.6 1.4 0.5 4.9 4.6 40.5 0.0 0.0 5.0 0.4 4.5 9.1 23.3 1.0Mixed 3.23 41 148 22.5 26.6 0.7 0.6 5.7 4.2 43.7 0.2 0.1 5.3 0.4 4.9 8.1 23.2 1.3Beech 4.64 55 184 32.7 41.1 1.7 1.0 9.8 6.5 58.8 0.1 0.0 7.3 0.7 6.6 9.6 24.4 1.2

Top soilSpruce 8.90 73 366 31.1 55.3 37.1 26.7 38.8 11.1 286.2 239.4 3.2 50.4 5.5 44.9 11.1 24.4 0.8Mixed 7.93 82 335 31.3 50.0 36.7 33.4 53.1 10.6 343.7 245.6 3.6 58.0 6.4 51.6 11.2 23.8 1.0Beech 8.35 87 356 40.5 61.5 35.4 30.4 52.2 11.6 293.9 187.8 3.4 49.0 6.1 42.9 12.6 23.2 1.0

FACTOR BEDROCK (Top soil)Flysch (All) 3.00*** 46*** 164*** 25.4* 26.6*** 59.5(*) 16.3** 31.3* 10.3ns 112.3*** 51.9*** 4.7ns 21.0*** 5.6ns 15.5*** 32.0** 19.8(*) 1.6**Molasse (All) 8.39 81 352 34.3 55.6 36.4 30.2 48.0 11.1 307.9 224.3 3.4 52.5 6.0 46.5 11.7 23.8 1.0

A one-way ANOVA (factor species composition)was performed for each bedrock and horizon separately and results of a Duncanmultiple range test are given only, if differenceswere significant (different letters indicate significant differences, p < 0.05; a represents the lowest mean). Another one-way ANOVA (factor bedrock) was done to test meandifferences between Flysch and Molasse for the top soil; level of significance is shown as: ns: not significant, p > 0.10; (*): p < 0.10; *: p < 0.05; **: p < 0.01; ***: p < 0.001.

T.W. Berger et al. / Soil Biology & Biochemistry 42 (2010) 986e997 989

Microbial C (Cmic) was determined using the chloroform fumiga-tion-extractionmethod according to Schinner et al. (1996). MicrobialC was calculated as the differences in organic C between fumigatedand non-fumigated (control) samples. Two replicates of each sample,2.5 g fresh forest floor or 5 g fresh mineral soil, were fumigated for24 h with ethanol-free chloroform at 25 �C. Subsequently the chlo-roformwas removed by evacuation. Fumigated samples and controlswere extracted with 25 ml 0.5 M K2SO4 and filtered; extracts werekept frozen until analysis. Total dissolved organic carbon wasanalyzed in the extracts with a Shimadzu TOC-5050 Total CarbonAnalyzer, Japan. Non-extracted amounts of microbial C werecompensated for by a correction factor of kEC ¼ 0.35.

2.3. Decomposition

Fresh spruce litter was collected by spreading nets from midSeptember to end of October 2005 under the pure spruce stands. Inaddition, we sampled already microbiologically conditionedcurrent year spruce litter from the forest floor (Oi horizon), whichwas carefully sorted out by hand and admixed (one third) to the netcollected litter (two thirds). Freshly fallen beech litter was collectedunder the pure beech stands from the forest floor end of October2005. Collected foliage litter was dried at 50 �C for 48 h, however,all data given in this paper are related to 105 �C dry weight, esti-mated from subsamples not used for the decomposition study.

End of November 2005, we inserted 15 plastic rings (innerdiameter: 20 cm; height: 12 cm on Flysch, 15 cm on Molasse) downto approximately 5 cm into the mineral soil at each of the six mixedspecies stands. These rings were placed in a randomized blockdesign with three 0.7 � 0.7 m blocks per stand. Each block con-tained 5 rings, one control and the following four litter manipula-tions: the Oi e and Oe horizons were removed and replaced byadditions of beeche, sprucee andmixed (1:1) litter of the adjacentpure stands and one zero addition (blank). Added litter amountswere 62.4 g litter per ring corresponding to an average dry weight(OiþOe) of 1987 gm�2 (2.0 kgm�2), deduced from litter collections(sampling frame of 900 cm2 area) of an earlier study by Berger et al.(2002) for these six sites. Hence, adding the same litter amounts forall sites overestimated natural accumulation of Oi þ Oe on Flysch(1.3 kg m�2) while these additions were below average for Molasse(2.7 kg m�2). All rings, except the controls, were covered withmeshes (1 mm mesh size) to exclude fresh litter fall and openedonly for soil respiration measurements. Litter that fell on themeshes was removed every three weeks. In mid October 2007, justbefore the onset of autumnal litter fall, the litter within all rings,including the control, was carefully separated from the Oa layer andmineral soil (mull humus forms on Flysch do not have an Oa

horizon), respectively. Mass loss (%) was calculated as the differ-ence between dry masses at the beginning and the end of theexperiment. Initial and final contents of Corg, Ntot, P, S, Ca, Mg, K, Na,

T.W. Berger et al. / Soil Biology & Biochemistry 42 (2010) 986e997990

Al, Fe and Mn were analyzed in spruce-, mixed- and beech litter asdescribed for the soil (forest floor) samples above. Nutrient release(g m�2) was estimated as initial content minus final content aftertwo years of decomposition in the field (exactly: 22.5 months).All data given in this paper as release in g m�2 can easily be con-verted to mg g�1 litter by multiplying with the factor 0.5 (exactfactor: ¼ 1000/1987). Area related release seemed more appro-priate for combining decomposition and soil respiration measure-ments within the current research, while mass related release maybe useful for comparing decomposition studies using differentmethods.

