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Microbial Processes andFeatures of the Microbiotain Histosols From a BlackAlder (Alnus glutinosa (L.)Gaertn.) ForestOliver Dilly, Hans-Peter Blume, LudgerKappen, Werner L. Kutsch, Ulrike Middelhoff,Jorg Wotzel, Francois Buscot, Klaus Dittert,Hans-Jurgen Bach, Bernhard Mogge, KarinPritsch, Jean Charles MunchPublished online: 29 Oct 2010.

To cite this article: Oliver Dilly, Hans-Peter Blume, Ludger Kappen, WernerL. Kutsch, Ulrike Middelhoff, Jorg Wotzel, Francois Buscot, Klaus Dittert,Hans-Jurgen Bach, Bernhard Mogge, Karin Pritsch, Jean Charles Munch (1999)Microbial Processes and Features of the Microbiota in Histosols From a BlackAlder (Alnus glutinosa (L.) Gaertn.) Forest, Geomicrobiology Journal, 16:1,65-78, DOI: 10.1080/014904599270758

To link to this article: http://dx.doi.org/10.1080/014904599270758

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Microbial Processes and Features of theMicrobiota in Histosols From a Black Alder

(Alnus glutinosa (L.) Gaertn.) Forest

OLIVER DILLYHANS-PETER BLUMELUDGER KAPPENWERNER L. KUTSCHULRIKE MIDDELHOFFJ ÈORG W ÈOTZEL

ÈOkologie-Zentrum

Universit Èat KielKiel, Germany

FRANCË OIS BUSCOT

Institut f Èur Ern Èahrung und UmweltUniversit Èat Jena

Jena, Germany

KLAUS DITTERT

Institut f Èur P¯ anzenern Èahrung und Bodenkunde

Universit Èat Kiel

Kiel, Germany

HANS-J ÈURGEN BACHBERNHARD MOGGEKARIN PRITSCHJEAN CHARLES MUNCH

Institut f Èur Boden Èokologie

GSF±Forschungszentrum f Èur Umwelt und GesundheitNeuherberg, Germany

Microbiological features and in situ microbial activities were analyzed in soils at ablack alder forest adjacent to the eutrophic Lake Belau during the course of the in-terdisciplinary program, ª Ecosystem Research in the Bornh Èoved Lake District.º The

Received 13 January 1998; accepted 24 September 1998.

We thank Elke Erlebach, Friederike Sch Èutze, J Èorn Sprenger, Mirsad Haskovic, Cathrin Schmidt, Anke

Buckenauer, Birgit Vogt for their excellent technical assistance; Drs. P. Weppen, O. Heinemeyer, E.-A. Kaiser, and

T.-H. Anderson (FAL, Braunschweig) for the use of laboratory facilities; Dr. U. Schleuû for helpful discussion,

and Nancy A. Weider-Zehrbach for the improvement of the English. These studies were supported by the German

Ministry of Education, Science, Research and Technology (BMBF), project no. 0339077E, and the state of

Schleswig-Holstein.

Address correspondence to Oliver Dilly, ÈOkologie-Zentrum, Universit Èat Kiel, Schauenburgerstraû e 112,

24118 Kiel, Germany. E-mail: oliver@pz-oekosys.uni-kiel.de

Geomicrobiology Journal, 16:65±78, 1999

Copyright C° 1999 Taylor & Francis

0149-0451/99 $12.00 + .00 65

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66 O. Dilly et al.

microbiological data were combined to evaluate the functional status of the Histosols.It was hypothesized that carbon accumulation typical for Histosols would mainly takeplace at the wet part (ª wet siteº ) close to the lake shore and not at the drier part(ª dry siteº ) of the forest. Rates of leaf litter decomposition, in situ soil C mineraliza-tion, and in situ N2-® xation were higher at the wet site. Furthermore, the compositionof the bacterial communities and the presence of ectomycorrhizas indicated suf® cientO2 availability and high microbial vitality in the soil at the wet site. An anthropogeniclowering of the lake water table during the 1930s seems still to control the actual soilconditions, resulting in humus degradation in the two Histosols of the forest. The twosoils clearly differed in productivity and C and N cycling, being controlled either byupland, acid runoff or by eutrophic lake water. Lake water seems to buffer but also tointensify microbial transformationsat the wet site and to supply nutrients, although hu-mus decay may possiblybe deceleratedby a temporarilyhigh water table and refractoryhumic substances.

