Biological processes in two beech forest soils treated with simulated acid rain—a laboratory experiment with Isotoma tigrina (insecta, collembola)

Soil Biol. Biochrm. Vol. 23, No. 4. pp. 381-390. 1991 Printed in Great Britain. Ait rightsreserved

003&!3717;91 s3.00 + 0.00

Copyright f 1991 PergamoaPresspk

BIOLOGICAL PROCESSES IN TWO BEECH FOREST SOILS TREATED WITH SIMULATED ACID RAINA LABORATORY EXPERIMENT WITH ISOTOMA TIGRINA (INSECTA, COLLEMBOLA) v. WOLTERS Zoologisches Institut der Universitlt G6ttingen. Abtcilung Gkologie, Berlinerstrasse 28. D-3400 G6ttingcn. Germany (Accepted

I October

1990)

etTcct of simulated acid rain on soil biotic processeswas studied in a beech forest on moder soil using experimental chambers containing intact soil cores sampled in the Soiling research area (Germany). Substrate from a limed area was included in order to analyse the influence of a modified state of base saturation on the reaction of soil organisms to acid rain. Water adjusted to pH 4.15 with HsSO, was applied to the control treatments to simulate the proton deposition by precipitation, and water adjusted to pH 2.95 was applied to the acid rain treatments to simulate the proton input by stemflow water. In the acid treatment of the natural soil, CO,-C production, C mineralization of the litter colonizing microflora. microbial biomass in the F layer and leaching of nitrate-N were reduced. In contrast, the metabolic quotient of the F microflora as well as leaching of mineral-N (ammonium-N + nitrate-N) were increased by acid rain. According to its effect on CO,-C production in the L layer as well as on the mineralixation of “C-labelled beech litter, the negative effect of acid rain on the early stage of litter decomposition was especially strong. After termination of the acid rain regime, the litter colonizing microflora responded rapidly and their “CO1 production approached that of the controls within 3 weeks. The collembolan species /so(onra rigrina reduced C mineralization of the litter colonizing microflora. diminished the biomass and the metabolic quotient of the microflora in the F layer and accelerated the leaching of mineral-N in the pti 4. I5 trcatmcnt of the natural soil. For many propertics measured these etfccts were significantly ditfcrcnt in the lime and in the pH 2.95 treatments. In the acid rain treatment of the natural soil. I ri@u diminished the acid-induced leaching of mineral-N and accelerated the recovery of the litter coloniting microflora after acid rain had been terminated. The maofauna may thus form an important part of the biological buffering system in the organic layer of acid soils. In the limed soil, the elfcct of acid rain on many of the properties measured was less pronounced than in the natural soil. It was thus concluded that liming of acid forest soils increases the butTering capacity of biotic processes against acid stress. However, the buffering effect on the early stage of litter decomposition was diminished by the fact that in the limed soil, in contrast to the natural soil, the litter-colonizing microflora was not able to respond rapidly after termination of the acid rain regime. In contrast to its etlkct in the natural soil, I rigrinu did not accelerate recovery. Summary-The

lNlRODUCTlON Acid rain strongly affects soil processes (Alexander, 1980; Francis, 1986; Persson. 1988a). It is thus commonly accepted that the so-called “forest decline” in Central and Northern Europe can partly be attributed to the effect of increased rain acidity on soil (Wellburn, 1988). However, almost every property for the activity of soil organisms measured (e.g. rate of C and N mineralization or enzyme activity) has been demonstrated to be either reduced. stimulated. or remain unaffected by simulated acid rain (Will rf 01.. 1986). Similar conclusions have been drawn for the direct effect of experimental acidification on soil organisms (Baath er al.. 1980; Hagvar. 1984; Hagvar and Kjondal. 1981). In the beech forests of the Soiling area (Germany) the weight of the organic layer has almost doubled during the last IS yr. Thcrc is much evidence that this accumulation of organic material indicates an alteration of decomposition processes caused by acid loading of precipitation and associated trace metal toxicity (Matzner. 1988). However,

the doubling of the organic layer in the Soiling area was not accompanied by significant changes in the structure of the decomposer fauna (Schauermann. 1987). These seemingly contradictory results lead to the hypothesis that, in the beech forests of the Soiling area, acid rain alters the timecourse of litter decomposition indirectly by affecting functional relationships in soil rather than directly by affecting soil organisms (Wolters and Schaefer, 1991). To test this hypothesis. a research programme was started, including both field and laboratory experiments. In one simulated rain containing different approach, amounts of protons was applied to experimental plots to study the e&t of increased acid loading rates of precipitation on soil fauna in the field (Heiligenstadt et al., 1991). In a second approach, the effect of various animal species on microbial processes was studied in a number of substrata which had been exposed to diffcrcnt amounts of simulated acid rain in the field (Wolters, 1991). In a third approach, simulated acid rain was applied to laboratory systems to obtain an insight into processes affected by acid 381