2.4. Soil respiration

In addition to the 15 rings inserted down to 5 cm mineral soil atthe mixed species stands (see above) we randomly installed 3 ringsper plot at the adjacent pure beech e and spruce stands withoutany litter manipulations (corresponding to the control at the mixedstand) end of November 2005. During the summers 2006 and 2007we manually measured CO2 emission rates from the soil within therings, using the closed chamber method. Stainless steel cylinders(inner diameter: 20.25 cm, height: 20 cm), fitting exactly over sucha ring, were put 2 cm into the forest soils prior to gas sampling toensure air tightness and the same volumetric conditions at eachsampling date. To guarantee complete mixture of gas within thechambers, we flushed the air 3 times with a syringe (volume:60 ml) through a rubber stopper at the top of the chamber beforetaking the gas samples. Gas samples (20 ml) were then taken at0 and 2 h and transferred into evacuated headspace vials. The vialswere sealed with silicone grease, transported in cooling boxes tothe lab and stored under water at 4 �C until analysis. Gas sampleswere analyzed by automated headspace gas chromatography(Hewlett Packard 5890 Series II, USA), equipped with a FID (flameionization detector) to quantify CO2 (injector: 120 �C, detector:250 �C, oven: 80 �C, carrier gas: He at 10 ml min�1

flow rate). SoilCO2 emission rates were assessed as the linear increase of head-space CO2 concentration over the 2-h closer period and areexpressed as mg CO2eC m�2 h�1.

At each sampling event and site we used 21 chambers (corre-sponding to a total of 21 rings) simultaneously at the three adja-cent stands. In general, we finished all 2-h measurements at thethree stands within 3 h. Each of the 8 samplings was done at foursuccessive days and we always kept to the same time schedule(morning versus afternoon) to enable comparisons within site.CO2 measurements were performed around 1 April, 1 June, 16 Julyand 1 Sept of each summer. Mean temperature of air (1 m aboveeach control ring) and soil (adjacent to each control ring atapproximately 5 cm mineral soil depth) were calculated fromrecords (hand thermometer) at the beginning and the end of each2-h CO2 emission period (N per stand ¼ 3 rings � 2 times ¼ 6).Volumetric soil water content was measured once during a 2-hCO2 emission period (mean of 3 replications per stand) with TraseTDR-systems from Soilmoisture Euqipment Corp. using fixedwaveguides, buried adjacent to each control ring at 0e10 cmmineral soil depth.

2.5. Statistics

Statistical differences of mean soil CO2 emissions (control), air-and soil temperature and soil water contentwere tested by analysesof variances (ANOVA), factor species composition nestedwithin siteeffect, for each bedrock separately (N ¼ 3 sites � 3 species compo-sitions; plots � 1 plot mean of three replications � 4 samplingevents per year; treated as repeated measurements ¼ 36) andresults of multiple pairwise comparisons (Bonferroni adjustment)

are given. Soil CO2 emissions above fourfold manipulated littermixtures were tested by an ANOVA, factor litter composition at themixed stands nested within site effect (N ¼ 3 mixed plots � 4 littercompositions � 1 plot mean of three replications � 4 samplingevents per year; treated as repeated measurements ¼ 48). Nestedterms (species e or litter composition within site) are useful formodelling the effect of a factor (composition) whose values do notinteract with the levels of another factor (site). Similar nesteddesigns (factor litter composition at themixed stands nestedwithinsite effect) were applied to test differences of mass loss and nutrientrelease for each bedrock (N ¼ 3 mixed plots � 3 littercompositions � 3 replications per plot ¼ 27). Mean soil properties(N ¼ 3 species compositions � 3 sites per bedrock ¼ 9) and initialnutrient contents of manipulated litter mixtures per bedrock (N¼ 3mixed plots � 3 litter compositions � 1 replication per plot ¼ 9)were tested by a simple one-way ANOVA (factor composition) andresults of a Duncanmultiple range test are given due to a shortage ofdegrees of freedom for a nested design. In most cases, an additionalone-way ANOVA (factor bedrock) was performed to test differencesbetween the two bedrocks Flysch and Molasse. Bivariate linearregressions were performed between initial nutrient contents andnet nutrient release of exposed litter mixtures at the mixed stands(N per litter mixture and bedrock ¼ 3 mixed plots � 3 replicationsper plot ¼ 9). All statistics were performed with the packageSPSS 15.0 for Windows (Release 6 Sep 2006).

3. Results

3.1. Soils

Nutrient stores and soil properties of the top soil (forestfloor þ 0e10 cm mineral soil; forest floor ¼ Oi þ Oe þ Oa horizons)indicated significant differences between the soils on Flysch andMolasse for all listed parameters (Table 2) except Mn stores andsum of base cations. Due to retarded decomposition on Molasse,forest floor and consequently top soil contents were significantlyhigher for all nutrients except Ca. However, a higher base satura-tion, higher Cmic/Corg and lower Corg/Ntot ratios of the top soilindicated higher nutrient turnover on Flysch. By comparing indi-vidual base cation storages within the 0e10 cm mineral soil, it isjustified to call soils on Flysch base-rich and soils on Molasse base-poor.

As documented elsewhere (Berger et al., 2002, 2004), effects oftree species composition were much more pronounced on soilsformed over Flysch than on Molasse. On Flysch, stand compositionrevealed significant differences in the top layers for C, Al, Fe, sum ofacid cations and base saturation. The C/N ratio in the forest floor onFlysch tended to increase from spruce e over the mixed e to thebeech stand. Mean pH (H2O) at 0e10 cm increased from 4.3(spruce) over 4.7 (mixed) to 5.2 (beech; beech > spruce;mixed ¼ spruce, beech; not shown in Table 2). Spruce sequesteredmore C, Mg and K (compare higher CEC) in the forest floor onFlysch. On Molasse, however, the C/N ratio in the forest floordeclined from spruce to beech (beech < mixed ¼ spruce) since Cstores showed a similar pattern (beech < mixed ¼ spruce). Therewere no other significant differences in soil nutrient contentsamong tree species at Molasse. Mean pH (H20) at 0e10 cmwas thesame for all species compositions (4.1).

3.2. Soil CO2 emissions under the pure and mixed stands

Two-year mean soil CO2 emissions were significantly higher onMolasse (52 mg CO2eC m�2 h�1) than on Flysch (32 mg CO2eCm�2 h�1). Soil water contents were significantly higher on the clayey,frequently water logged, soils on Flysch than on the well-aerated

Table 3Mean soil CO2 emissions (mg CO2eC m�2 h�1) under the pure and mixed stands ofspruce and beech on the bedrocks Flysch and Molasse, measured above the originalOi and Oe horizons (controls: no litter manipulations were performed). Meantemperature (�C) of air (1m above ground) and soil (5 cmmineral soil depth), as wellas soil water content (%; 0e10 cm horizon) are given for four sampling events peryear (AprileSeptember).