Keywords blackalder forest,carboncycling,Histosol,microbialcommunity,nitrogen

cycling

Black alder (Alnus glutinosa (L.) Gaertn.) forests are widespread throughout Northern

Germany, occurring naturally near rivers and lakes mostly on soils with high organic carbon

contents. This type of ecosystem was selected for study because human impact regulating

the water table severely controls the structure of wetland ecosystems.Thus, this system ® lled

a signi® cant and distinct role during the interdisciplinary program, ª Ecosystem Research

in the Bornh Èoved Lake District,º which aims to analyze and model structures, dynamics,and functions of terrestrial and limnic ecosystems. Nineteen subprojects have been carried

out at the alder forests by 11 German research groups.

Histosols are soil types with net C accumulation over long periods attributable to

retarded biological degradation of assimilated C caused by high water levels. However, the

lake water table may not be the only indicator for estimatingnet C accumulationin soil sincemicrobiologicalprocesses are also affected by proton concentration,mineral nutrient status,

and vegetation type. Because whole soils, and their constituentparts, should re¯ ect general

properties of ecosystems (Elliott 1994), the broad spectra of microbiological data obtained

during the main research period from 1988 to 1995 were combined to elucidate the actual

functionalstate with reference to net C accumulationof the Histosols at the alder forest. Bothsoil biochemistry and the structure of the soil microbiota were considered and completed

by system theory because the data of every component represent only particular aspects,

may be restricted by the methodology, and, therefore, should not be applied separately for

drawing general conclusions.

Materials and Methods

Site and Soils

The research site is located 30 km south of Kiel in Schleswig-Holstein, Northern Germany

(54±060 N, 10±14 0 E; Figure 1). The landscape, formed during the Pleistocene, consists of

morainic hills and lakes. The climate is in¯ uenced by both the North Sea and the BalticSea. Long-term (1951 to 1980) mean annual total precipitation was 697 mm, and average

annual air temperature was 8.1±C, according to the local meteorological stations.

Along a transect from a kames hill to Lake Belau, a catena running west to east was

established with a sequence of forests: a beech forest (Fagus sylvatica), a sloping mixed

forest, and the black alder forest at the bottom of the catena (Figure 1). The soils of thebeech and the mixed forest were predominantly acidic and sandy; the soils from the black

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TABLE 1 Properties of the topsoils of the black alder forest along Lake Belau in the

Bornh Èoved Lake district of Northern Germany

Horizon Depth pHa Corga Cks

b C/Na C/Nksb C/NRoot

c

(cm) (H2O) [mg g ¡ 1 dry soil] (w/w)

Dry site H 0±20 4.1 275 0.35 15 4 36

Wet site H 0±20 6.0 230 0.18 15 16 32

aFrom Dilly and Munch (1995).bks, potassium sulfate-soluble C compounds of soil (control values of fumigation-extraction

method; sampling October 1994).cFrom Wachendorf et al. (1997).

alder had high organic matter content. The black alder forest contained single specimens of

beechand oak (Quercus roburL.) and some shrubs,mainlyhazel (CorylusavellanaL.). Twosites were separated for the investigations in the alder forest: The ª dry siteº was situated at

the bottomof the slope, and the ª wet siteº was connectedby a reed belt (PhragmitesaustralisTrin. ex Steud.) to Lake Belau. Soil properties of the two Histosols are presented in Table 1.

According to the FAO (1988), soils were classi® ed as Fibric Histosols with dystrophic and

eutrophic conditions at the dry and wet site, respectively. The organic horizons, L and Of,

could easily be differentiated, whereas underlying horizons (topsoil, up to 20 cm deep)could be separated extensively (Wachendorf 1996) and showed a high spatial heterogeneity

(Figure 1).