V. W0LrElts

382

rain. Results from the last approach are reported in this paper. Biotic interactions as well as their effect on nutrient turnover are strongly dependent on soil conditions (Hagvar. 1991; Walters, 1991). The conflicting results regarding the effects of acid rain on essential processes in soil may therefore partly be explained by the fact that the influence of increased loading rates of acid rain on functional relationships in soil are also significantly altered by soil conditions (Uhich, 1987; Schaefer, 1991). Thus. our study focussed on processes which have proven to be most important in the beech forest soils of the Soiling area: microbial colonization of leaf litter, size and metabolic activity of the microflora living in the highly rooted fermentation layer and the interaction between grazing animals and soil microflora (Ellenberg et al., 1986). To induce a strong but not unnatural acid stress, water with a proton content close to that of the stemflow water of Soiling beech trees (cf. Schafer. 1988) was applied to soil cores sampled in the field. The high input of protons via stemflow water has been proven to alter decomposition processes in the proximity of beech trees even in seemingly well buffered soils (Wolters, 1989a; S. Scheu, unpublished Ph.D. thesis. University of Giittingcn. 1989). To study the interaction bctwccn animal grazers and microflora. the collcmbolan spccics kofon~a rigrim (Nicolct, 1842) was introduced to some of the expcrimcntal chambers. The lcaching of mineral-N was mcasurcd to rclatc the cffcct of acid rain on soil biotic proccsscs to its potential cffcct on nutrient transfer to primary producers. Substrate from a limed arca was included in this study in order 10 analyst the influcncc of a modilicd state of base saturation on the reaction of soil organisms to acid rain. ,MATEWlAlS AND METIIODS

In 1986. soil substrate was sampled from the BI area of the Solling research project (cf. Ellenberg cl ul.. 1986) and from an area close to BI which had been limed in 1975 and 1980 with 6 t ha-’ slag and 4 t ha-’ dolomite respectively (Matzncr. 1985). The study site is situated on the plateau of the Soiling mountains (Germany), 500 m above sea level (9’ 30’ east, 51” 40’ north). The soils in this area are Typic Dystrochrepts (Acid Brown Earth with the humus type moder) developed from a 40 to 60 cm deep loess overlying New Red Standstone. The area is covered by a 147.yr-old beech forest (Fugus sylmico L.). The annual mean tempcraturc is 6.2 C with mean annual rainfall of 1000 mm year-‘. The experiment was carried out in the hbOrdtOry for 132 days. The 32 experimental chambers consisted of IS cm lengths of pcrspex 1ubing (6 cm i.d.) scaled onto ceramic plates (see Woltcrs, 1989b for a more detailed description). The cylinders were capped with a gas-tight lid fitted with tubes to flush the hcadspacc for CO: determinations. By creating subpressure in a chamber below the ceramic plate it was possible to simulate the matrix potential of soil. In both areas. the organic layer was removed from 3 randomly selected plots (1 m?). The uppermost 3 cm of the A, horizon (including the very thin H layer) were cut directly into the pcrspex tubes of the experimental

chambers without disturbing soil structure. Substratum from the L layer and from the F layer was collected separately into plastic bags and transported to the laboratory. All non-beech leaf litter was removed from the samples of the L layer. The field moist samples from the F layer were sieved ( c 4 mm). Visible animals were removed by hand sorting. Fresh substratum equivalent to 18.4 g dry wt (L layer) and 27.6 g dry wt (F layer) were filled into the respective experimental chambers of both soil treatments to simulate the natural stratification within the organic layer of the investigated soils (cf. Ellenberg et 01.. 1986). Rainfall was simulated by applying 7.75 ml of acidified water to the experimental chambers daily, corresponding to an annual input of IOOOmm. The control treatments were watered with distilled water adjusted to pH 4.15 with H$O, to simulate the proton deposition by precipitation in the Soiling area (0.8 kg H+ ha-’ yr-‘, Matzner. 1988). Acid rain treatments were watered with distilled water adjusted to pH 2.95 with H,SO, to increase the proton content of the simulated acid rain to I6 times that of the controls. Although two acid rain treatments are in fact compared, only the treatments watered with rain of pH 2.95 are referred as to “Acid rain treatments” later in the text. Acid rain was applied in two intervals from day I to day 20 and from day 55 to day 132 to simulate the temporal variability in the input of protons in the field. In the interval bctwccn acid rain applications, wnlcr adjusted to pH 4.15 was applied to all cxpcrimcntal chambers. At the beginning of 1hc cxpcrimcnt, 40 adult 1. rigrinu from laboratory cultures which had been set up with spccimcns collcctcd close to the B I arca wcrc transfcrrcd to each of the 16 cxpcrimcntal chambers. The mean density of 1hc collcmbolan population in the Soiling arca, which is dominated by lsotomidac and Onychiuridae, is 63,000 Ind. m-r (C.V. =44%; Ellenbcrg et ul.. 1986). The addition of 40 1. tigrinu per experimental chamber corresponds to an increase of collcmbolan density by half of the variation observed in the ficld (14.000 Ind. m’ = 22% of annual mean density). It was assumed that carrying capacity was not exceeded by this increase. The experimental design can bc summarized as follows: (I) natural soil, pH of simulated rain: 4.15; (2) natural soil, 40 1. rigrina, pH of simulated rain: 4.15: (3) natural soil, pH of simulated rain: 2.95; (4) natural soil, 40 1. rigrinu, pH of simulated rain: 2.95; (5) limed soil. pH of simulated rain: 4.15; (6) limed soil. 40 I. ri.qrinu. pH of simulated rain: 4.15; (7) limed soil, pH of simulated rain: 2.95; (8) limed soil, 40 1. ti+qrinu, pH of simulated rain: 2.95. Each combination of factors was set up with 4 rcplicatcs. The cxpcrimcntal chambers were kept in permanent darkness at a tcmpcrature of 10-C and at 100% air humidi1y. The C content of soil material was determined using a Carlo Erba clcmcntal analyser. The C content of the substratum from the natural soil amounted to