Site CO2 Air temp. Soil temp. Soil water

FLYSCH2006Spruce 5.2a I 13.8a I 10.3a I 29.6a IIMixed 5.9a I 14.9a I 10.5a I 29.8a IIBeech 5.4a I 15.0a I 10.7b I 32.5b I

2007Spruce 54.1a II 16.9a II 13.0ab II 23.0a IMixed 63.8b II 17.3b I 13.1a II 23.2a IBeech 54.7a II 17.6b I 13.6b II 29.4b I

2006e2007Spruce 29.7a 15.3a 11.6a 26.3aMixed 34.9b 16.1a 11.8a 26.5aBeech 30.0a 16.3a 12.2b 30.9b

MOLASSE2006Spruce 7.0a I 14.3a I 10.2a I 19.7a IMixed 8.7b I 15.0ab I 10.4b I 23.1b IIBeech 9.1c I 15.1b I 10.4b I 22.1ab I

2007Spruce 75.6a II 16.4a I 12.5ab II 21.1ab IMixed 100.6b II 16.6a I 12.7b II 19.5a IBeech 108.5b II 16.6a I 12.5a II 22.5c I

2006e2007Spruce 41.3a 15.4a 11.4a 20.4aMixed 54.6b 15.8ab 11.6b 21.3abBeech 58.8b 15.8b 11.5ab 22.3b

FACTOR BEDROCK (2006e2007)Flysch (All) 31.5** 15.9ns 11.9ns 27.9***Molasse (All) 51.6 15.7 11.5 21.3

A one-way ANOVA (factor species composition nested within site effect) was doneto test differences of CO2 emissions, air- and soil temperature and soil water contentfor each bedrock separately (N ¼ 3 sites � 3 species compositions; plots � 1 plotmean of three replications � 4 sampling events per year; treated as repeatedmeasurements ¼ 36) and results of multiple pairwise comparisons (Bonferroniadjustment) are given (different letters indicate significant differences, p < 0.05;a represents the lowest mean). Another one-way ANOVA (factor year) was per-formed to test differences between the two study years 2006 and 2007 for eachspecies composition (roman letters indicate significant differences, p < 0.05;I represents the lowest mean). A third one-way ANOVA (factor bedrock) was done totest mean differences between Flysch and Molasse for all species; level of signifi-cance is shown as: ns: not significant, p > 0.10; **: p < 0.01; ***: p < 0.001.

T.W. Berger et al. / Soil Biology & Biochemistry 42 (2010) 986e997 991

sandy soils onMolasse (Table 3). However, air- and soil temperatureswere not different between the bedrocks at the time of monitoring.Bivariate correlations revealed that CO2 efflux was positively corre-lated with air- and soil temperature but negatively with soil watercontent (for all cases p < 0.001; N ¼ 2 bedrocks � 3 sites� 3 speciescompositions; plots � 3 replications per plot � 4 samplingevents � 2 years ¼ 432; not shown in Table 3). The summer period2007 was characterized by significantly higher air- and soiltemperatures and lower soil water contents than 2006, probablycausing huge significant differences of soil CO2 emissions.

Two-year mean soil CO2 emissions increased from spruce (41)over the mixed (55) to the beech (59) stands on Molasse, whiletree species effects were lower on Flysch (30e35,mixed > beech ¼ spruce; all data in mg CO2eC m�2 h�1). Soil CO2emissions of the individual sampling events are plotted in Fig. 1(bar charts, control). Soil CO2 efflux on Molasse increased fromspruce e over the mixed e to the beech stand at all eight eventsexcept 1 Jun 07 (2006: spruce < mixed < beech; 2007:

spruce < mixed ¼ beech). On Flysch, there was a clear trend thatsoil CO2 efflux was highest at the mixed stand (7 out of 8 events,except 1 Jun 06; 2006: ns; 2007: mixed > beech ¼ spruce).

3.3. Decomposition study at the mixed stands

3.3.1. Initial litter qualityInitial nutrient contents of manipulated litter mixtures (exposed

in November 2005) and of the undisturbed controls (originalOi þ Oe horizons sampled in October 2007) at the mixed spruce-beech stands are given in Table 4. Nutrient contents of the mixedspruceebeech litter were exactly between those of the singlecomponent litter (corresponding to a linear response or an additivereaction), since spruce e and beech litter were mixed 1:1(dry weight) in the lab. Base cation contents (Ca, Mg and K)increased from spruce to beech. The acid cations Al (both bedrocks)and Fe (Flysch) were excellent marker of species composition,declining from spruce to beech. A striking feature is the fact thatN litter contents declined from spruce to beech on both bedrocks,although Berger et al. (2009a) measured significantly higherN foliage contents for beech (23.82 vs. 23.89) than for spruce (13.57vs. 13.82) at the same 6 mixed stands (Flysch vs. Molasse; differ-ences were not significant; data in mg g�1). Hence, in accordance toKristensen et al. (2004) and Berger et al. (2009b) retranslocation ofN prior to senescence is higher in beech than in spruce foliage,which seemed to be more efficient on Flysch than on Molasse(spruce litter: Flysch ¼ Molasse; beech litter: Flysch < Molasse;Table 4). Litter differences between the two bedrocks for Ca, Mg, Kand Al matched nutrient contents of the 0e10 cm mineral soil(preferred horizon of nutrient uptake) but were hardly reflected bytop soil nutrient contents (decoupling of nutrient cycling due toaccumulation in the forest floor; compare Table 2). Differencesbetween litter C contents on the two bedrocks were small buthighly significant in accordance to higher soil C stores in acid soilson Molasse. A higher N retranslocation capacity of beech maybe one reason, why the initial C/N ratio of the litter mixturesincreased from spruce e (35 vs. 37) over mixed e (48 vs. 45) tobeech (77 vs. 58) litter (Flysch vs. Molasse; for both bedrocks:spruce < mixed < beech, p < 0.05; compare Fig. 2).

