Effects of tree species stand age and so

EFFECTS OF TREE SPECIES, STAND AGE AND SOIL TYPE ON SOIL MICROBIAL BIOMASS AND ITS ACTIVITY IN A SOUTHERN BOREAL FOREST

J. BAUHUS, 1 * D. PAREÂ 2,3 and L. COÃTEÂ 3

1 Department of Forestry, Australian National University, Canberra, ACT 0200, Australia, 2 UniversiteÂ

du QueÂbec aÁ MontreÂal, DeÂpartment des Sciences Biologiques, Groupe de Recherche en EÂcologie

ForestieÁre, C.P. 8888, Succursale Centre-ville, Montreal, Que., Canada H3C 3P8 and 3 BiodoÃme de

MontreÂal, 4777 avenue Pierre-de-Coubertin, Montreal, Que., Canada H1V 1B3 (Accepted 18 August 1997) Summary ÐMicrobial C (C mic ) and N (N mic ), the C mic -to-organic C (C org ) and N mic -to-total N (N t )

ratios and the speci®c respiration of microbial biomass were investigated in a southern boreal mixed forest. The forest stands were 50 and 124 years old and consisted of trembling aspen, paper birch and mixed conifers comprising white spruce and balsam ®r. Stands were growing on soils derived either from clay (89% average clay content) or till (46% average clay content) deposits in the clay belt region of northern Quebec. In the forest ¯oors the relative concentrations of microbial C and N and the C mic - to-C org and N mic -to-N t ratios, regarded as measures of organic matter quality, declined with stand age whereas the speci®c microbial respiration increased, indicating decreasing C assimilation eciency. In the mineral soils, in contrast, C mic -to-C org and N mic -to-N t ratios increased with stand age. The C mic -to- N mic ratio widened with stand age in both the forest ¯oors and mineral soils, suggesting that the proportion of fungi had increased. Concentrations of microbial C and N were on average lower in forest ¯oor beneath conifers (C mic -to-C org 1.9%, N mic -to-N t 7.5%) than beneath the deciduous species birch (C mic -to-C org 2.2%, N mic -to-N t 8.6%) and aspen (C mic -to-C org 2.4%, N mic -to-N t 9.2%). Average

C mic -to-N mic ratios were only slightly di€erent in the forest ¯oors beneath the di€erent tree species (C mic -to-N mic : conifers 8.9, birch 7.2, and aspen 8.3). In both forest ¯oors and mineral soils, average concentrations of C mic and N mic were generally higher in the clay than in the till soils, but the C mic -to-

C org ratios were similar in both soil types. The average N mic -to-N t ratios were lower in till than in clay

soils only beneath conifers. The average speci®c microbial respiration (qCO 2 =mg CO 2 -C mg C mic ÿ1 d ÿ1 )

in clay soils (22) was approximately half that in till soils (41). Since the microbial parameters measured were sensitive to the factors stand age, tree species and soil type, they may have the potential to be used as indicators of the in¯uence of forest management on soil organic matter quality. # 1998 Elsevier Science Ltd. All rights reserved

enced by forest management (Ohtonen et al., 1992; The soil micro¯ora is a small but signi®cant com-

INTRODUCTION

Bauhus and Barthel, 1995; PietikaÈinen and Fritze, ponent in most terrestrial ecosystems. Soil microbial

1995) and increase proportionally with forest pro- activity contributes to the regulation of soil carbon

ductivity (Myrold et al., 1989). From the above it storage, soil respiration and ecosystem productivity.

has been suggested that concentrations of microbial The role of soil microbial biomass as a relatively

C and N may be regarded as sensitive indicators for labile nutrient pool in the cycling of C, N and P is

changes in the soil ecosystem. Normally the quan- well established (Marumoto et al., 1982b; Van Veen

tity and quality of the bulk of SOM changes only et al. , 1987; Duxbury et al., 1989, Jenkinson and

slowly and because of the large background value Parry, 1989). Amounts of microbial biomass are

of soil C and the spatial heterogeneity of soils, changes are dicult to measure. Therefore,

in¯uenced by soil texture and soil organic matter microbial biomass and, in particular, the ratio of (SOM) quality (Wardle, 1992; Ross and Tate, 1993, microbial C and N to total organic C and N have Bosatta and AÊgren, 1994; Hassink, 1994). The

quantity and composition of microbial biomass is been suggested as indicators of the state and modi®- cation of SOM (Anderson and Domsch, 1989;

sensitive to changes in the soil chemical and physi- Wolters and Joergensen, 1991; Bosatta and AÊgren, cal environment (Wolters and Joergensen, 1991; 1994). Soil microbial biomass is usually resource Wardle, 1992; Bauhus and Khanna, 1994; Beck et limited and thus microbial C and N concentrations al. , 1995). In addition, it has been shown that

are generally related to amounts of soil C and N amounts of soil microbial C and N can be in¯u-

(Wardle, 1992). The ratio of microbial C to soil or- ganic carbon has thus been used as an indicator for

*Author for correspondence.