Biological processes in two beech forest soils dry wt (L layer) and 31.6% dry wt (F layer). The respective values for the timed soil were 44.8 and 29.5%. The decomposition of freshly fallen litter was studied by means of beech leaves homogeneously labelled with “C (27 nCi mg-’ C; cf. Walters, 1989b). The soluble carbohydrates were removed from the particulated leaves by extraction with a methanolchloroform-water mixture (Dickson, 1979). 60 mg of the remaining leaf particles were placed on top of the L layer at the beginning of the experiment. There was a continuous flow of COZ-free air above the soil cores. The CO*-free air was passed through the experimental chambers, and the effluent gas was bubbled through 10 ml I N NaOH with a constant gas Row rate. Soil respiration was determined every 2 weeks in 0.S ml aliquots by precipitating the carbonate with BaCl, and titrating the excess base with 0.1 N HCI. The production of r4COZ was determined in 0.5 ml aliquots by liquid scintillation counting (see Fig. 2 for points at which “COr-C was determined). The C mineralization rates (IOC) in the L, the F and in the A, horizon were determined at the end of the experiment. To reduce effects of disturbance, the L and the F layers were removed one after another and the CO& production of the remaining substratum was measured. The CO: produced was absorbed in KOH during 48 h after each layer had been removed and the CO,-C production of each horizon was calculated by subtraction. The “C content of each layer was dctcrmincd by means of an oximat oxidizer. Microbial biomass in the F layer was mcasurcd by the fumigation-incubation method (Jcnkinson and Powlson, 1976). Following Vance ef al. (1987). biomass in the F layer of the natural soil was dctcrmincd without subtracting a control to overcome the bias of the fumigation-incubation method in acid soils. The horizon-specific C mineralization rate in the F layer was used to calculate the metabolic quotient 4 (unit CO,-C unit-’ biomass-C h-r; Anderson and Domsch, 1985). The soil solution passing through the ceramic plate was collected in glass vessels and was analysed every 2 weeks for NH,+-N and NO;-N by 46.6%

383

steam distillation using the MgO-Devarda alloy method (Keeney and Nelson. 1982). Organic-N was not determined and no corrections were made for possible interference with NH,+ yield from organic-N compounds. Tbe sum of ammonium-N and nitrate-N is referred to as mineral-N,, later on the text. The pH of the soil substrates was measured in water at the end of the experiment (soil-water = l/2.5). All measurements were replicated at least twice. Unless otherwise stated, the results are expressed on an oven dry weight (OD) basis of soil (IOS’C). Statistical analysis was performed by means of a 3-way ANOVA. Treatment means were compared using the Tukey test (0.05 level). RESULTS

General comparison between treatments

Theresultsofthe3-way ANOVAshowninTable I indicate that a very large part of variance of the measured variables is explained by the acid rain treatment or by the lime treatment. The effects of 1. tigrina explained only a small proportion of the total variance. However, the 3-way ANOVA showed significant main effects of I. rigrinu on the biomass of the F microtlora, on total CO,-C liberation and on cumulative leachate concentration of nitrate-N. Lime x 1. tigrina interactions as well as acid rain x 1. tigrina interactions wcrc significant for the leaching of mineral-N,,,,. In addition, lime x I. tigrina interactions wcrc significant for cumulative CO:-C production and acid rain x I. tigrina interactions wcrc signiticant for the metabolic quotient 4 of the microflora in the F layer. Main acid rain elTcxts wcrc significant for almost every variable measured. The only cxccption was the cumulative mineral-N,“,, loss. With the exception of C and r4C mineralization. the main ctrccts of lime treatment were significant for every variable listed in Table I. All main effects of the lime treatment were accompanied by significant lime x acid rain interactions. Significant second-order interactions were limited to cumulative “CO1-C production.