Comparing the actual forest litter (Oi þ Oe horizons) chemistryat the mixed stands (control) with fresh self-made mixed litter(Table 4) gives rough hints on decomposition patterns: contents ofN and Al were higher in the original litter (e.g., caused by microbialimmobilization), while lower contents of all other elementsreflected nutrient release during early stages of decomposition.

3.3.2. Mass lossMass loss within two years amounted to 61e68% on Flysch and

36e44% on Molasse (Table 4). Mixed litter revealed the highestmass losses, indicating trends of non-additive mixed species effects(higher than expected from mass losses of the single-species litter;results of multiple pairwise comparisons are given in Table 4).Higher mass loss of all litter mixtures on Flysch than on Molasse isin accordance with the general fact that nutrient cycling is muchfaster on mull than on moder humus type. At the end of theexperiment, after 22.5 months, the remaining dry masses ofthe appliedmixed litter were in the same range as the drymasses ofthe natural (control) Oi þ Oe horizons (Flysch: 636 vs. 534, differ-ence: ns; Molasse: 1110 vs. 1099, difference: ns; data in mg g�1),indicating that the decomposition study was stopped very close tonatural steady state conditions.

3.3.3. Nutrient releaseNet nutrient release of manipulated litter mixtures at the mixed

stands over two years are given in g m�2 in Table 5. Carbon release

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1 Apr 1 Jun 1 Sep16 Jul 1 Apr 1 Jun 1 Sep16 Jul

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Fig. 1. Bar charts: mean soil CO2 emissions for each sampling event above the original Oi þ Oe horizons (undisturbed controls) at the spruce e, mixed e and beech stands and abovethe blanks (no litter: removed Oi e and Oe horizons) at the mixed stands on Flysch and Molasse (N ¼ 3). Point plots: litter composition/blank ratios of soil CO2 efflux at the mixedstands on Flysch and Molasse. All figures: standard errors were calculated for 3 replications per species.

T.W. Berger et al. / Soil Biology & Biochemistry 42 (2010) 986e997992

showed exactly the same patterns as mass losses (Table 4): themixed litter released the highest C amounts (Flysch: differenceswere ns; Molasse: spruce<mixed; beech¼ spruce, mixed). BesidesC release, only S release was closely related to mass loss. Phos-phorus and N release were greatest from spruce, intermediate frommixed and lowest from beech litter at both bedrocks. Thesesdifferences were probably caused by a relatively higher microbialimmobilization rate of beech litter, as clearly indicated by a nega-tive N release of beech litter on Molasse. The differences inN release (retention) resulted in a characteristic converging trend inC/N ratios for all litter compositions on both bedrocks duringdecomposition (Fig. 2).

Frequently, nutrient immobilization during the early phases ofdecomposition, especially on nutrient poor soils, is followed byrelease of the same nutrient during later phases of decomposition(e.g., Prescott et al., 1993; Albers et al., 2004). Since we did notcollect subsamples of decomposing litter during the study period,

we can only report so-called net nutrient releases after two years.Negative net releases of Al were clear indicators of microbialimmobilization as well. Net release of Ca, Mg and K were signifi-cantly different between litter mixtures on both bedrocks andincreased from spruce e over mixed e to beech litter. Again(see above), Fe was an excellent marker of species composition inaccordance to initial contents of the litter mixtures althoughabsolute differences were very small (see Table 4; note that therounded values 0.1 on Molasse stand for: spruce: 0.139, mixed:0.141, beech 0.143, data in mg g�1).

Linear regressions between initial nutrient contents (mg g�1;independent variable) and net nutrient release (mg g�1 litter;dependent variable) for two-year exposed litter mixtures are givenin Table 6. Expressing nutrient release in mg per g litter and not ing m�2 seemed more appropriate, since then, the same units wereused for the dependent and independent variable. Increasing initialC contents of litter caused decreasing C releases as indicated by

Table 4Initial nutrient contents of manipulated litter mixtures (November 2005) and of the undisturbed controls (original Oi þ Oe horizons) at the mixed spruceebeech stands(mg g�1). Litter manipulations were performed by replacing the natural Oi e and Oe horizons with additions of fresh spruce e, beech e and mixed (1:1) litter of the adjacentpure stands. Initial and final (October 2007) dry masses of litter (g m�2) enabled the calculation of mass loss (%).

Litter mixture C N P S Ca Mg K Na Al Fe Mn Dry massNov 2005

Dry massOct 2007

Mass loss

FLYSCHSpruce 490a 14.1b 0.9b 1.2a 9.7a 0.5a 1.7a 0.1a 0.8c 0.6c 1.4a 1987 777 61aMixed 492a 10.5ab 0.6ab 1.2a 13.5ab 0.9ab 3.5b 0.2a 0.6b 0.5b 1.2a 1987 636 68aBeech 493a 6.9a 0.3a 1.1a 17.3b 1.3b 5.3c 0.2a 0.3a 0.3a 1.1a 1987 674 66a

MOLASSESpruce 508a 14.2b 0.7a 1.0a 3.0a 0.4a 1.7a 0.2a 0.2c 0.1a 0.5a 1987 1269 36aMixed 511a 11.6ab 0.6a 1.0a 5.0a 0.7ab 2.7ab 0.2a 0.2b 0.1a 0.7a 1987 1110 44bBeech 513a 9.1a 0.5a 1.0a 6.9a 0.9b 3.7b 0.1a 0.1a 0.1a 0.9a 1987 1258 37ab

FACTOR BEDROCKSpruce *** ns * * *** * ns ns *** *** ** ns * *Mixed *** ns ns ** *** * ** ns *** *** ** ns * *Beech *** * ns *** *** * *** ns *** *** ns ns ** **

CONTROL (Mixed)Flysch 426 13.5 0.7 1.4 13.6 1.2 2.1 0.2 3.3 0.0 1.4 534Molasse 473 15.9 0.5 1.0 2.7 0.4 0.9 0.1 0.6 0.0 0.5 1099Difference ** ** (*) ns *** *** *** ** *** *** ** **

A one-way ANOVA (factor litter composition) was performed to test initial chemical differences for each bedrock separately and results of a Duncan multiple range test aregiven (N ¼ 3 mixed plots � 3 litter compositions � 1 replication per plot ¼ 9; different letters indicate significant differences, p < 0.05; a represents the lowest mean). Meandifferences between Flysch and Molasse were tested for each manipulated and natural (Control) litter composition by one-way ANOVAs as well (N ¼ 2 bedrocks � 3 mixedplots � 3 litter compositions ¼ 18); level of significance is shown as: ns: not significant, p > 0.10; (*): p < 0.10; *: p < 0.05; **: p < 0.01; ***: p < 0.001. Different mass loss wastested for each bedrock separately by another one-way ANOVA (factor litter composition nested within site effect; N ¼ 3 mixed plots � 3 litter compositions � 3 replicationsper plot ¼ 27) and results of multiple pairwise comparisons (Bonferroni adjustment) are given (different letters indicate significant differences, p < 0.05; a represents thelowest mean).