C availability (Anderson and Domsch, 1986; Insam C availability (Anderson and Domsch, 1986; Insam

The amount of information on soil microbial bio- mass in forested ecosystems is limited when com- pared to agricultural systems (Wardle, 1992). This is especially true for the vast area of boreal forests. In this study we wanted to determine how microbial biomass and its activity in the southern boreal for- est is a€ected by the dominant tree species, soil type and stand age. Our hypotheses were:

(a) The ratio of microbial C to total organic C (C mic -to-C org ) and the ratio of microbial N to total N (N mic -to-N t ) are related to substrate quality. If this were true, the C mic -to-C org and the N mic -to-N t ratios would be expected to be higher in a rich soil substrate than in a poor one, in a forest ¯oor beneath deciduous trees than beneath conifers and they should also decrease with stand age in aggrad- ing forests. The availability of nutrients in the forest ¯oor decreased with the stand age after ®re and was higher in birch and aspen than in coniferous stands (Pare et al., 1993). Forest ¯oor decomposition rates were related to fungal biomass and were higher in deciduous than in coniferous boreal forests (Flanagan and Van Cleve, 1983). Since the pro- portion of woody litter increases with stand age substrate quality in the forest ¯oor should then decrease (Klinka et al., 1995).

(b) The speci®c microbial respiration (qCO 2 =mg

CO 2 -C mg C mic ÿ1 d ÿ1 ) decreases with increasing sub- strate quality. Several studies (e.g. Anderson and Domsch, 1993; Wardle and Ghani, 1995) have shown that speci®c microbial respiration is higher under unfavourable than under favourable con- ditions. Thus speci®c microbial respiration should increase with stand age and should be higher in a poor soil and a coniferous forest ¯oor than in a rich soil and a deciduous forest ¯oor.

MATERIALS AND METHODS

Field sites The study area was located in the southern boreal

forest of Quebec, in the Abitibi region around Lac Duparquet (48830'N, 79820'W). The continental cli-

mate is characterized by a mean annual temperature of 0.68C, mean annual precipitation of 823 mm and

a frost-free period of 64 days. The study area is part of the northern Clay Belt, where most soils originate from glaciolacustrine clay

deposits

(Vincent and Hardy, 1977). Forests in the region are mostly composed of trembling aspen (Populus tremuloides ) and paper birch (Betula papyrifera) in young successional stages and converge gradually towards dominance of conifers such as balsam ®r

(Abies balsamea), white spruce (Picea glauca) and cedar (Thuja occidentalis) in later successional stages (Bergeron and Dansereau, 1993). The age of forest stands in the area has been determined in dendro- chronological studies (Dansereau and Bergeron, 1993).

Field plots were located in stands originating from ®res in 1944 and 1870. They included 50 and 124 year old stands of aspen, birch and conifers (®r and spruce) on clay and till soils. The patchy occur- rence of conifers in the forest did not allow us to locate plots in pure stands of only one coniferous species. Therefore Abies balsamea and Picea glauca were combined into one conifer group. Thus, although in the following text, plots will be named according to the dominant species or species group, the composition was not always entirely monospeci- ®c. Aspen stands in the 124 year old forest were in their second natural rotation (Pare and Bergeron, 1995). Aspen stands disintegrate and rejuvenate at around 70±80 years of age and aspen trees in the 124 year old forest were thus approximately of the same age as in the 50 year old forest. The clay soils were Grey Luvisols with moderate to good drainage and the till soils developed on moraine deposits were classi®ed as Humo-Ferric Podzols (Agriculture Canada Expert Committee on Soil Survey, 1987). Within a randomized design for the factors stand age, tree species and soil type, each treatment was replicated four times amounting to 48 plots of

Field sampling Mineral soil (0±10 cm) and forest ¯oor (O F - and

O H -layer) samples were collected from four lo- cations at each plot in October 1994. These four samples were combined to form one composite sample for each plot. The forest ¯oor was not sep-

arated into O F - and O H -layers, because the layers were closely attached by a dense ®ne root mat. Mineral soil samples were collected using a 50 mm diameter soil corer. Samples were refrigerated (48C) until further processing in the laboratory. We sampled soils for microbial biomass determinations only once because other studies had shown that amounts of microbial C and N ¯uctuate to only a small extent throughout the year (Patra et al., 1990; Bauhus and Barthel, 1995). We also assumed that potential seasonal ¯uctuations would not disguise di€erences between tree species, soil type or stand age. Forest ¯oor samples were sieved (5 mm mesh) to remove coarse woody parts and roots. The till soils were also sieved (2 mm), whereas the clay soils were homogenized by hand because they were too moist for sieving.

Soil analysis Moisture content was determined on sub-samples

by drying to constant weight at 1058C. On sub-

1078 J. Bauhus et al.

1079 samples, pH was determined in bi-distilled water

Soil microbial biomass and activity

a correction factor of k C =2.86 (Sparling et al., and in KCl solution; substrate-to-water ratios were

1990) for soils with a high organic C content and 1-to-2 for mineral soil and 1-to-3 for forest ¯oor

k N =1.85 (Brookes et al., 1985; Joergensen and material. Organic matter and C org concentrations

Mueller, 1996), respectively. were determined by loss on ignition after ashing at

The speci®c respiration rate (Anderson and 5508C for 2 h; C org was derived by multiplying the

Domsch, 1990) (qCO 2 =mg CO 2 -C mg C mic ÿ1 d ÿ1 ) organic matter concentration by the factor 0.58

was determined during incubation of fresh soil at (Allen, 1989). Total nitrogen (N t ) was determined

room temperature (228C) in sealed tubes after colorimetrically (Tecator FIA Star 5020 Analyzer)

adjusting the water content to 300% in forest ¯oor following wet digestion in sulphuric acid using mer-

material and to 40% in mineral soil 3 days prior to cury oxide as a catalyst (Allen, 1989). Bray II-

measurement. The respiration rate for these extractable P (McKeague, 1976) was analyzed spec-

samples, measured as CO 2 evolution, was deter- trophotometrically

mined by injecting samples of the head space gas of 1001plus) at 880 nm. Exchangeable cations were

soil samples incubated for 3 h in Vacutainers into a

gas chromatograph (Licor 6200). Values for speci®c and measured using atomic absorption spectropho-

determined in 1 M NH 4 NO 3 (Stuanes et al., 1984)

respiration represent the means of two measure- tometry. Particle size distribution of mineral soil

ments.

samples was determined by the hydrometer method (Sheldrick and Wang, 1993).