Sampling and Methods

In situ CO2 emission rates were determined between 15 May and 15 October 1992 with

a continuous-¯ ow inverted-box system and an infrared gas analyzer (Kutsch 1996). Four

boxes were used in parallel, covering an area of 16 £ 12.5 cm = 200 cm2 and a volume

of » 2800 cm3. Each box was equipped with 2 Pt100 temperature sensors. Concentrationsof CO2 and mean Pt100 signals at each box were recorded at least 3 times per hour. Data

were integrated for the measurement period with a model that is dependent on temperature

(Kutsch and Kappen 1997). After ® nishing CO2 measurements in October 1992, the mass

of roots with diameter <5 mm and of nodules below the chambers as deep as 20 cm of soil

was determined. More detailed investigations on structural properties and growth rates ofroots were carried out by using the methods by B Èohm (1979) and Flower-Ellis and Persson

(1980), which had been adapted for our purposes: Soil monoliths measuring 70 cm long

and 10 cm in diameter were sampled during February 1992. Thereafter, root growth was

measured by the ingrowth-core method, in which the holes of the sampling in February

are re® lled with a peat±sand mixture in a nylon net of 4-mm mesh size and the cores are

harvested throughout the year. Monoliths were also taken from the Of horizon, where theroots were apparently concentrated.

In situ N2-® xation was estimated for 1989 to 1991 based on ® eld data of the active

volume of nodules (peripheral part of the nodules), the 15N natural abundance method, and

the acetylene-reduction method, i.e., incubating nodules in a PVC cuvette in the presence

of 10% (v/v) C2H2 for a maximum of 0.5 h (Dittert 1992).In situ N2O emission rates were measured with the ª closed-soil-coverº system

(Hutchinsonand Mosier 1981)on 2 consecutivedays in December1994 with use of 6 closed

tubes covering an area of 707 cm2 and a volume of » 16 000 cm3. Sampling was done after

maximum enrichmentof 1.5 h. The gas was analyzedaccording to Heinemeyer et al. (1991).

Microbiologicalfeatures were measured in soil samples taken according to the internalsampling schedule (every 4 weeks) started on 26 April 1988. Unless otherwise noted,means

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Microbiota in Histosols From a Black Alder Forest 69

of all sampling between May and October 1992 (6 samplings) were taken and summarized

for the period.Microbiological features of the litter were derived from the experiment with leaf litter

described in detail by Dilly and Munch (1996), in which microbial biomass content was es-

timated by using substrate-induced respiration, and basal respiration rates were determined

at 22±C both without modifying the water content ( ¡ ) and after adding deionizedwater (+ )

to a maximum of 2.5 g of H2O per gram of dry litter.

For analysis of the topsoils,multiple cores were taken at each site with a drill and mixedtogether,gentlysieved, stored at 4±C, and analyzedwithin4 weeks. The fraction<2 mm was

used. Soil microbial biomass content was estimated by the fumigation±extraction method

(Vance et al. 1987), applying a conversion factor kEC of 0.38 (kEC = 1/ 2.64 = 0.38,

where microbial biomass (Cmic) = (lysable) microbial C/ kEC). The fumigation±extraction

method was used for the estimation of microbial biomass C because it is the most reli-able technique for use with these at least seasonally waterlogged soils. For a more de-

tailed description, see Dilly and Munch (1995). Cmic/Nmic ratios in extracts were analyzed

with an automated TOC/TNb analyzer (Maihak, Hamburg, Germany). The basal respira-

tion rate was determined at 22±C by using the apparatus described by Heinemeyer et al.

(1989). Soil samples were analyzed at ® eld water content after preconditioning for 3 daysin the laboratory, and the mean respiration rate after 15 to 24 h was calculated. The mi-

crobial metabolic quotient, qCO2, was determined by dividing the basal respiration rate

(mg CO2-C L ¡ 1 dry soil h ¡ 1) by the microbial C content (g Cmic L ¡ 1 dry soil).

Arginine ammoni® cation was measured according to Alef and Kleiner (1987), with

minor modi® cations as described by Dilly and Munch (1995). Protease activity was de-

termined according to Ladd and Butler (1972) as modi® ed by Dilly and Munch (1996):Fresh soil (1 g, instead of the 0.4 g of litter used) was incubated with 5 ml of buffer (pH

8.1) and 5 ml of casein (Sigma C 8654) suspension. The reaction was stopped with 5 ml

of trichloroacetate solution. Filtrate (3 ml instead of 1 ml) was colored by mixing with the

Folin reagent.

The bacterial communities were isolated by spreading appropriate diluted soil suspen-sion onto complex oligotrophic agar media containing 0.25 g of standard-I-nutrient broth,

5.94 g of NaCl, and 0.1 g of cycloheximideper liter at pH 6.0 in October 1992, March 1993,

and October 1993. Media containing predominantlygelatin, xylan, and CM cellulose as the

source for C were also used (Bach 1996).

