Plant and Soil 168-169: 563-570, 1995. © 1995 Kluwer Academic Publishers. Printed in the Netherlands.

Soil respiration in a poor upland site of Scots pine stand subjected to elevated temperatures and atmospheric carbon concentration

Brita Pajari University of Joensuu, Faculty of Forestry, PO Box 111, 80101 Joensuu, Finland

Key words: boreal forests, increased CO2, increased temperature, open top chamber, soil respiration

Abstract

Soil respiration rates under elevated temperature and atmospheric CO2 concentrations were studied in eastern Finland (62°47'N, 30°58'E, 144 m.a.s.1.) around naturally regenerated 20 - 30 years old Scots pine trees, enclosed in open top chambers. The production of CO2 varied spatially and temporally, but clearly followed the changes in temperature measured at the soil surface. However, soil respiration in the open control was higher than that in chambers; i.e. the chamber itself changed the conditions by increasing the temperature, altering the movement of water, and thereby soil moisture. Nevertheless, an elevation in the concentration of atmospheric CO2 raised soil respiration and brought it nearer to the level in the open control. An increase in temperature seemed to inhibit this rise, possibly because of an imbalance between temperature and moisture.

Introduction

Within the next hundred years, the mean annual tem- perature in Finland is expected to rise by 2 - 4 °C due to increases in the concentrations of atmospher- ic CO2 and other greenhouse gases (Holopainen and Carter, 1992; Kettunen et al., 1987). The mean annu- al precipitation is predicted to increase by 10 - 15% (Holopainen and Carter, 1992; Kettunen et al., 1987). Changes in climate will evidently lead to changes in the dynamics of forest ecosystems.

The activity of microorganisms performing decom- position is mainly regulated by the chemical compo- sition of the organic material, as well as by soil tem- perature and moisture. Increases in the concentration of atmospheric CO2 have been found to cause litter to decompose more slowly than under the ambient CO2 concentration, because of the higher C/N ratio and higher concentrations of soluble phenolics and struc- tural compounds (Melillo, 1983). Furthermore, an ele- vated CO2 concentration stimulates growth (Dahlman et al., 1985; Kramer, 1981; Warrick, 1988). Thus, pro- duction of low quality litter may be promoted and the activity of microorganisms may be inhibited. On the other hand, warmer and wetter conditions may stimu- late decomposition and counter the effects of rises in CO2 concentration alone (Melillo, 1983).

The response of biological activity to temperature is often expressed using the temperature coefficient Q10; i.e. the fold-increase in response to a 10°C change in temperature (Linder and Troeng, 1981; Swift et al., 1979). The response to temperature change depends on the ability of microorganisms to acclimatize to the change (Swift et al., 1979). Maximum decomposer activity, however, depends both on the rise in soil tem- perature and the change in soil moisture, as well as on changes in substrate quality (Swift et al., 1979). More optimal moisture conditions, especially in early- stage decomposition, can result in higher decomposi- tion rates than under same temperature, but different moisture conditions (Rout and Gupta, 1989; Virzo de Santo et al., 1993).

Soil warming experiments have been observed to result even in radical increments in the mortality of fine roots, decomposition, and nutrient availability (Red- mond, 1955; Van Cleve et al., 1990). Thus, CO2 evo- lution in the soil has increased, indicating more inten- sive decomposition and microbial activity in the soil (Peterjohn et al., 1993).

Soil respiration consists mostly of the CO2 pro- duced by soil microorganisms and the roots of plants. Rises in the concentration of atmospheric CO2 have been found to substantially enhance root growth (Nor- by et al., 1987; Rogers et al., 1992) and thus, to improve

564

the water uptake efficiency of plants and to increase root activity (Kimball and Idso, 1983; Rogers et al., 1992).

While elevated CO2 concentrations both stimulate root activity and decrease the quality of decomposing substrate, but higher temperatures promote decompo- sition, there is the possibility that total soil respiration may increase compared to the ambient situation. Fur- thermore, if CO2 evolution from the soil increases, it can enhance global warming even further (Jenkinson et al., 1991).

The aim of this study is to find out, how soil respi- ration changes in the forest soil under elevated temper- ature and atmospheric CO2 concentration conditions.