Statistics

Microbial C (C mic ) and N (N mic ) were determined The e€ect of the experimental factors, stand age, using the chloroform fumigation±extraction method

tree species and soil type on microbial and soil vari- (Brookes et al., 1985, Vance et al., 1987). C mic and

ables was tested by means of ANOVA using the N mic were calculated as the di€erences in organic C

SYSTAT package (Systat Inc., 1992). Homogeneity and total N between fumigated and non-fumigated

of variances was examined with Levene's test (control) samples. Two replicates of each sample,

(Snedecor and Cochran, 1980). In only two cases

5 g from the organic layer or 10 g mineral soil, were were the variances between factors not hom- fumigated for 24 h with ethanol-free chloroform at

ogenous. In one of the cases, homoscedasticisty was 258C. Subsequently the chloroform was removed by

achieved by hyperbolic transformation of the data. evacuation. Fumigated samples and controls were

In the other case, data were rank transformed since

none of the regular transformations yielded hom- (Whatman 42); extracts were kept frozen until ana-

extracted with 50 ml 0.5 M K 2 SO 4 and ®ltered

ogeneity of variances (Conover and Iman, 1981;

Potvin and Ro€, 1993). An ANOVA on these by dichromate digestion (Walkley and Black, 1934;

lyzed. Organic C in K 2 SO 4 extracts was determined

ranked data provided similar results for signi®cance Vance et al., 1987) followed by potentiometric

and non-signi®cance of factors and interactions as (Metrohm, Herisau) end point titration (Raveh and

the ANOVA on the non-transformed data. Avnimelech, 1972). Total nitrogen in K 2 SO 4 Regression analysis (general linear or non-linear

extracts was determined colorimetrically (Tecator models) was used to assess the in¯uence of soil FIA Star 5020 Analyzer) following alkaline persul-

properties on microbial biomass and the interaction fate oxidation (Cabrera and Beare, 1993). The per-

between microbial variables. sulfate oxidation procedure oxidizes both organic N and ammonium, with N being analyzed as nitrate. Our evaluation of the method using ammonium-

nitrate and urea standards showed full recovery of RESULTS N in K 2 SO 4 extracts. Non-extractable amounts of

Chemical properties of the forest ¯oor and min- microbial C and N were compensated for by using

eral soil materials are given in Tables 1 and 2, re-

Table 1. Some chemical properties of forest ¯oor material from stands of di€erent age, soil type and species composition. Averages for

n = 4 samples, standard deviation is given in parentheses. Values for pH are medians and the range is given in brackets Age (year)

Soil Species

K (mg g ÿ1 ) Mg (mg g ÿ1 ) P (mg g ÿ1 ) 50 clay

Corg (g kg ÿ1 ) Norg (g kg ÿ1 ) pH (in H 2 O) Ca (mg g ÿ1 )

clay aspen

1.40 (0.20) 0.74 (0.17) 22.7 (5.3) birch

1.84 (0.25) 0.77 (0.14) 20.4 (4.4) conifer

1.34 (0.14) 0.48 (0.09) 18.5 (4.1) till

aspen

1.34 (0.22) 0.54 (0.03) 14.1 (6.4) birch

1.34 (0.26) 0.61 (0.06) 19.1 (2.2) conifer

J. Bauhus et al.

Table 2. Some chemical properties and clay contents of the mineral soil from stands of di€erent age, soil type and species composition. Averages for n = 4 samples, standard deviation is given in parentheses. Values for pH are medians and the range is given in brackets

Age (year) Soil

K (mg g ÿ1 ) Mg (mg g ÿ1 ) P (mg g ÿ1 ) Clay (%) 50 clay

Species C org (g kg ÿ1 )N org (g kg ÿ1 ) pH (in H 2 O)

clay aspen

spectively. In the following the results are presented stands than in old stands. The C mic -to-C org ratio separately for forest ¯oor and mineral soil.

decreased from aspen (2.41%) to birch (2.18%) and to conifers (1.94%) and was higher in forest ¯oors

Forest ¯oor over clay than over till soils. Organic C in the forest ¯oor ranged from 325 to

The best multiple linear regression model for the 539 g kg ÿ1 and was not a€ected by stand age, soil

forest ¯oor data explained only 24% of C mic vari- type, or tree species. Forest ¯oor N org contents,

ation, using K and P concentrations as independent however, were, on average, signi®cantly lower

variables. Microbial C concentrations were posi- under conifers (13.2 g kg ÿ1 ) than below a birch

tively related to K beneath aspen, to P beneath (15.9 g kg ÿ1 ) or aspen (15.4 g kg ÿ1 ) canopy. The

birch and to Mg and P beneath conifers (Fig. 1). average pH was also lower in the forest ¯oor

beneath conifers than beneath either deciduous

Microbial nitrogen

species. However, pH was more strongly in¯uenced Microbial N ranged between 542 and 1848 mg g ÿ1 by soil type. The forest ¯oor overlaying clay soils

in the forest ¯oors. The contribution of N mic to N t (pH 5.4) was less acidic than forest ¯oor over till

was 4.3±12.4%. Microbial N concentrations and soils (pH 4.9). Concentrations of base cations and P

the N mic -to-N t ratio were signi®cantly in¯uenced by were higher in the forest ¯oors on clay than on till

all experimental factors (Table 5). Microbial N soils (Table 1).