Table 1. Results of the 3-way ANOVAS on the influence of liming (LM). simulated acid rain (SAR) and 1. rigrlno (I) on various variables for microbial C turnover as well as on cumulative mineral-N,,, and NH, -N lossesin leachatn (“CO& IOI~I:cumulative CO,-C production from “C-lab&d beech litter; CO,-C total: cumulative CO& production; metabolic quotient 9: unit CO,-C unit-’ biomass-C h-‘; biomass: mg biomass-C g-’ soil: N min. to~rl: cumulative mineral-N loss in Icachato: NO, -N total: cumulative NO, -N loss in Icachalcs) Main CITCCIS Factor d.f. “CO& co,-c

SAR I ... ... lolaI

R3P .....

101al

69.0 .....

.....

IS.6 .....

51.2 .....

22.4

65.8 .....

MetabolicC quotient 9 Biomass’ N min. total NO,-N

LM I

total

lnleraclions

I I

SARxLM I

IxSAR I

IxLM

I

IxSAR x LM I ..

. 3:7

...

.

8.8 ..*

2.4

.. 2.1

3.6 .....

.

.....

64.9 .....

.

9.2

84.5

0.6

13.9 ..

2.6

3.0

2.8

Error 24 10.6 20.9 14.9 5.2

.

13.4

2.6

3.8

1.2

‘Asterisks indicate significance lcvcls of F values: l **** = P < O.ooOl; l ** = P < 0.001; l = P c 0.05. bNumbcn indicate percentage of explained variance. cMetabolic quotient and microbial biomass-C in the F layer.

l

* = P < 0.01;

v. WOLTEIU

384

LM

M

mgC/gC/d 1.0

0.6

0.6

mgC/gCtU

0.4

0.2

0

0

0.2

0.6

0.4

0.6

1.0

1

+

SAI?

0

control

Fig. I. Rate of C mineralization (10°C) in the L layer (L), in the F layer (F) and in the A,, horizon(A) of a natural (M) and of a limed (LM) soil (points represent means of 4 replicates).

The injlucnce of acid rain in the natural soil

As mcasurcd at the end of the experiment, the dcprcssion of C mineralization by acid rain in the L layer was greater than in the F layer (P < 0.05; Fig. I). The strong etfect on the early stage of litter decomposition was accompanied by a significant reduction in “COI-C liberation [Fig. 2(a)]. At the beginning, however, “CO& production was stimulated by acid rain [Fig. 3(a)]. After about IO days “CO2 liberation decreased in the acid rain treatment and rapidly reached a minimum of 20% of the control. When the pH of the rain solution was changed from 2.95 to 4.15, “CO#Z production recovered to control levels within 3 weeks [Fig. 3(a)]. When acid rain was reapplied to the chambers, the rate of “C mineralization decreased once again. As in the beginning, this decrease was preceded by a stimulation of “CO& production.

The rate of C mineralization rapidly dccrcascd in the natural soil with increasing depth (Fig. I). It can bc calculated from the data shown in Table 2 that about 60% of the mineral-N,,,, leached from the soil cores was released as ammonium. Acid rain decreased the pH in the L. F and A, horizons. As mcasurcd at the end of the experiment, increased input of protons depressed the rate of C mineralization in the L and in the F layer but increased it in the Ah horizon (Fig. I). Acid rain diminished the cumulative CO2 production by 35% and reduced the biomass, but increased q of the F microflora (Table 2). Nitrate loss was strongly reduced in the acid rain treatment. In contrast, acid rain strongly stimulated the leaching of NH:-N and thus increased cumulative leachate concentrations of mineral-N,,,, .

2. Influenceof simulated

Table and

mineral-N,,,

(NH;-N

acid rdin

(DH

2.95) on oH.

+ NO<-Nj’losres

various

in k&hates

variables

(L:

L layer;

for microbial F: F layer;

Cumulative

Microbial

c0r-C mt3 C per microcosm

~Hn,o

(mg) in leachatc

(me) in lcachatc

mciln

mean

SE

SE

SE

9.08

3.45 0.92

mean SE

SE

I: M

4.59

4.28

4.1 I

I354

0.0032

IM+SAR

4.27

3.74

3.78

natural h-‘)

and

treatment

267

791

0.84

0.004X’

1613’

13.75’

23

0.0008

265

0.88

0.28

7391’

16.34’

14.61’

0.00

I?

5.10

4.32

1261 43

0.000 I

5.07

4.20

1033”.