M a t e r i a l a n d m e t h o d s

Study area and experimental set-up

The field experiments were carried out close to the Mekrijarvi Research Station, University of Joensuu in eastern Finland (62°47'N, 30°58'E, 144 m.a.s.1.) in a naturally regenerated 20 - 30 years old stand of Scots pine (Pinus sylvestris L.). The heights of the trees varied between 2 and 4 m. The forest type of the experimental site is Vaccinium type (Cajander, 1949), dominated by Hylocomium, Polytrichum and Pleuroz- ium mosses and Calluna vulgaris and Vaccinium vitis- idaea dwarf shrubs. The soil type is of iron podzol, relatively poor in available nutrients and with fine sand texture.

During the summers of 1990 and 1991, twenty experimental saplings of Scots pine (age 20 - 30 years, height 2 - 3 m) were enclosed in open-top chambers (2.5 m x 2.5 m x 3.5 m), one in each chamber, and four saplings designated as control plots (2.5 m x 2.5 m) (H~inninen et al., 1993). The walls of the chambers were inserted 0.5 m into the soil in order to cut the root connections and to prevent the soil from freezing in the chambers of elevated temperature.

The experiment was a factorial design including the following treatments: (1) open control, (2) cham- ber control with no elevation in temperature or concen- tration of atmospheric carbon, (3) increased air tem- perature, (4) increased atmospheric CO2 concentra- tion, and (5) increased air temperature and atmospher- ic CO2 concentration. Treatment (3) was divided into two levels of temperature treatments i.e. (3) increased air temperature and (6) further-increased air tempera- ture (H~inninen et al., 1993). Each treatment had four

replicates. In the temperature treatments, increased air temperature followed a simulated warming program as described by H~nninen et al. (1993).

In the temperature-treated chambers, the tempera- ture varied between 0 - 5 °C in the winter, and it was occasionally even 30°C higher than outside during the coldest months. In the summer, the chambers were 2 - 3 °C wanner compared to the ambient temperature (Fig. 1).

The concentration of atmospheric CO2 in treat- ments (4) and (5) was increased in the daytime (6.00 a.m. - 6.00 p.m.) during the period May 15, 1992 - September 15, 1992, so that it varied between 400 and 800 ppm (Fig. 2). The mean value was 500 - 550 ppm.

Measurements

The environmental conditions for decomposition were described by measuring soil temperature and mois- ture. This paper deals with temperature effects only. Temperature was measured simultaneously with soil respiration measurements being made from the air, at ground level, and 5 and 10 cm below the surface of the humus. The period over which measurements

30

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-30

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8.

01.10.91 01.01.92 01.04.92 01.07.92 01.10.92

Date

30

b

-15 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

-30

01.10.91 01.01.92 01.04.92 01.07.92 01.10.92

Date

Fig. 1. A i r t empera tu re du r ing the pe r iod J u n e 26 - S e p t e m b e r 26,

1992. a) open control (1), b) further-increased temperature (6).

900 a

800 ...............................................................

700

600

500 O~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3O0

26.06.92 26.07.92 26.08.92 26.09.92 Date

900

-~ 80O

700

_~ 600

500

400

300

b

26.06.92 26.07.92 26.08.92 26.09.92 Date

Fig. 2. A t m o s p h e r i c CO2 concen t ra t ion o f the C O 2 t rea ted c h a m b e r s

and open control during the period June 26 - September 26, 1992. a) open control, b) increased atmospheric CO2 concentration (4).

were made of both temperature and soil respiration was September 1991 - December 1992.

Soil respiration was followed by means of a cuvette method, whereby a cuvette (volume 3.35 x 10 -3 m 3) was placed over the soil, cutting through the humus layer entering into the mineral soil. All the green parts of vegetation were removed at least 30 minutes before the sampling took place. Gas samples were tak- en from the cuvette using syringes immediately after the cuvette was in place, and 2 - 6 minutes and 4 - 12 minutes following adjustment, the sampling inter- vals being shorter during the growing season. The gas samples were analyzed by means of an infra-red gas analyzer (Hartman and Brown Uras 3E) and the results were expressed in mg CO2 m -2 h - l .