decreased, on average, from young (1385 mg g ÿ1 ) to old (1083 mg g ÿ1 ) stands, it was higher in forest

Microbial carbon ¯oors overlaying clay soils (1366 mg g ÿ1 ) than in for- Values for C mic were between 5570 and

est ¯oors over till soils (1103 mg g ÿ1 ), and lower 13540 mg g ÿ1 (Table 3). C mic contributed 1.33±

beneath conifers (1032 mg g ÿ1 ) than beneath decid- 3.44% to total organic C in the forest ¯oors. Both

uous species (average 1335 mg g ÿ1 ). The interaction

C mic and the C mic -to-C org ratio were signi®cantly between age and soil for the N mic -to-N t ratio was a€ected by stand age, tree species and soil type in

signi®cant (Table 5), with a decrease in relative the forest ¯oors (Table 5). C mic , in absolute and

N mic concentrations with stand age occurring only relative terms, was on average higher in young

in forest ¯oor overlaying till soils. In 124 year old

Table 3. Microbial biomass C and N, C mic -to-N mic ratio, C mic -to-C org and N mic -to-N t ratio and the speci®c microbial respiration in forest ¯oor material from stands of di€erent age, soil type and species composition. Averages for n = 4 samples, standard deviation is given in

parentheses

Age (year) Soil Species

C mic -to-C org (%) N mic -to-N t (%) q CO 2 * 50 clay

C mic (mg g ÿ1 )

N mic (mg g ÿ1 )

C mic -to-N mic

clay aspen

4.8 (0.8) 74.3 (12.3) *qCO 2 =mg CO 2 -C mg C mic ÿ1 d ÿ1 .

Soil microbial biomass and activity

Fig. 2. The relationships between litterfall N concen- trations (mg g ÿ1 ) and the N

Fig. 1. Correlation coecients for the relationships mic -to-N t ratio in the forest

¯oor. Signi®cant regressions were found only for till soils. between forest ¯oor chemical properties and some mi-

crobial parameters for selected tree species. Only signi®- cant correlations were included (P < 0.05).

and r 2 =0.68 for linear regressions between C mic stands the N mic -to-N t ratio was, on average, 6.4

and N mic in the forest ¯oors and the mineral soils, and 8.9% in forest ¯oors over till and clay soils, re-

respectively. The C mic -to-N mic ratio increased from spectively.

young (7.6) to old stands (8.6) in the forest ¯oor. It The pH, organic N and C explained 31% of the

was higher for conifers (8.9) and aspen (8.3) than variability of N mic . Only in the coniferous forest

for birch (7.1) and it was higher on the till (8.5) ¯oors, a signi®cant relationship between N mic and

than on the clay soils (7.7) (Table 5). However, the soil chemical variables was found (Fig. 1), with

ANOVA model explained only 44% of the C mic -to- N mic positively related to pH, Mg and P concen-

N mic variation in the forest ¯oor. trations.

Multiple regression analysis showed that only a In the forest ¯oors over the till soils we found a

small proportion of the variation of the C mic -to- positive correlation between litterfall N concen-

N mic ratio could be explained by the variables trations (Brown, unpublished data) and the N mic -

measured. In the forest ¯oor pH, N t and C org con- to-N t and the C mic -to-C org ratios, whereas no corre-

centrations explained 31% of the variation. The lation between these variables was found in the for-

C mic -to-N mic ratio in coniferous forest ¯oors was est ¯oors over the clay soils (Fig. 2). The goodness

negatively correlated with pH and exchangeable Ca of ®t for the correlation between litterfall N concen-

(Fig. 1).

trations and the Cmit-to-C org ratio in forest ¯oor over till soils was R 2 =0.34.

Speci®c microbial respiration Speci®c respiration (qCO 2 ) was generally higher

in the forest ¯oors than in the mineral soils The microbial C-to-N ratio

(Tables 3 and 4). In the forest ¯oors, it was strongly

C mic -to-N mic ratios were highly variable, as indi- in¯uenced by the stand age and to a lesser extent

cated by small goodness of ®t values of r 2 =0.41

by tree species and soil type (Table 5). The qCO 2

Table 4. Microbial biomass C and N, C mic -to-N mic ratio, C mic -to-C org and N mic -to-N t ratio and the speci®c microbial respiration in the mineral soil from stands of di€erent age, soil type and species composition. Averages for n = 4 samples, standard deviation is given in

parentheses

Age (year) Soil Species

N mic -to-N t (%) q CO 2 * 50 clay

C mic (mg g ÿ1 )

N mic (mg g ÿ1 )

C mic -to-N mic

C mic -to-C org (%)

clay aspen

2.3 (0.3) 36.9 (11.8) *qCO 2 =mg CO 2 -C mg C mic ÿ1 d ÿ1 .