T.W. Berger et al. / Soil Biology & Biochemistry 42 (2010) 986e997 993

negative slopes. The fact, that C release declined from beech e overmixed e to spruce litter (see slopes, Table 6), supports the theorythat spruce litter contains a higher fraction of recalcitrant C thanbeech litter (see Introduction). For those elements showing netretention (N and Al; see Table 5), regressions were not significant inmost cases. However, for all other nutrients, initial contents of litterwere positively correlated with the release of the correspondingnutrient. It is striking that positive slopes declined from spruce e

over mixed e to beech litter (linear responses of mixed litter) for P,Ca, K and Na, suggesting that the effect of litter quality on nutrientrelease was higher for spruce than for beech. Highly significantslopes for Fe of exactly 1.00 for all litter mixtures revealed that Ferelease is merely a function of initial litter quality (which variedstrongly and typically between species; see above), not effected byother internal or external factors.

3.3.4. Soil CO2 e and litter CO2 emissionsMean soil CO2 e and litter CO2 emissions (mg CO2eC m�2 h�1)

and annual ratio (%) of litter CO2 to soil CO2 are given in Table 7. In all

Fig. 2. Mean initial (Nov 05) and final (Oct 07) C/N ratios of exposed litter mixtures (spruce,plot may show minemax (error bars), 25%e75% (box), median (vertical line) and e in a sin

cases soil CO2 emissions from the no litter (blank) variants weresignificantly lower than from the litter addition variants, indicatingthat litter CO2 can actually be estimated as net CO2 efflux (littermixture additions minus blank). At generally higher soil CO2 effluxrates during the summer 2007 (compare Table 3), soil CO2 emissionsincreased from spruce e over mixed e to beech litter and conse-quently, in this order, the two-year mean litter CO2 emissions. LitterCO2 emissions in percent of soil CO2 emissions were slightly higher(difference: ns) at the base-rich sites on Flysch and amounted to22e32% (mixed litter equals spruce litter). On Molasse, decompos-ing litter contributed 11e28% of total soil respiration (mixed litter:intermediate between spruce e and beech litter).

Although the undisturbed control variant was not part of thelitter manipulations, it is worth noting that the original Oi þ Oehorizons contributed 25% (two-year means) of total soil respirationon both bedrocks. According to Fig. 1 (bar charts) mean CO2 effluxesfrom the no litter variants (mixed blank) were lower than from themixed controls for each individual sampling event on bothbedrocks.

mixed, beech) at the mixed spruceebeech stands on Flysch and Molasse. An individualgle case e extremes (circle).

Table 5Net nutrient release of exposed litter mixtures (g m�2) at the mixed spruceebeech stands on Flysch and Molasse over two years (Nov 2005eOct 2007).

Litter mixture C N P S Ca Mg K Na Al Fe Mn

FLYSCHSpruce 655.1a 16.8c 1.2c 1.3a 12.6a 0.2a 2.0a 0.17a �2.5a 1.269c 1.6bMixed 708.6a 12.2b 0.8b 1.4a 18.1b 1.0b 5.9b 0.22b �1.2a 0.913b 1.6bBeech 702.4a 4.2a 0.2a 1.2a 22.0c 1.4c 9.4c 0.22b �1.6a 0.555a 1.0a

MOLASSESpruce 406.5a 5.4b 0.6b 0.2a 2.4a 0.4a 2.5a 0.13a �0.4b 0.276a 0.4aMixed 499.5b 4.7b 0.6b 0.8b 6.7b 0.9b 4.6b 0.15a �0.8b 0.279b 0.8bBeech 462.3ab �1.6a 0.2a 0.5a 9.8c 1.1c 6.1c 0.11a �2.7a 0.281c 1.0b

FACTOR BEDROCKSpruce * * (*) * ** ns ns ns (*) *** *Mixed * (*) ns (*) ** ns * ns ns *** (*)

Beech ** ns ns ns * ns *** ns ns *** ns

A one-way ANOVA (factor litter composition nested within site effect) was performed to test differences of nutrient release for each bedrock separately and results of multiplepairwise comparisons (Bonferroni adjustment) are given (N ¼ 3 mixed plots � 3 litter compositions � 3 replications per plot ¼ 27; different letters indicate significantdifferences, p < 0.05; a represents the lowest mean). Mean differences between Flysch and Molasse (factor bedrock) were tested for each manipulated litter composition byanother one-way ANOVA (N¼ 2 bedrocks� 3mixed plots� 3 replications per plot¼ 18; level of significance is shown as: ns: not significant, p> 0.10; (*): p< 0.10; *: p< 0.05;**: p < 0.01; ***: p < 0.001).

T.W. Berger et al. / Soil Biology & Biochemistry 42 (2010) 986e997994

Litter composition/blank ratios of soil CO2 efflux at the mixedstands on Flysch and Molasse are plotted in Fig. 1 (point plots).Ratios for the individual sampling events normalize for fluctuationsin absolute numbers and enable comparisons between sites andyears. All ratios were higher than one. Since carbon loss duringdecomposition is accompanied by CO2 production and conse-quently a reduction in C resources, we expected declining ratiosover time, which were visible during the first year on Flysch.On Molasse soils, characterized by retarded decomposition rates,no temporal patterns were found.