O.OOl4’~

microbial

soil; SAR.

biomass-C

I; “significantly

in

diRerent

simulated the from

F

NO;-N

0.0009

5.27

limed

I

N

IO1

4.25

sod: LM.

mineral

$77’

98 ‘M.

Cumulative

rl3C 8” soil mcun

A,

+ SAR

Cumulative

NH ;-N crro;)

quotient**

F

7: LM

SE: standard

biomass*’

L

5: LM

and cumulative

horizon;

Metabolic

9 mean

Treatment’

C turnover

A: A,

layer.

Rcsuhs

2.95). of

the

5: %gnificantly

I.73

43 I S’.b.C

0.0001

acid rain (pH

treatment

741

Tukey

test

different

(P

I I .3P.’

0.39

quotient from

2.72

14.78’.b.’

197

l*Mctabohc

1.74’

cO.OS):

(unit

CO&

‘significantly

treatment

3.

0.28 unit-’

biomass-C

different

from

Biological

processes in two beech forest soils

The influence of I. tigrina in the natural soil

385

(a)

In the natural soil. I. rigrina diminished total CO& production (NS) but significantly stimulated the leaching of NO;-N and mineral-N,, (Fig. 4). According to the measurements made at the end of the experiment, the effect on total C02-C production was largely due to a marked reduction in the rate of C mineralization in the F layer (- 25%; P > 0.05). This effect was accompanied by a negative effect on both the biomass and the metabolic activity of the F microflora. In addition, 1. tigrina reduced the rate of “C mineralization from the very beginning, effecting a reduction in total “CO& production by 14% [Fig. 2(a)]. During both periods of acid rain application. the negative effect of I. tigrina on the mineralization of labelled leaf litter was less pronounced than in the control. The reduction of the microbial biomass in the F layer, in contrast. was stronger in the acid rain treatment than in the control (Fig. 4). In the experimental chambers treated with acid rain, opposite to its effect in the control, 1. rigrina increased q and reduced the leaching of NO; and mineral-N,,, . For a limited period of time and in contrast to effects otherwise ohscrvcd. I. figrintl stimulated “CO? production after acid rain had been terminated [Fig. 3(a)]. This elfect was so strong that more “COz-C was produced in the acid rain trcatmcnt containing 1. Qtyinn than in the acid rain trcntmcnt without 1. rigrina during the course of the cxpcrimcnt [Fig. 2(a)]. (a) - Control _ + I. (lgflna

240 -

a11 0

I

I

t

I

20

40

60

80

1

I

100

I

l20

140

loo

120

la

Time (days) 1.1 r

0

tb)

20

40

60

60

Time (days I Fig. 3. lnflucnce of simulated acid rain (pll 2.95) on “CO,-C production from labclled bce%h litter components placed on top of the L layer of IWO beech forest soils: (a) natural soil and (b) limed soil (deviation from the rcspcctivc treatments without acid rain application and without added springtails). Points rcprcstmt means of 4 rcplicatcs: the first arrow indicates the point a; which acid rain application was tcrminatcd, the second arrow indicates when it was restarted.

160 -

f g

Eflects of lime treutments

120 m40-

60

60

100

120

140

Time tdoysJ

0

20

40

60

60

100

120

140

TlmeCdaysJ Fig. 2. Cumulative CO& production from structural components or “C-1abelled beech-leaf litter placed on top of the L layer of two beech forest soils: (a) natural soil and (b) limed soil (points represent means of 4 replicates). Control: experimental chambers were watered with artificial rain of pH 4.15: +SAR: expcrimcntal chambers were watered with artificial rain of pH 2.95; I. rigrina:expcrimental chambers to which 40 individuals of this collcmbolan species were added at the beginning of the experiment.

DifTerences in the pH value between the natural and the limed soil were limited to the L and to the F layers (Table 2). The cumulative CO& production was significantly lower in the limed soil than in the natural soil. As measured at the end of the experiment, however, the horizon-specific C mineralization rates were very similar in both soils (Fig. I). The leaching of mineral-N,,,, was significantly increased in the limed soil (Table 2). According to the timecourse of “CO* production, the initial colonization of beech leaf litter was slightly promoted in the limed soil [Fig. 2(b)]. In the limed soil, in contrast to the natural soil, the decrease in the pH value induced by acid rain was limited to the L layer (Table 2). However, the suppressive effect of acid rain on C mineralization in the F layer was greater in the limed than in the natural soil (Fig. I). With a dccreasc of 18%. the depression of total CO, production was less pronounced in the acid rain treatment of the limed soil than in that of the natural soil (Table 2). Nevertheless, the effect of acid rain on the biomass and on y of the microflora of the F layer, though different in extent, was basically identical to the natural soil (Table 2). In the limed soil, in contrast to the natural soil, acid rain diminished the leaching of N. Nitrate loss only decreased from 89 to 77%. The rate of C mineralization in the L layer (Fig. I) as well as the time-course