J. Bauhus et al.

Table 5. The e€ect of stand age, dominant tree species and soil type on C mic , the ratio of microbial C to total soil organic C (C mic -to- C org ), N mic , the ratio of microbial N to total soil N (N mic -to-N t ), the C mic -to-N mic ratio and the speci®c microbial respiration qCO 2 (mg

CO 2 -C mg C mic ÿ1 d ÿ1 ) in the forest ¯oor F -ratios

C mic

C mic -to-C org

N mic

N mic -to-N t

C mic -to-N mic q CO 2

2.1 0.8 1.0 1.5 0.1 1.2 0.1 0.6 1.1 1.3 0.5 0.3 1.9 5.3* 2 1.4 1.9 0.9 1.5 0.2 Multiple r 0.6 0.50 0.60 0.59 0.62 0.44 0.72

*P < 0.05. **P < 0.01. ***P < 0.001.

increased for all combinations of soil and species

Mineral soil

from young (=50.3) to old stands (qCO 2 =76.3)

Organic C in the mineral soils ranged between 15

and 94 g kg ÿ1 , and organic N between 0.9 and higher in the forest ¯oors overlaying till than over

(Table 3). Beneath aspen and birch, qCO 2 was

2.9 g kg ÿ1 . Concentrations of both C org and N org the clay soils. It was lower in the aspen forest ¯oor

were, on average, signi®cantly (P < 0.01) higher in (55.2) than in that of birch (66.2) and conifers

the clay than in the till soils. However, the di€er- (68.5) (Table 3).

ence was more pronounced in old stands (Table 2). The speci®c microbial respiration decreased with

The pH and base cation and P concentrations were increasing concentrations of C mic [Fig. 3(a)]. A

also higher on average in the clay than in the till negative relationship between exchangeable Ca con-

soils. Particle size analysis showed that, in Luvisols centrations and qCO 2 was found in the forest ¯oors

developed from lacustrine clay deposits, the clay under birch and aspen (Fig. 1).

and silt fractions contributed, on average, to 89 and 11% of the mineral soil, respectively, whereas the clay, silt and sand fractions in the Podzols devel- oped on till, comprised 46, 28 and 26% of the min- eral soil, respectively (Table 2).

Microbial carbon

In the mineral soil C mic ranged from 224 to 1333 mg g ÿ1 (Table 4). C mic contributed to 0.53± 2.41% of C org in the mineral soils. C mic and the

C mic -to-C org ratio in the mineral soils were in¯u- enced by stand age. However, the trend was inverse compared to the forest ¯oors (Table 4); higher ab- solute and relative C mic concentrations occurred in old stands compared to young stands. The C mic -to-

C org ratio was 1.05% in 50 year old stands as com- pared to 1.63% in 124 year old stands. C mic signi®- cantly higher in the clay soils than in the till soils. However, soil clay content explained only a small proportion of the variation in C

(R =0.26). Since the C mic -to-C org ratio takes account of the di€erent C content of soils, this di€erence between clay and till disappeared when C mic was expressed in relative terms (Table 6). The C mic -to-C org ratio was higher beneath deciduous than coniferous trees.

2 mic

The interaction between age and soil for C mic showed that C mic increased with stand age in the clay but not in the till soils. This was not apparent

Fig. 3. The relationship between the concentration of microbial C (mg C mic kg ÿ1 ) and the metabolic quotient

for the C mic -to-C org ratio, because of the increase in (mg CO 2 -C mg C mic ÿ1 d ÿ1 ) in (a) the forest ¯oor and (b) the

C org from young to old stands in the clay soils. mineral soil.

Interactions in the C mic -to-C org ratio occurred

1083 Table 6. The e€ect of stand age, dominant tree species and soil type on the ratio of microbial C to total soil organic C (C mic -to-C org ), the

Soil microbial biomass and activity

ratio of microbial N to total soil N (N mic -to-N t ), the C mic -to-N ÿ1 mic ratio, and the speci®c microbial respiration qCO 2 (mg CO 2 -C mg

C mic ÿ1 d ) in the mineral soil F -ratios

C mic

C mic -to-C org $

N mic

N mic -to-N t

C mic -to-N mic q CO 2

1.9 8.3** Multiple r

0.65 0.74 0.52 0.60 0.63 0.74 *P < 0.05.

**P < 0.01. ***P < 0.001. $

For comparison with other variables ANOVA results for the C mic -to-C org ratio are from untransformed data, although variances were not homogenous for the three tree species groups. However, ANOVA on rank-transformed data gave similar results for signi®cance and non-signi®cance of factors and interactions.

between age and species and also between species

Microbial nitrogen

and soil type. Whereas the C mic -to-C org ratio was,

in the mineral soils were between on average, higher in clay than in till beneath aspen

Values for N

mic

24 to 168 mg g ÿ1 and the contribution of N and conifers, the inverse was found for birch

mic to N t was 2.1±7.5%. Stand age showed no in¯uence on

(Table 4). In young stands the C mic -to-C org ratio was highest under birch, whereas it was smallest

N mic concentrations (Table 6), but N mic and the under birch in old stands. Increasing C mic -to-C

mic -to-N t

ratio were signi®cantly in¯uenced by all

ratios with stand age were found only for aspen other experimental factors. As found also for C mic , and conifers. The variability of the C

org

mic -to-C org

the e€ect of age on the N -to-N ratio in the min-

mic t

ratio was signi®cantly lower in soil under birch eral soils was inverse to the age e€ect in forest than in soil under aspen and conifer.