4. Discussion

4.1. Does tree species composition affect soil respiration?

Yes, according to our results tree species composition affected soilrespiration. However, patterns on Flysch andMolassewere different:total soil respiration increased from spruce e over mixed e to beechstands on Molasse, while on Flysch, soil CO2 emissions were similarfor spruce e and beech stands and elevated at the mixed stands.

Table 6Linear regressions between initial nutrient contents (mg g�1; independent variable) andmixtures at the mixed spruceebeech stands on Flysch and Molasse.

Litter mixture C N P S Ca

DETERMINATION (r2)FlyschSpruce ns 0.45* 0.59* ns 0.56*Mixed �0.68** 0.80** 0.88*** 0.68** 0.85***Beech ns 0.60* 0.71** 0.40(*) ns

MolasseSpruce �0.47* ns ns �0.84** 0.69**Mixed ns 0.68** 0.93*** ns 0.98***Beech 0.36(*) ns 0.90*** 0.40(*) 0.99***

AllSpruce �0.22(*) ns 0.52** ns 0.72***Mixed �0.32* 0.37** 0.87*** 0.45** 0.68***Beech �0.29* ns 0.70*** 0.44** 0.69***

SLOPE (All)Spruce �5.55* ns 2.01*** ns 0.82***Mixed �5.39* 1.50** 1.90*** 2.15** 0.78***Beech �4.49* ns 1.42*** 2.95** 0.73***

N per litter mixture and bedrock ¼ 3 mixed plots � 3 replications per plot ¼ 9. Level of sisignificant, p > 0.10; (*): p < 0.10; *: p < 0.05; **: p < 0.01; ***: p < 0.001.

Hence, theses results were in accordance with most studies statingthat soil respiration is higher for deciduous than for coniferous treespecies (e.g., review by Raich and Tufekcioglu, 2000). Unfortunately,interpretation of soil respiration data without separation into auto-and heterotrophic respiration is limited. We may roughly concludefrom our data that trends of higher soil respiration at the mixedstands on Flyschweremainly caused by higher root respiration, sincethe highest soil CO2 emissions were coupled with the lowest littercomposition/blank CO2 efflux ratios of the mixed litter (compare barand point charts of Fig. 1; summer 2007). On Molasse, stand speciescomposition and litter species composition showed the same CO2emission trend for spruce, mixed and beech, suggesting that treerelated differences of total soil respiration were mainly driven byheterotrophic respiration in forest floors. In fact, Schmid (2002)measured significantly higher fine root biomasses (live fine roots)at the mixed stand at Kreisbach (Flysch, see Table 1) than at the purestands due to a two-story root development and over-representationoffine roots of beech (mixed: 482> beech: 255¼ spruce: 251; data ing m�2 down to 1 m soil depth). However, at Frauschereck (Molasse,see Table 1), differences of fine root biomasseswere smaller between

net nutrient release (mg g�1 litter; dependent variable) for two-year exposed litter

Mg K Na Al Fe Mn

ns ns �0.54* ns 1.00*** 0.69**0.57* 0.77** 0.86*** 0.48* 1.00*** 0.76**0.56* 0.86*** 0.92*** ns 1.00*** ns

0.70** 0.89*** 0.79** ns 1.00*** 0.79**0.91*** 0.99*** 0.87*** 0.53* 1.00*** 0.88***0.84*** 0.83** ns ns 0.90*** 0.83**

ns 0.59*** 0.68*** �0.23* 1.00*** 0.75***0.48** 0.79*** 0.80*** ns 1.00*** 0.78***0.56*** 0.92*** 0.80*** ns 1.00*** ns

ns 1.35*** 1.32*** �1.96* 1.00*** 0.70***1.31** 1.04*** 1.11*** ns 1.00*** 0.99***1.12*** 1.07*** 1.07*** ns 1.00*** ns

gnificance is shown for the determination of the model (r2) and the slope as: ns: not

Table 7Mean soil CO2e and litter CO2 emissions (mg CO2eCm�2 h�1) and annual ratio (%) oflitter CO2 to soil CO2 at the mixed spruceebeech stands on the bedrocks Flysch andMolasse. Litter manipulations were performed by replacing the natural Oi e and Oe

horizons with additions of fresh spruce -, beech e and mixed (1:1) litter of theadjacent pure stands and one zero addition (blank ¼ no litter). Litter CO2 wasestimated as net CO2 efflux (litter mixture additions minus blank).

Litter mixture SoilCO2

LitterCO2

Litter/soilCO2 (%)

FLYSCH2006Spruce litter 7.8bc 3.1ab 39Mixed litter 7.5c 2.8b 37Beech litter 6.6b 1.8a 28No litter 4.8a

2007Spruce litter 58.6b 11.2a 19Mixed litter 59.3b 11.9a 20Beech litter 70.6c 23.2b 33No litter 47.4a

2006e2007Spruce litter 33.2b 7.1a 22Mixed litter 33.4b 7.3a 22Beech litter 38.6b 12.5a 32No litter 26.1a

MOLASSE2006Spruce litter 8.4bc 1.6ab 18Mixed litter 10.0c 3.1b 31Beech litter 9.5b 2.6a 27No litter 6.9a

2007Spruce litter 83.8b 8.6a 10Mixed litter 94.1c 18.9b 20Beech litter 104.1d 28.8c 28No litter 75.2a

2006e2007Spruce litter 46.1b 5.1a 11Mixed litter 52.0c 11.0b 21Beech litter 56.8c 15.7b 28No litter 41.0a

FACTOR BEDROCK (2006e2007)Spruce litter ** ns nsMixed litter ** ns nsBeech litter ** ns nsNo litter **

A one-way ANOVA (factor litter composition nested within site effect) was done totest differences of litter CO2 e and soil CO2 emissions for each bedrock separately(N¼ 3mixed plots� 3e4 litter compositions� 1 plot mean of three replications� 4sampling events per year; treated as repeated measurements ¼ 36e48) and resultsof multiple pairwise comparisons (Bonferroni adjustment) are given (differentletters indicate significant differences, p < 0.05; a represents the lowest mean).Mean differences between Flysch and Molasse (factor bedrock) were tested for eachmanipulated litter composition by another one-way ANOVA; level of significance isshown as: ns: not significant, p > 0.10; **: p < 0.01.