386

Biomass

Mctobollc

quot~cnt

Mineral-N

*3

Parametrr Fig. 4. Influence of the collembolan species 1. tigrino on various variables for microbial C turnover as well as on cumulative mineral-N and NO;-N losses in leachotes from a natural and a limed beech forest soil. All values arc expressed as pcrccntagc deviation of the I. figrina treatment from the respective trcatmcnt without added 1. rigrinu shown in Table 2 (CO&: cumulative CO& production; biomass: C pool of the microflora in the F layer: mctubolic quotient: metabolic quotient of the microflora in the F layer: mineral-N: cumulative mineral-N loss in Icachatcs; NO,: cumulative NO;-N loss in lcachatcs).

of “COr liberation [Fig. 3(b)] indicate that the litter colonizing microllora of the limed soil was less alfcctcd by acid rain than that of the natural soil. In contrast to the natural soil. acid rain caused no initial stimulation of “C mineralization. According to the time-course of “CO,-C production, the litter dwelling microflora of the limed soil, in contrast to litter colonizers in the natural soil. was unable to respond rapidly after termination of acid rain application. At the end of the experiment, the effect of acid rain on cumulative “‘CO2-C production was thus almost identical in both soils (Fig. 2).

The in~utwcnceof I. tigrinu in rhe limed soil In the limed soil, similar to the natural soil. 1. rigrinu reduced the biomass and y of the F microflora and also increased cumulative leachate concentrations of NO;-N and mineral-N,,, (Fig. 4). In contrast to the natural soil, 1. rigrku significantly accelerated CO& production. The fact that this species reduced “CO,-C production in the natural soil but stimulated it in the limed soil reveals that the ctkct of 1. tigrina on the litter colonizing microflora was complctcly altered in the lime trcatmcnt [Fig. 2(b)]. In the limed soil, as in the natural soil. application of acid rain significantly modified the intluencc of 1. rigrino. Howcvcr. the acid-induced modification of the 1. figrinu effect on q of the F microflora as well as on leaching of NO;-N and mineral-N,,, was less pronounced than in the natural soil (Fig. 4). It can bc concluded from the effect 1. rigrina had on the mineralization of “C-labelled litter (Fig. 2). on total CO:-C production by soil respiration and on micro-

bial biomass in the F layer (Fig. 4) that cxpcrimcntal acidification of the limed substratum diminished the diffcrcncc bctwccn the limed and the natural soil. In contrast to the natural soil. however. I. rigrina did not accelerate rccovcry of the litter colonizing microflora after acid rain application had been tcrminatcd [Fig. 3(b)]. DlSClXXON The eflect of ucid ruin on microbid

C turnover

The strong suppression of CO?-C liberation by simulated acid rain in my study supports the conclusion of Matzner (1988) that increased acid loading rates of precipitation might be responsible for the accumulation of organic material observed in the Solling area. In general, these results are consistent with results of other authors which have shown that increased rain acidity affects the decomposition of organic matter in forest soils (cf. Francis. 1986). As indicated by the strong depression of “CO? liberation and CO,-C production in the L layer. the early stage of leaf litter decomposition is cspccially sensitive to acid stress in the investigated modcr soil. However, our cxpcrimcnt was carried out over a relatively short period under continually moist conditions as well as with a strongly increased rate of proton input. The results cannot thcreforc simply bc extrapolated to long-term effects of acid rain in the field. There is evidcncc that variations in the proton input might significantly alter the cffcct of acid rain in the field. The litter colonizing microflora was shown to be capable of relatively fast recovery after termination of acid stress. In addition, acid rain application effected

Biological processes in two beech forest soils a short-term stimulation of “CO,-C production. In the field, where periods of increased input of protons by rain may be very short. both processes may diminish the negative effect of acid rain on litter decomposition. From 1 to 3% of soil organic-C is usually contained in the microbial biomass (Jenkinson and Ladd. 1981). With a C content of about 0.9%. the microbial biomass in the F layer of the natural soil was thus rather small. The values presented here are similar to those given in my earlier paper (Walters, 1989b; fumigation-incubation method) but lower than the values given by Ellenberg ef al. (1986; SIR method). This could be partly accounted for by the different methods used. The determination of microbial biomass as well as the calculation of ecophysiological variables have provided very fruitful information about the correlation between microbial performance and environmental conditions (e.g. Insam and Haselwandter, 1989). The metabolic quotient seems to be an especially good indicator for environmental effects on the soil microflora (Anderson and Domsch, 1990; Wardle and Parkinson, 1990). In my study, acid rain reduced the biomass and increased the metabolic quotient of the microflora in the F layer. The increased metabolic quotient in the acid rain treatment is consistent with the results of Killham (1985). who suggested that the soil microflora reacts to cnvironmental stress with an increase in metabolic activity. The microflora is a most important sink for mobile plant nutrients (Jcnkinson. 1988; Paul and Clark. 1989). In the highly rooted Flayer of the Soiling arca, a reduction of the microbial biomass conncctcd with an increased metabolic activity might thus srriously affect nutrient transfer to the primary producers. The eflecf of acid rain on rhe leuching of mineral-N