¯oors, with the N mic -to-N t ratio increasing with Multiple linear regressions showed that in the

stand age in the mineral soils. On the other hand, mineral soils 53% of the variation in C mic could be

the e€ects of species and soil type were the same as explained by the organic C concentration and the

in the forest ¯oors. That is, on average, lower ab- clay content (P < 0.05). However, the in¯uence of

solute and relative microbial N concentrations were soil chemical and physical variables on microbial

found beneath conifers (55 mg g ÿ1 ) than beneath properties di€ered for di€erent tree species groups.

deciduous species (83 mg g ÿ1 ) and concentrations Whereas no signi®cant relationship could be found

were higher in the clay soils (88 mg g ÿ1 ) than in the between soil factors and C mic for conifers, C mic was

till soils (60 mg g ÿ1 ). In contrast to the C mic -to-C org positively in¯uenced by K in soils under birch and

ratio the N mic -to-N t ratio was higher in clay soils it was related to soil texture in soils under aspen (Fig. 4).

Fig. 4. Correlation coecients for the relationships between mineral soil chemical and physical properties and

Fig. 5. The e€ect of tree species and soil type on speci®c some microbial parameters for selected tree species. Only

microbial respiration (mg CO 2 -C mg C mic ÿ1 d ÿ1 ) in mineral signi®cant correlations were included (P < 0.05).

soil. Error bars represent the standard error.

J. Bauhus et al.

conifers (Fig. 4). The C mic -to-N mic ratio increased with clay content beneath aspen.

The speci®c respiration In mineral soils, there was

a strong e€ect of soil type, a small e€ect of stand age and signi®cant interactions between soil and species, and between all factors. The qCO 2 was, on average, almost twice as high in the till (39.6) as in the clay soils (22.2), and higher in young stands

than in old. In the till soils, the qCO 2 was highest under aspen but in the clay soils it was lowest in soil under aspen (Fig. 5).

The speci®c microbial respiration decreased with increasing concentrations of C mic [Fig. 3(b)]. It was negatively correlated with N t , P and exchangeable

Fig. 6. Correlation coecients for the relationships Mg concentrations; these factors contributed to between mineral soil chemical and physical properties and

51% of its variation. Soil chemical and physical the speci®c microbial respiration for tree species. Only sig-

ni®cant correlations are included (P < 0.05). variables in¯uenced qCO 2 beneath di€erent species

to di€erent degrees (Fig. 6). Although increasing (0.61%) than in till soils (0.50%). In mineral soils,

concentrations of exchangeable base cations and interactions between species and soil and between

clay reduced qCO 2 in mineral soils under all species, all factors were also found for the N mic -to-N t ratio

q CO 2 was negatively related (P < 0.05) to soil pH (Table 6). The average N -to-N

and P concentrations beneath aspen only.

mic

t ratios were 5.1,

4.9 and 4.0% for birch, aspen and conifers respect- ively. In the clay soils, there was no di€erence

between species, but in the till soils the N DISCUSSION

mic -to-N t

ratio was, on average, signi®cantly smaller under The concentrations of microbial C and N, C mic - conifers (2.7%) than under a deciduous canopy

to-N mic ratios, C mic -to-C org and N mic -to-N t ratios (5.5%). The signi®cant interaction between age and

and qCO 2 were all within the range of values soil type for N mic (Table 6) was caused by the

reported for boreal forests (Visser and Parkinson,

1989; Martikainen and PalojaÈrvi, 1990; Fritze et al., soil from young to old stands in the clay soils and a

increase from an average of 74 to 103 mg N mic g ÿ1

1994; Smolander et al., 1994; Scheu and Parkinson,

decrease from 67 to 55 mg N mic g ÿ1 soil from young

to old stands in the till soils. Anderson and Domsch (1993) showed that soil Organic N concentrations and clay contents

acidity had a signi®cant in¯uence on the availability explained 61% of the variability of N mic in the min-

of soil C to the micro¯ora. An analysis of many eral soils. Beneath conifers N mic was positively re-

data sets showed that pH had a strong in¯uence on lated to exchangeable base cations (Ca, K, and Mg)

the C mic -to-C org ratio (Wardle, 1992). The pH and and also to clay content (Fig. 4). In aspen soils,

base cation concentrations could not explain the N mic also showed a positive relationship to clay

variation in the C mic -to-C org ,N mic -to-N t and the content.

C mic -to-N mic ratios for the entire data set in our study. This result agrees with the ®ndings of Joergensen et al. (1995) and Beck et al. (1995) for

The microbial C-to-N ratio large data sets. However, when our data were The C mic -to-N mic ratio in mineral soil was

divided into tree species groups, pH showed a sig- a€ected by stand age and tree species. It increased

ni®cant in¯uence on the N mic concentration and on from an average of 6.4 in young stands to an aver-

the C mic -to-N mic ratio in coniferous forest ¯oor and age of 8.3 in old stands, and was higher in soil

mineral soil samples. Thus the above relationship beneath conifers (8.8) than under deciduous species

may be found only for samples of comparable sub- (aspen = 6.8, birch = 6.4). Interactions between

strate and for data that show a wide range of soil species and soil, and between species and age

pH extending into strongly acidic values, such as revealed that the C mic -to-N mic ratio was higher in

beneath conifers in our study. Joergensen et al. the till than in the clay soils, and was signi®cantly

(1995) also suggested the existence of a threshold higher in old stands (10.9%) than in young stands

for the relationship between pH and the C mic -to- (6.7%) in the conifer plots only. The C mic -to-N mic

N mic ratios, and found little variation above pH ratio beneath aspen was smaller in the till than in

the clay soils (Table 4). The negative relationship between qCO 2 and C mic The pH, P and exchangeable K concentrations

concentrations, which has also been reported by explained 27% of the variation in the C mic -to-N mic