Diversity and vitality of ectomycorrhizas of monthly sampled material of the Of hori-

zon to as deep as 5 cm below the surface, where the roots are concentrated, were investi-gated during 1993 and 1994 by using the methods described in detail by Pritsch (1996).

Ectomycorrhizal morphotypes were distinguished by their macroscopic appearance and

were grouped into clusters of similar morphology, i.e., color and surface structure of the

mantle. The vitality of the ectomycorrhizas was determined by vital staining with ¯ uores-

cein diacetate100-l m-thick longitudinalsectionsof a representativeportionof mycorrhizalroots after the methodof Ritter et al. (1986),modi® ed by Pritsch (1996).Analyses were done

at least in triplicate. The 95% con® dence limit was used to isolate signi® cant differences

(t-test).

Results

C Cycling

During a 5-month period (May±October 1992) in situ CO2 emission rates (CO2-C) were0.5 kg m2 at the dry site and 1.4 kg m2 at the wet site. A similar trend was observed

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for the root mass directly below the CO2 measuring system (dry weight including root

nodules, to a depth of 20 cm), which was 425 and 976 g of dry roots per meter2 at thedry and wet sites, respectively (Figure 3). In agreement to this ® nding, more detailed

investigations showed higher amounts and growth rates of roots in the soil at the wet site

(Table 2). Root growth rates were higher at the dry site only in soil depths between 10 and

70 cm.

During the period from May to October, the microbial C content was signi® cantly

higher in leaf litter and topsoil at the dry site than at the wet site (Figures 4 and 5). Incontrast, basal respiration rates and qCO2 did not differ signi® cantly between the two sites.

However, respiration rates and qCO2 of the leaf litter that had ® eld moisture content were

signi® cantly lower than basal respiration rates at the two sites. The basal respiration rates

were slightly higher in the leaf litter at the dry site than at the wet site, whereas the opposite

was observed for data under ® eld moisture content. The qCO2 was slightly higher in thetopsoil at the wet site.

N Cycling

N2 ® xation by the Alnus±Frankia symbiosis was lower at the dry site than at the wet site(Figure 2). The respective amounts, as determined from 15N natural abundance,were 40±45

and 70±85 kg N ha ¡ 1 a ¡ 1 (Dittert 1992).

In situ N2O emission rates from soil in the alder forest were higher than those from soil

in the nearby beech forest (Mogge et al. 1998). Within the alder forest, N2O emission rates

varied signi® cantly, being highest at that site reported by Mogge et al. (1998). The emissionrates at that site (near the sloping mixed forest), at the dry site, and at the wet site were 1.03,

0.66, and 0.09 mg N2O-N m ¡ 2 d ¡ 1, respectively. This estimation was made only once and

emission rates may change during the year. However, a similar pattern was also determined

at Lake Belau by Rusch (1996). The modeled denitri® cation losses of 60 kg (N) ha¡ 1 a¡ 1

were similar at the two sites (Wetzel et al. 1996).

The Cmic/Nmic ratio differed slightly between the dry and wet sites (Figure 5). Arginineammoni® cation rates were slightly higher in the soil at the wet site during the observa-

tion period, whereas protease activity was signi® cantly higher in the topsoil at this site

(Figure 6).

Structure of the Microbiota

The dry site was not investigated for bacteria. The dominant culturable bacteria isolated

on complex heterotrophic media from the topsoil at the wet site were Pseudomonas,Flavobacterium, Cytophaga, Alcaligenes, Arthrobacter, Promicromonospora, and other

unidenti® ed organisms. With use of selective media, Pseudomonas ¯ uorescens biotypeswere found on gelatin medium (proteolysis); actinomycetes, Bacillus, and Favobacterium /Cytophaga were found on xylan-containing medium; and Cellulomonas, Pseudomonas,and nocardioformes were found on cellulose-containingmedium (Bach 1996).

The roots of the alder trees were extensively colonized by ectomycorrhizal but not by

vescicular±arbuscular mycorrhizal fungi. The vitality of mycorrhizal rootlets was generallylower at the dry site. In addition, the potential for root regeneration was obviously higher

at the wet site and was particularly evident during the rapid root growth in spring. The total

length of the mycorrhizal roots was threefold longer at the wet site than at the dry site. The

diversity of mycorrhizal types was reduced at the dry site.