The leaching of mineral-N was used as a measure for the relationship between the ellcct of acid rain on microbial carbon turnover and the mobilization of plant nutrients. In the natural moder soil investigated, the application of acid rain stimulated the leaching of mineral-N corresponding to an annual loss of 17 kg N ha-’ yr -I. This calculation is most probably an underestimation, because the loss of organic-N has not been taken into account in my study. The mobilization of N by acid rain application is consistent with the effect of simulated acid rain in other soils (e.g. Tamm. 1977; Haynes and Swift, 1986). Field measurements in the Solling area, however, gave no indication of an acid-induced loss of N (Matzner. 1988). A possible explanation for this contradiction to our laboratory experiment is that an acid-induced mobilization of N was not detectable in the field. because N was kept within the system due to uptake by plants (cf. Anderson and Leonard. 1988). In addition, a loss of N may have been compensated by N deposition which amounts to 42 kg N ha-’ in the Solling area (Matzner. 1988). The following discussion of the possible relationship between the measured effects of simulated acid rain on microbial-C turnover and the increased leaching of mineral-N will be confined to the biotic aspects of N mobilization. The effects of acid rain on factors such as cation exchange capacity and nutrient balance will be reported (Wolters and Meiwes. in

387

preparation). One reason for the leaching of mineral-N from the experimental chambers treated with acid rain was most probably the decrease in microbial biomass in the F horizon. On the one hand, this led to a mobilization of N rich compounds from the microorganisms killed (cf. Aber et al., 1982). On the other hand, it reduced the storage capacity of the microflora and thus diminished the microbial immobilization of N (cf. Persson. I988a). In addition, the stress-induced increase in metabolic activity may have shifted the microflora in the F layer to a state of less economic use of N. A further, very important reason for the increased leaching of mineral-N may have been the strong suppression of the litter colonizing microflora. This effect may have hindered both the transfer and the storage of N in the carbon rich litter layer. The increased C: N ratio in the litter layer of the acid rain treatment (Walters and Meiwes, in preparation) confirms this conclusion. Nitrate is primarily produced by heterotrophic nitrification in the beech forests of the Soiling area (Runge. 1973). Heterotrophic nitrification is less sensitive to acid rain than autotrophic nitrification because it is carried out by a wide range of microorganisms differently adapted to the acidity of their environment (Alexander, 1980). The strong suppression of hcterotrophic nitrate production by acid rain thus also points to gcncral damage to the microflora. The eflbct of acid ruin on the inlaraction I. tigrinu ond edaphic microjloru

between

I. tigrinu lives in various microhabitats and under a great variety of soil conditions (Fjcllberg, 1979). It is thus well suited for comparative studies. In the modcr soil of the Solling arcu, increasing feeding pressure of I, rigrinu has been shown to eticct a gradual decrease in the C mineralization of the litter colonizing microflora (Woltcrs. 1990). There is much evidence that the influence of collcmbola on the mobilization of NH,+, Na+ or Ca” (Anderson ef ul.. 1983; Teuben and Roelofsma. 1990) is due to their influence on microorganisms in soil (Anderson CI (11.. 1985; Verhoef er al.. 1989). Springtails can efficiently alter the growth conditions of decomposer microorganisms (Newell, 1984; Visser, 1985) and may exhibit a highly selective feeding pressure on soil fungi (Bengtsson et al., 1988). Ineson et ul. (1982) have shown that significant increases in the leaching of N and Ca occurred as a consequence of a depression of fungal biomass by grazing springtails. Ncverthelcss. only little is known about the correlation between the effect of springtails on the edaphic microflora and their effect on nutrient turnover. In the expcrimcnt presented here, the increased mobility of mineral-N in the 1. rigrina treatment can partly be cxplaincd by the stimulation of nitrification (cf. Anderson and Leonard, 1988). The animal induced leaching of mineral-N can also be related to the dcprcssion of the litter colonizing microflora as well as to the reduction of the microbial biomass in the fermentation layer. In addition to the direct contribution to the N pool via excretory N, I. tigrino may have hindcrcd the microbial immobilization of N. The correlation between the strong depression of the microflora in the F layer of the limed soil, which is dominated by bacteria