Wolters and Joergensen (1991) and Wardle and ratio. It was negatively correlated with pH beneath

Ghani (1995), indicates that microorganisms were

1085 more C ecient on substrate supporting high con-

Soil microbial biomass and activity

Qualls et al. (1991) showed that the ¯ux of dis- centrations of micro¯ora. Wolters and Joergensen

solved organic matter from the forest ¯oor to the (1991) described a negative relationship between

mineral soil can be substantial. At ®elds sites of this

study Pare (unpublished data) found that N t con- interpreted as metabolic stress of the micro¯ora

soil Ca concentrations and the qCO 2 , which was

centrations in seepage water below the forest ¯oor induced by acidity. In the aspen mineral soils and

overlaying clay stands were twice as high in forest ¯oors, qCO 2 was negatively correlated with

124 year old stands as in 50 year old stands. Thus a both nutrients (K and Ca) and pH, thus supporting

decrease in organic matter quality in the forest ¯oor the interpretation of Wolters and Joergensen (1991).

may increase the input of soluble organic matter to However, in birch forest ¯oors and mineral soils,

the mineral soil.

q CO 2 was negatively correlated only with concen- The increases of C mic -to-C org and N mic -to-N t trations of Ca and K. Thus high speci®c microbial

ratios with stand age in the mineral soils were ac- respiration may be indicative of nutrient stress

companied by a highly signi®cant widening of the rather than acid stress. Under most ®eld conditions

C mic -to-N mic ratio, indicating a growing importance this will be dicult to distinguish because acid soils

of fungi, which have a higher C-to-N ratio than have usually a poorer nutrient status than soils with

bacteria (Marumoto et al., 1982a), in microbial higher pH.

communities. This trend in C mic -to-N mic ratios, which also occurred in the forest ¯oors, was most

The e€ect of stand age prominent in coniferous stands. In the mineral soils, We had hypothesized that the C mic -to-C org and

it was also accompanied by a decrease in activity as N mic -to-N t ratios would decrease with stand age,

expressed by the speci®c respiration. Thus the assuming that substrate quality was higher in young

increase in C mic -to-C org and N mic -to-N t ratios may stands than in old stands. Our ®ndings are of inter-

be attributable to an increase in microbial eciency est because we observed contrasting trends for mi-

or to a larger portion of inactive biomass in the crobial variables in the forest ¯oors and the mineral

mineral soils of old stands. Nitri®cation in these soils. Although no di€erence was found between

forests declines sharply from 50 to 124 year old

C org and N org concentrations in the forest ¯oors of stands (Brais et al., 1995; Pare and Bergeron, 1996) young and old stands, C mic -to-C org and N mic -to-N t

pointing also to a change in substrate and microbial ratios decreased from young to old stands, indicat-

communities.

ing a decline in substrate quality. A decrease in the In the forest ¯oor, qCO 2 increased with stand

C mic -to-C org ratio in organic horizons with increas- age. This decrease in microbial C eciency may ing successional stage of vegetation communities

again be related to a decline in organic matter qual- has also been observed by Wardle (1993). Neither

ity from young to old stands. The decrease in the the decrease of C mic -to-C org and N mic -to-N t ratios

C mic -to-C org ratio with stand age in the forest ¯oors in the forest ¯oors, nor their increase in the mineral

discussed above may be a direct result of the soils, from young to old stands was accompanied

decrease in microbial eciency. by a change in N t , or exchangeable base cation con-

Odum's theory of ecosystem succession (Odum, centrations. However, concentrations of BrayII-

1969) says that in ecosystems approaching a steady extractable P in the mineral soils increased signi®-

state or ``climax'' stage of development, the ratio of cantly from an average of 5.5 mg g ÿ1 in young

respiration to biomass declines as the biomass stands to 8.7 mg g ÿ1 in old stands. In another study

becomes more energy ecient. Studies testing this in the same forests Pare et al. (1993) demonstrated

theory for microbial biomass either corroborated that the availability of P in the F/H layer over clay

this hypothesis (Insam and Haselwandter, 1989; soils decreased with stand age, whereas N avail-

Anderson and Domsch, 1990) or found no di€er- ability, determined by aerobic laboratory incu-

ence between early and late successional stages bation, did not change with time. Microbial

(Wardle, 1993). Insam and Domsch (1988) found biomass in the mineral soils of our study may have

that after reclamation of open-pit mine sites the been P limited in younger stands.

speci®c respiration decreased with time in agricul- Towards the end of the aggradation period in the

tural soils but not in forest soils. However, none of mixed boreal forest more litter occurs in the form

the above studies looked at both mineral soil and of woody material (Pare and Bergeron, 1995). If the

forest ¯oor, for which we found inconsistent trends. decrease in microbial concentrations with stand age

Wardle and Ghani (1995), who investigated L, F/H was caused by a decline in organic matter quality,

and mineral soil layers from a primary succession decomposition and mineralization processes in the

chronosequence concluded that the speci®c respir- forest ¯oor might be less complete in old stands.

ation does not necessarily decline during succession This could not only lead to the often-observed ac-

and that it was not a suitable measure for assessing cumulation of forest ¯oor mass but it could also

ecosystem development. Ecosystems in which the leave more material that could potentially be lea-

quality of the C input changes during succession ched from the forest ¯oor into the mineral soil.

may not follow Odum's theory when applied to mi-

J. Bauhus et al.

crobial biomass and respiration. If there are equili- (1995) showed that C mic and the C mic -to-C org ratio bria between microbial C use and C mic during suc-

declined with aggregate size for aggregates smaller cession of plant communities, the forest ¯oors and

than 0.5±1.0 mm.