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FIGURE 3 Mean in situ CO2 emission and root mass at the wet and dry sites of the alder

forest for the growth period from May to October 1992 (bars extend to the 95% con® dence

limits).

Discussion

C Cycling

A high soil water table suggests retarded mineralization rates and, consequently, the forma-

tion of peat. However, in situ CO2 emission rates were higher at the wet site, thus indicating

a higher loss of C despite the high water table. In addition, rates of CH4 emission were also

higher near the lake (Rusch 1996). Emission rates of both CO2 and CH4 indicated higher C

loss at the wet site. The higher respiration rates were connected with a greater root biomass.If we assume a constant respiration rate per unit of root biomass, a considerable gap in the

C budget becomes evident that can be explained by higher rates of humus decay (Table 3).

The C may derive from the higher litter input (Wachendorf et al. 1997) and the more rapid

decomposition of leaf litter (Dilly and Munch 1996). As observed by Prescott (1996), the

faunal activity also strongly affected litter decomposition in the experiment of Wachendorfet al. (1997). Displacement of litter by the soil macrofauna into deeper horizons, as indi-

cated by Wachendorf et al. (1997), cannot be excluded at the wet site, but decomposition

of alder leaf litter was also rapid in the adjacent littoral zone, with <10% of the original

C remaining after 32 days (Gessner et al. 1996). The rates of leaf litter decomposition at

the alder forest sites were much higher than in the neighboring beech forest (Irmler 1996).

TABLE 2 Root biomass [g dry roots m ¡ 2] during the growing season at the dry and wetsites of the alder forest along Lake Belau in the Bornh Èoved Lake district of Northern

Germany

Fraction Dry siteb Wet site

Fine roots of alder 170a 390bAll roots, 0±70 cm 530a 1440b

Root growth, Of±horizon 467a 863b

Root growth, ingrowth-core; 0±70 cma 182a 322b

Root growth, ingrowth-core; 10±70 cma 84a 55b

aFor the 1992 growing season.bDifferent letters indicate signi® cant differences when comparing data from dry and wet sites

(P < 0.05).

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Microbiota in Histosols From a Black Alder Forest 73

FIGURE 4 Microbial biomass content (Cmic), basal respiration rate (BAS), and metabolic

quotient (qCO2) of leaf litter during decomposition period from May to October 1992 at

the wet and dry sites of the alder forest (+ , suf® cient with >7.5 g (H2O) g ¡ 1 dry litter,

n = 6; ¡ , at ® eld moisture content, n = 4. Different letters indicate signi® cant differences

for applied t-test, P < 0.05).

FIGURE 5 Microbialbiomass content (Cmic), Cmic/Nmic ratio, basal respiration rate (BAS),

and metabolic quotient (qCO2) of topsoils at the wet and dry sites of the alder forest for

the growth period from May to October 1992 (n = 6; different letters indicate signi® cant

differences for applied t-test, P < 0.05).

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FIGURE 6 Arginine-ammoni® cation rates (ARG) and protease activities(PRO) of topsoils

at the wet and dry sites of the alder forest for the growth period from May to October 1992(n = 6; different letters indicate signi® cant differences for applied t-test, P < 0.05).

Microbial basal respiration rates did slightly but not signi® cantly differ in litter and top-soil at the dry and the wet sites, respectively (Dilly and Munch 1996), whereas microbial

biomass C in the two topsoils differed (Dilly and Munch 1995). The ratio between respira-

tion rates and microbial C, qCO2, indicates that the microbiota intensively but inef® ciently

mineralized C compounds in the litter (Dilly and Munch 1996) and in the topsoil at the wet

site.

All these observations make unlikely the possibility that organic matter accumulatesin the upper soil horizons at the wet site during the period of investigations in comparison

to the dry site. Particularly at the dry site, the soil surface seems to be lower, as indicated

by uncovered trunk base of the trees. Site and soil-speci® c conditions seem to favor humus

degradation at this interface of atmosphere, hydrosphere, and lithosphere. Only refractory

litter, e.g., wood, or litter that is displaced in deeper soil horizons may accumulate, butthese were not included in our studies. Our conclusion that C might at present be miner-

alized at the wet site may only be critical when signi® cant C accumulation occurs at the

dry site, because this would not fall within our procedure for evaluating the status of the

wet site by comparing with the situation at the dry site, for which humus degradation is

assumed.