T.W. Berger et al. / Soil Biology & Biochemistry 42 (2010) 986e997 995

the mixede (535 g m�2) and the spruce stand (432 g m�2; no beechstand was studied by Schmid, 2002).

In addition, we hypothesize that the acidic moder on Molassecauses a relative shift of heterotrophic respiration (decomposition)from the Oi/Oe horizon to the Oa horizon. E.g., assuming a hetero-trophic respiration of 50% of total soil respiration (seeIntroduction), the contribution of the mineral soil þ Oa horizon toheterotrophic respiration amounted 18e28% (beech: 18%, mixed:28%, spruce: 28%,) on Flysch, but 22e39% (beech 22%, mixed: 29%,spruce 39%) on Molasse. Linear relations between Corg stores in thetop soil (Table 2) and two-year mean soil CO2 emissions for all 18stands were positive and significant, yielding higher coefficientsof determination with the forest floor (Oi þ Oe þ Oa; r2 ¼ 0.44,p ¼ 0.002;) than with the 0e10 cm mineral soil (r2 ¼ 0.23,

P ¼ 0.045), strengthening the role of the Oa horizon as CO2 sourceon base-poor sites.

4.2. Does admixture of beech to spruce increase mass lossand nutrient release during decomposition?

i) Mass loss of beech litter was not higher than mass loss ofspruce litter (however, mixed spruceebeech litter showed highermass losses than the corresponding pure species litters).ii) Nutrient release of the base cations (Ca, Mg, K) increased withadmixture (meaning from spruce over mixed to beech) of beech.iii) However, nutrient release of N and P declined with admixture ofbeech (see Tables 4 and 5). The fact, that there are three differentanswers emphasizes the importance of focusing on partialprocesses within the complex patterns of decomposition and onindividual elements.

4.3. How is litter decomposition related to mass loss,nutrient release and CO2 efflux?

Except for C and S, nutrient release of decomposing litter was notrelated to mass loss. The observed net immobilization of Al (Table 5)was in accordancewith Schlesinger (1997), reporting that plant litterappears to absorb Al, perhaps in compounds that are precursors tothe fulvic acids. A net immobilization was also found for N in beechlitter on Molasse. As reported above, differences in N release(immobilization) resulted in a characteristic converging trend in C/Nratios for all litter compositions on both bedrocks during decompo-sition (Fig. 2). In all these cases the C/N ratios decreased, suggestingthat N was limiting decomposition regardless of initial nutrientcontents in accordance to Vesterdal (1999). Berger et al. (2009b)estimated mean residence times (Tn ¼ forest floor content dividedby annual inputs) of individual nutrients within the forest floor forthe stands at Kreisbach (Flysch) and Frauschereck (only spruce andmixed stands; Molasse), which increased in all cases from beech e

over the mixed e to the spruce stands. While in that research turn-over was a function of litter quality and tree species composition(stands), our study on decomposition excluded stand effects byexposing different litter mixtures (and associated qualities) at themixed stands only. Nevertheless, individual residence times accord-ing to Berger et al. (2009b) at the mixed stands only were quitedifferent (Kreisbach: N: 1.9; Ca: 1.6; Mg: 2.2; K: 1.0; Frauschereck:N: 12.0; Ca: 6.0; Mg: 4.6; K: 1.1; data are mean residence times inyears). That is why, we cannot expect that nutrient cycling (e.g., Ca,Mg and K) is solely determined bymass loss, as our data have shown(compare Tables 4 and 5).

There was a not significant two-year trend that during decom-position higher C release of mixed than of beech litter (Table 5) wasaccompanied by lower litter CO2 emissions from mixed than frombeech litter (Table 7). This inverse relationship, more pronouncedon Flysch than on Molasse, indicated that C (mass) loss was notnecessarily coupled with equivalent release of C via CO2. Our onlyexplanation is that this missing C was transferred to deeper soilhorizons. Hence, it may be possible that mixing spruce and beechlitter contributes to increased C sequestration via storage in deepersoil horizons. While forest floor C pool (compare lowest C loss ofspruce litter, Table 5) is more volatile thanmineral soil C and can belost upon changing site conditions, incorporation of C in mineral-associated soil C pool is a long lasting process (DeGryze et al., 2004).

4.4. Do decomposing spruceebeech litter mixtures revealadditive or non-additive effects?

Mass losses of mixed litter revealed non-additive effects (higherthan expected from the pure stands, Table 4). However, net nutrient

T.W. Berger et al. / Soil Biology & Biochemistry 42 (2010) 986e997996

releases (and retentions, respectively; Table 5) of the mixed littershowed linear (additive) reactions as expected from net nutrientreleases of the pure litter fractions (except for C and S, see above).

As stated in the introduction, we were interested to know howmuch beech should be admixed to increase nutrient cycling, sincethe formation of thick organic layers in monocultures of spruce isassociated with reduced tree growth and therefore hampers forestproductivity. According to these results nutrient cycling is speededup proportionally (linear) to admixture of beech. However, wesuggest that the same effect can be reached by a relatively smallerproportion of beech due to another mechanism. Studies by Bergeret al. (2004, 2009b) at the same sites of this study and elsewhere(e.g., Rothe et al., 2003) revealed that admixture of beech improvesnutrition of associated spruce. Significant slopes of linear regres-sions between initial litter nutrient contents and net nutrientrelease declined (except for C increased) from spruce e over mixede to beech litter (linear responses of mixed litter, Table 6) for allnutrients, suggesting that the effect of litter quality on nutrientrelease was higher for spruce than for beech. Consequently,admixture of beech will additionally hasten decomposition viaimpacts on spruce litter in mixed spruceebeech forests. Hence, weconclude that detailed decomposition studies should be seen in thecontext of a larger field scale.

4.5. Does different respiration between tree speciescomposition affect net soil C sequestration?

As already pointed out above, it is concluded that mixing spruceand beech litter may contribute to a net soil C sequestration viatransfer of C into the mineral soil, since the highest C losses ofdecomposing litter were not accompanied by corresponding litterCO2 emissions.