mineral soils in our study appear to be moving Soil type had a highly signi®cant e€ect on the towards distinctly di€erent equilibria; for a better

speci®c respiration in the mineral soils, with qCO 2 understanding of the e€ect of stand age on mi-

in the till soils being, on average, twice as high as crobial biomass and activity, more age classes

in the clay soil. This observation conforms with the should have been included. Our hypothesis that

above theory that microbial C turnover is more stand age would a€ect substrate quality negatively

rapid in coarse-textured soils than in ®ne-textured and thus decrease the C mic -to-C org and N mic -to-N t

soils (Van Veen et al., 1987; Verberne et al., 1990). ratio while the speci®c respiration increased was

Interactions between species and soil type (Fig. 5) supported only by results for forest ¯oor materials.

indicated that the di€erence in speci®c microbial Our results for the minerals soil suggest the need

respiration between clay and till was most pro- for a revision of the common belief (which stems

nounced in mineral soil beneath aspen. In the aspen from studies of the forest ¯oors) that nutrient avail-

stands, small roots were more concentrated at the ability in boreal forests decreases with stand age.

surface in the till than in the clay soils (Bauhus, unpublished data). In the more nutrient limited

E€ect of soil type soils in till the relative input of C from roots to Our common understanding is that clay soils ac-

stimulate N mineralization might be higher than in cumulate more organic matter than sandy soils

the clay soils and result in a larger portion of the because organic matter is stabilized to a higher

micro¯ora being metabolically more active (Bradley degree and less accessible to microbial decompo-

and Fyles, 1995).

sition in clay soils (Van Veen et al., 1987; Oades, The higher C mic -to-C org and N mic -to-N t ratios 1988). However, most of our knowledge is derived

and a lower C mic -to-N mic ratio and speci®c respir- from studies on agricultural soils which are, in con-

ation in forest ¯oors overlaying the clay soils com- trast to forest soils, frequently perturbed. The clay

pared to those in the forest ¯oor over the till soils soils in our study contained more C org (53 g kg ÿ1 )

can be interpreted as the indirect in¯uence of soil and

nutritional status on forest ¯oor nutrient concen- (C org =36 g kg ÿ1 ,N t =1.5 g kg ÿ1 ); the C-to-N ratios

N t (2.0 g kg ÿ1 )

trations via litter quality. Although litterfall N did not di€er. Since the turnover of microbial bio-

explained only 48% of the variation in the N mic -to- mass is slower in ®ne-textured soils, they contain

N t ratio (Fig. 2), this relationship is surprisingly higher concentrations of C

mic and N mic than coarse-

strong considering that leaf litterfall comprises only

textured soils (Van Veen et al., 1987). The Luvisols

a fraction of the total litter input into the forest in the boreal forest of our study contained on aver-

¯oor. The input of ®ne root litter is estimated to be age 55% more C mic and 48% more N mic than the

high in this boreal forest, where dense root mats Podzols. Whereas some ®ne-textured soils also have

are found in the forest ¯oors. The above relation- higher C -to-C

and N -to-N ratios than ship indicates that microbial biomass in the forest

coarse textured soils (Hassink, 1994; Beck et al., ¯oors on the poorer soil substrate may be N lim- 1995), there is generally no positive relationship

between C mic -to-C org and soil clay content (Wardle, 1992). This indicates that microbial biomass is pri-

The e€ect of tree species

marily correlated to the higher C content in clay Only a few studies have looked at the e€ect of soils. This was also true in our study. However, the

di€erent tree species on soil microbial biomass and N mic -to-N t ratio was higher in the clay than in the

activity under comparable soil and climatic con- till soils. In ®ne-textured soils the proportion of

ditions (Flanagan and Van Cleve, 1983; Sparling et bacteria, which have a lower C-to-N ratio than

al. , 1994; Bradley and Fyles, 1995; Billore et al., fungi, may be higher because they are better pro-

1995; Scheu and Parkinson, 1995). Although our tected against desiccation than in coarse-textured

results di€ered for forest ¯oor and mineral soil, in soils (Grin, 1981). Smaller microbial C-to-N ratios

general they substantiate the hypothesis that forest in ®ne textured soil than in to coarse textured soil

¯oor and mineral soil supported fewer micro¯ora were also reported by Hassink (1994).

beneath conifers than beneath deciduous species. The absence of a strong relationship between

Scheu and Parkinson (1995), who compared aspen

C mic and clay content has also been reported by soil pro®les with those of Pinus contorta, also found Hassink (1994) and Beck et al. (1995). Conditions

that less C was incorporated into microbial biomass for microbial biomass can be unfavourable at high

in the coniferous forest ¯oor than in that of aspen. soil clay contents, possibly because of insucient

At the same time, relatively more C, expressed as drainage and aeration. Aggregate size itself, which

speci®c respiration, was released from the forest clearly in¯uences soil porosity, can also have signi®-

¯oors under conifers than under aspen. The main- cant e€ects on microbial biomass. Miller and Dick

tenance of a relatively high C mic -to-C org ratio

1087 despite a high speci®c respiration in birch forest

Soil microbial biomass and activity

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Acknowledgements ÐWe are very thankful to Claire Cabrera M. L. and Beare M. H. (1993) Alkaline persulfate Vasseur for laboratory assistance. The project received

oxidation for determining total nitrogen in microbial funding from the Natural Sciences and Engineering