TABLE 3 Budget of soil C ¯ ows [kg C m ¡ 2] during the growing season at the dry and

wet sites of the alder forest along Lake Belau in the Bornh Èoved Lake district of Northern

Germany

Fraction Dry site Wet site

(1) In situ CO2-C emissiona 0.50 1.36

(2) In situ CH4-C emissionb 0 0.007

(3) CO2-C derived from leaf litterc 0.10 0.18

(4) Rhizomicrobial respiration

and CO2-C derived from soil organic matterd 0.40 1.173

aFor the 1992 growing season.bFrom Rusch (1996) for the 1994 growing season measured at a shore of Lake Belau similar to

the dry and wet sites.cFor the growing season of 1992; litter decomposition rate of kDry site = 0.6 a ¡ 1 and kWet site =

1.0 a ¡ 1 (Dilly and Munch, 1996); litter input of 0.56 kg (C) m ¡ 2 a ¡ 1 and 0.76 kg (C) m ¡ 2 a¡ 1 at thedry and wet sites (Wachendorf et al., 1996) for the period of 150 days (135 days after starting theexperiment).

d (1) minus (2) minus (3).

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Microbiota in Histosols From a Black Alder Forest 75

N Cycling

The highest N2-® xation rates were observed at the wet site near the lake shore wheresoil pH was higher in comparison to the dry site. The soil at the wet site seemed to be

buffered by lake water resulting in favorable conditions for N2-® xation. The high biological

N2-® xation increased N content and, consequently, reduced the C/N ratio in the roots

(Table 1). In contrast, in situ N2O emission seemed to be highest at the dry site of the

alder forest adjacent to the sloping mixed forest. These results are consistent with Rusch’s® ndings (1996) that in situ emission rates for N2O, NO, and NO2 were signi® cantly lower

at wet sites of the same lake. However, the modeled N budgets indicate that N losses

derived from denitri® cation at the two sites of the forest were similar, 60 kg (N) ha¡ 1 a¡ 1

(Wetzel et al. 1996). Consequently, the N2/N2O ratio would increase under wet conditions.

Under such conditions, denitri® cation, in addition to the alkaline and eutrophic lake water,

may in¯ uence soil pH at the wet site because denitri® cation increases the pH value in soil(10[H] + 2H+ + 2NO3

¡ ! N2 + 6H2O; Schlegel 1985; van Miegrot and Cole 1985).

High denitri® cation rates apparently could not increase the soil pH at the dry site. Thus,

C mineralization and nitri® cation, and to a minor extent sulfurication and lateral input of

acidic soil solutions from the beech and mixed forest at the hill and the slope, controlled

the soil pH. Whereas the water regime at the dry site was independent of the lake, the lakewater had obviously supplied nutrients (e.g., Ca, P) and stabilized the pH value in the soil

at the wet site, which thus favored humus degradation.

High N2O emission rates from the soil surface in the alder forest (Mogge et al. 1998)

revealed both an open N cycle typical for early stages of succession, and microbial commu-

nities that were less ef® cient for N conservation. With respect to succession, these ® ndingssuggest that submerged and semiterrestrial soils tend to shift to terrestrial soils. In conclu-

sion, both assimilated N and high pH values may favor the degradation of organic matter in

the alder forest. However, low N2-® xation rates at the dry site may be controlled by a low P

supply, which is indicated by a high phosphatase activity (H Èaussling and Marschner 1989).

The phosphatase activity was signi® cantly higher in the soil from the dry site than in that

at the wet site, being 9.5 mg and 6.8 mg of phenol per gram of dry soil in 3 h, respectively,between May and October 1992 (unpublished data). Although alder may alter biological

P transformation (Zou et al. 1995) increasing P supply seems not to prevent P limitation

(Giardina et al. 1995).

Arginine ammoni® cation rates and protease activitiesare microbiologicalfeatures with

reference to N cycling.Whereas arginine ammoni® cation is related to active microbial cells,protease activitymay also be stabilizedat abioticsoil componentsand may thusbe present in

the absence of active microbial cells (Alef and Nannipieri 1995). The two microbiological

features, however, indicate higher activity at the wet site and thus suggest the intensive

degradation of proteineous compounds at the wet site.