The applied method of separating forest litter and total soilrespiration was successful, since a contribution of 25% from theoriginal Oi þ Oe horizons to total soil respiration on both bedrockswas well within the range of other studies. E.g., soil respirationrates were reduced by 10e20% after removal of the L and Of layer(corresponding to Oi and Oe horizons), and by 30e40% afterremoval of the L and most of the Of and Oh layers (corresponding toOi þ Oe þ Oa) in Picea abies stands of Northeastern Bavaria(Buchmann, 2000). The mean contribution of litter to the total soilrespiration rate at northern hardwood e and aspen forests ofNorthern Wisconsin amounted 9% and 21%, respectively (Martinand Bolstad, 2009). Average contributions of leaves (correspond-ing to Oi þOe) and leaves þ humus (corresponding to Oi þ Oe þ Oa)of a mixed beech-oak stand near Darmstadt, Germany, were 12%(up to 22%) and 16% (up to 25%) of total soil respiration (Dong et al.,1998). Mean CO2 emissions of manipulated forest litter (replacingOi þ Oe horizons) within this research were slightly higher and arenot directly comparable to these data, since the decompositionstudy was started with higher litter additions than ambientconditions (especially on Flysch) but were stopped within thenatural range of forest litter masses.

Due to the fact that we do neither know C litter inputs, nor soilrespiration (on an exact yearly basis), nor root respiration (all termson the right side of equation (1); see Introduction), we cannotanswer this question for the study sites. However, rough estimatesand some conclusions can be drawn from this research.

We tried to estimate net soil C increment as follows. Litter inputsaccording to equation (1) are the sum of above (litter fall) andbelowground (fine roots turnover) litter. Foliage litter fall seemedmore appropriate than total litter fall for short-term calculationswhich were given by Berger et al. (2009b) for Kreisbach (spruce:414, mixed: 343, beech: 270) and Frauschereck (spruce: 223,mixed:247; dry masses in g m�2 year�1). Dry masses of dead fine roots

(necromass in g m�2) were given by Schmid (2002) for theseextensively studied sites as well: Kreisbach: spruce: 191, mixed:372, beech: 84; Frauschereck: spruce: 282, mixed: 188. Assumingthat dead fine roots (d < 2 mm) decompose within one year, thesedatawere good estimates of annual belowground inputs. Total litterinputsweremultipliedwith 0.5 (corresponding to amean C contentof 500mg g�1; compare Table 4), yielding C inputs in g Cm�2 year�1.Missing inputs for the pure beech stand at Frauschereck werelinearly extrapolated from the spruce and the mixed stand(according to Berger et al., 2009b, most mixed species effects atFrauschereck were additive in contradiction to those at Kreisbach).Mean soil CO2 emissions during two summer periods (April toSeptember) at Kreisbach (16.9, 23.5, 16.4) and Frauschereck (36.7,50.1, 49.2 for spruce, mixed- and beech stands; data in mg CO2eCm�2 h�1; note that data in Table 3 are means of three sites perbedrock) were multiplied with the factor 0.46 to yield meanwinterrespirationfluxes. This factorwas deduced frommonthly data (April2002eMarch 2004) for the mixed stand at Kreisbach, given by Pörtl(2005). We are aware that the estimate of yearly soil CO2 emissions(Kreisbach: 1.1e1.5 t C ha�1; Frauschereck: 2.4e3.2 t C ha�1) basedon 8 sampling events is crude. Then, we subtracted half of yearlyrespiration fluxes (C losses) from C inputs, assuming that 50% oftotal soil respiration is heterotrophic, yielding net soil C balances.

Our final results suggest that annual net soil C sequestration atFrauschereck was highest in the sprucee (135), intermediate in themixed e (57) and lowest in the beech (25) stand (data in g C m�2

year�1). At Kreisbach, themixed stand (282) yielded the highest netsoil C increment, followed by the spruce e (249) and beech (125)stand (data in g C m�2 year�1). This rough estimate may help tounderstand why Berger et al. (2002) found higher organic soil Cstores (down to 50 cm) under pure spruce than under adjacentmixed spruce-beech stands on the same two bedrocks, based on soildata of 18 pairs. Since their definition of mixed: “at least 40%broadleaf trees, mainly beech” included also almost pure beechstands, our results at Kreisbach were in accordance to Berger et al.(2002) as well. The mixed stand at Kreisbach showed the highestnet soil C balance since veryhigh soil C inputswere coupledwith lowto moderate soil C losses as already concluded from the decompo-sition study above (CO2 emissions from decomposing foliagemixtures). It seems likely that the estimated net soil C balance waseven higher for the mixed stand at Kreisbach, since due to the highfine root biomass the heterotrophic respiration was probably lessthan 50% (and autotrophic root respiration higher than 50%). At thenutrient poor site Frauschereck, the high fine root necromasses ofspruce coupledwith low respiration losses caused a higher net soil Cbalance for the sprucee than for themixed stand. However, on bothbedrocks, contributions of beech stands to net soil C sequestrationwere lower thanof the corresponding (mixed) spruce stands, sinceClosses were relatively high compared to small C inputs.

5. Conclusions

Since soil respiration is the largest terrestrial source of CO2 to theatmosphere, small changes could have a large effect. Conversion ofsecondary pure spruce stands to mixed stands is a current issue inlarge areas of Europe and our data show that soil respiration isaffected by tree species composition. Hence, we conclude that planteffects on soil respiration play an important role within the globalcarbon cycle. However, net soil C sequestration is primarilycontrolled by C inputs to the soil.

Acknowledgements

This research was supported by the Austrian Science Fund (FWF,project number P18208; granted to T.W. Berger). We thank Anita

T.W. Berger et al. / Soil Biology & Biochemistry 42 (2010) 986e997 997

Gruber, Gerlinde Mistlberger, Michael Pfeffer, Brigitte Schrauf-städter, Monika Sieghardt and Karin Wriessnig for performing thechemical analyses at the laboratories. We thank the forest owners(Lilienfeld Abbey, Kremsmünster Abbey, Austrian Federal Forests)for the possibility to perform this research on their properties.Torsten W. Berger thanks his family, especially his wife Pétra andhis sons Ralf and Joachim for their support in the field during manyextended weekends. Finally, we thank two anonymous reviewersfor their critical comments for the improvement of this paper.

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