388

v. W0LTERs

(Lang and Beese, 1985). and the remarkable mobilitation of mineral-N confirms this conclusion. It seems very likely that a reduction of the bacterial biomass by I. rigrina of the order observed in the limed soil is rather due to a modification of the microbial growth conditions than to a direct suppression by grazing. 1. figrina shows very specific reactions to experimental acidification (Huang and Winter, 1986). However, despite the numerous investigations on soil acidification on the population structure of edaphic animals (cf. Hagvar. 1987). only little is known about the effects of acid rain on the interaction between soil fauna and microflora. In the natural soil investigated, acid rain reduced the effect of 1. tigrina on the litter colonizing microflora and increased its effect of the biomass of the microflora in the F layer. The modification of the effects I. tigrina had on microbial performance in the acid rain treatments of the natural soil reversed the effect of this species on nitrification and cumulative mineral-N lossesin leachate. Though this might point to a serious alteration of the effect of grazing animals on the turnover of plant nutrients. it essentially counteracts the effect acid rain had on the leaching of mineral-N. In addition, I. tigrina ncccleratcd the recovery of the litter colonizing microflora when the H+ content of the simulated rain was lowered. Obviously, stimulatory effects of 1. rigrina (c.g. substrate inoculation; cf. Visscr. 1985) wcrc more dominant than inhibitory efTccts during the microbial recolonization of beech leaf litter. The mohilc mcsofauna may thus form an important part of the biological bufTcring system of acid soils.

substratum by autotrophic nitrifiers (Lang and Beese. 1985) which are assumed to be very sensitive to acid rain (Alexander, 1980). Additionally, the fact that acid rain application, in contrast to the natural moder soil. diminished the leaching of mineral-N from the limed soil confirms the conclusion of an increased buffering capacity: presumably acid rain depressed microbial activity but, in contrast to the natural soil, did not effect a mobilization of N-rich compounds from microorganisms killed. However, the negative effect of acid rain on the decomposition of freshly fallen litter and on the microflora in the F layer were fundamentally the same in both the natural and the limed soil. The fact

that simulated acid rain strongly affected the microbial biomass but not the pH value in the F layer confirms the conclusion of Ulrich (1987) that acid stress may have strong effects even in seemingly well buffered soils. This conclusion is further confirmed by the slow response of the litter colonizing microflora to the termination of acid rain application. The negative effect of acid stress on leaf litter mineralization in the limed soil was thus prolonged. even during periods when no acid rain was applied. With respect to the cumulative CO& production from freshly fallen leaf litter, the buffering effect liming had on acute acid stress was virtually eliminated by the modified reactivity of the litter colonizing microflora in the limed soil. It is known that there are considerable difTcrenccs in the adaptation of dccomposcr communities to the acidification of their environment (Baath ef al., 1984). The diflcrcnt reaction of litter colonizing microflora in the natural and in the limed

The data prcscntcd hcrc confirm that liming significantly affects biological processes in soil (Persson, 1988a. b). My results support previous observations that liming of forest soils leads to an increased leaching of N as well as to a stimulation of nitrification. The marked increase of the microbial biomass in the F layer as well as the significant decrease of the metabolic quotient confirm the results reported by Lang and Bcese (1985) and Wolters (l989b). However, the principal effect of liming on soil processes lies beyond the scope of this paper. The discussion will thus bc confined to the effect of acid rain on the limed soil.

The numerous interactions between the acid rain treatment and the lime treatment revealed by the 3-way ANOVA indicate that the sensitivity of the decomposer organisms to acid rain was significantly altered in the limed soil. Liming of acidified forest soils is an increasingly important silvicultural method (Dcromc CI al., 1986; Andersson and Persson. 1988). It is thus surprising that the influcncc of acid rain on limed forest soils has ncvcr been invcstigatcd. According to the results prcscntcd in this paper, liming of acid beech forest soils may increase the buffering capacity of many soil biotic proccsscs against acid stress. This conclusion is confirmed by the determination of soil respiration. C mineralization of the litter colonizing microflora and the metabolic quoticnt of the F microflora. The low effect of acid rain on nitrification also indicates an increased buffering capacity. because NO; is produced in the limed

soil to the termination of acid rain points to the importance of factors such as inertia. stability and rcsilicncc for the evaluation of the cffcct of environmental stress on biological processes (cf. Underwood, 1989). The fact that 1. tigrino, in contrast to its effect in the natural soil. had no influence on the recovering capacity of the litter colonizers might indicate that acid soils treated with lime may lose an important part of their biological buffering capacity. Acknow(eclge~tenrs-This work was supported by the Bundesministerium fiir Forschung und Technologie (Germany). 1 would like to thank Professor Dr M. Schaefer for his continuous support; R. G. Joergensen and K. J. Meiwes for their critical comments on the manuscript; K. Winter and P. Huang for providing the I. rigrinacullures: Mrs M. FrankeKlein and the stad of the “Zentrales lsotopenlaboralorium, G&ingen” for their technical assistance.

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