Structure of the Microbiota

The structure of culturablebacterial communities in the soil at the wet site was similar to that

reported for the rhizosphere by Acero et al. (1993). The high proportion of gram-negative

bacteria typical for the rhizosphere (Richards 1987; Acero et al. 1993) indicates that the

roots play an important role in the bulk soil. The bacterial communities seem to be adjustedto the presence of readily usable organic substances. In comparison with the situation at

a temporarily waterlogged site with a high content of orgnic matter, the proportion of

bacteria that decompose high-molecular-mass compounds is relatively low (Bach 1996).

Bacteria that decompose refractory compounds make up only a small fraction of the

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76 O. Dilly et al.

community. The dominance of so-called r-strategists indicates an inef® cient use of C com-

pounds at the wet site. Flavobacterium and Cytophaga are typical culturable bacteria inwell-drained agricultural and grassland soils of the district. Flavobacterium is a widespread

genus in the rhizosphere (Alexander 1981), a soil compartment with high amounts of avail-

able carbon (Cheng et al. 1996).However, the readily usable C compoundsmay even induce

primingeffects and consequentlyincrease the decompositionof refractory humic substances

(Shen and Bartha 1996). The high abundance of earthworms may also have stimulated the

growth of Promicromonospora (Tret’ yakova et al. 1996).The low contribution of facultative anaerobic organisms and the similarity to commu-

nities of other well-drained sites of the Bornh Èoved Lake district suggest suf® cient oxygen

in the soil at the wet site (Bach 1996). Effective biologicalmineralizationalso occurs anaer-

obically (Ehrlich 1993). Instances in which organic matter is not completely degraded may

be due to limited availability of external electron acceptors and the relative refractorinessof some organic matter (Ehrlich 1993). Currently, the two possibilities seem unlikely, con-

sidering the eutrophic and highly productive nature of Lake Belau (Gessner et al. 1996) and

the nitrogen-rich alder litter. The high C/N ratio in soil extracts at the wet site (Table 1)

additionally indicates that the soluble compounds have a C/N ratio similar to that of fresh

leaf litter from the alder forest (Dilly and Munch 1996). Finally, high contents of solubleand N-poor carbohydrates may increase decay rates (French 1988). The narrow C/N ratios

in the soil extracts at the dry site suggest that C is already mineralized and lost via CO2

production, whereas N-rich humic substances remain.

The appearance of fungi at the two sites may indicate suf® cient oxygen supply. When

moisture content is excessive, diffusion of the O2 necessary for aerobic metabolism is

inadequate to meet the microbial demand. The fungi are among the ® rst to suffer andare therefore virtually absent from the lower levels of poorly drained peat (Alexander

1981). Mycorrhizal fungi react sensitively to insuf® cient O2 and poor aeration (Skinner

and Bowen 1974, Theodorou 1978). Both the vitality and the diversity of mycorrhizal

and also saprophytic fungi were higher at the wet site (Rosenbrock et al. 1995; Boyle

1996; Pritsch 1996). Thus, humus decay may be higher at the wet site, although soil waterconditionsshould accelerate the decayof organicmatter at the dry site. In terms of ecological

succession (Odum 1969), high vitality corresponds to short and simple life cycles, which

is typical of the juvenile stage, whereas high species diversity is typical of the mature

stage. With reference to the development of internal symbiosis, the data on N2-® xation and

mycorrhiza suggest that the soil at the wet site of the alder forest is at the mature stage

(Odum 1969).

Conclusions

Although high spatial and temporal variations occurred for most of the microbiological

features and thus hindered validation of the data, the results apparently indicate that the an-

thropogenic lowering of the lake water table during the 1930s, and the eutrophic conditions

of Lake Belau, control the actual matter and energy ¯ uxes. The peat was degraded at the twosites of the forest tested, resulting in a lowered soil surface (Figure 1). Whereas the pH value

declined in Dystri-® bric Histosol at the dry site, the pH value in the Eutri-® bric Histosol

at the wet site was stabilized by the adjacent lake. In addition, nutrients imported by the

lake water may intensify microbial activities at the wet site only, whereas the water regime

at the dry site was independent of the lake. Although possibly in¯ uenced by a temporarily

high water table, the degradation rates we observed in the upper soil horizons were high,indicating that the humus accumulation there is unlikely.

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Microbiota in Histosols From a Black Alder Forest 77

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