A few problems were encountered when measur- ing soil respiration using the cuvette method. First, the CO2 production of plant roots remained unknown; therefore, the soil respiration values presented include both root respiration and the CO2 production of decom- posers. Second, the results of sampling were also inac- curate when the temperature was below 0°C because moisture could condense into the needle and prevent using the sample, and/or the cuvette could not be

565

inserted properly into the ground because of the frozen topsoil. Thus, the lowest reliable measurements made were approximately 10 mg CO2 m -2 h - 1

Analyses

The CO2 production in the soil was expressed using the mean values obtained by treatment and season. The differences between the treatments were analyzed with analysis of variance using least-squares means for imbalanced design (Searle et al., 1980). The analysis by season was conducted using the following classi- fication; ( i )Fal l 1991 (September 1991 - N o v e m b e r 1991), (ii) Winter 1992 (December 1991 - February 1992), (iii) Spring 1992 (March 1992 - May 1992), (iv) Summer 1992 (June 1992 -August 1992), and (v) Fall 1992 (September 1992 - November 1992).

Equation (1) was fitted to the soil respiration data (Running and Coughlan, 1988).

R = a w e k'r., (1)

where R = soil respiration (mgCO2 m -2 h- l ) , a = respiration coefficient (kg °C - l h - l kg-1), e = Neper value (,-~2.718), k = temperature coefficient, T = temperature at ground level (°C), and w = mass of CO2-producing component (kg m-2).

Temperature coefficient (k) is expressed as a function of temperature (Larcher, 1975) (Eq. 2;).

K = (In Qlo)/10, (2)

where Qlo expresses an increment in reaction rate per 10°C rise in temperature (Linder and Troeng, 1981) (Eq. 3).

Qlo = b - cT, Qlo > 0 (3)

where b and c = parameters, and

T = temperature at soil surface (°C) ,T < b/c

Estimates of parameters a, b and c were calculat- ed by using data obtained from each treatment. The differences in parameter estimates between treatments were tested with Student's test (Snedecor and Cochran, 1980). The results give only the trends for analyzing the differences in parameter estimates, because the real risk for rejecting a real Ho hypothesis in pairwise com- parisons increases (Snedecor and Cochran, 1980).

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Results

The production of CO 2 in the soil varied spatially and temporally (Fig. 3), but clearly followed the changes in temperature measured at the soil surface (Fig. 4.). The only treatments that remained statistically similar over the whole time period observed were the chamber control (2) and the treatment of increased temperature (3) (Tables la - e).

All of the treatments behaved similarly also during the winter (Fig. 5.). Soil respiration was significant- ly higher (about 7 mg CO2 m -2 h -n) in the winter

only in treatments of increased temperature and CO2 concentration (5) and further-increased temperature (6) compared to soil respiration in the chamber control (2). During the summer, the production of CO2 was clearly the highest in open control (1).

Furthermore, soil respiration in the treatment of further-increased temperature (6) was intensive at the beginning of the observation period, and then decreased by the fall of 1992 to the same level as in the fall of 1991 instead of remaining at a higher level as it did in the open control (1 ) and especially in the increased-CO2 treatment (4) (Table 2).

Figure 6 shows the curves fitted to the data obtained from each treatment. Estimates for parameters a, b and c and results of Student's tests are presented in Tables 3a - c. The degree of determination varied between 52 - 87%, except in the increased-CO2 treatment (4), where it was between 20 - 68%; this was probably due to the small amount of data under low tempera- tures. Furthermore, the fitted curves gave slight under- estimates for soil respiration under low temperatures. However, residual plot analysis showed that the Equa- tion (1) fitted quite well to the data.

The open control (1) showed the highest soil res- piration rates of all the treatments in relation to tem- perature (Fig. 6.). Increased CO2 concentration (4) was the highest of all the chamber treatments (Fig. 6). Statistical tests showed that there were differences in the value of parameter a (intercept) between the open control (1) and temperature-treated chambers ((3), (5) and (6)), as well as between the chamber control (2) and both increased temperature and CO: concen- tration treatments (5) and further-increased tempera- ture (6). Parameters b and c (slope) differed statisti- cally between increased temperature (3) and further- increased temperature (6).

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23.08.91 01.12.91 11.03.92 19.06.92 27.09.92 05.01,93

Date

• open ctrl * chamber ctrl • T]

o CO2 * TI & CO2 ~ Tf

Fig. 3. Soil respiration during the period September 23, 1991 - December 31, 1992. Legend: open ctrl = open control (1), chamber ctfl = chamber control with no elevation in temperature or atmospher- ic carbon (2), T1 = increased air temperature (3), CO2 = increased atmospheric CO2 concentration (4), T1 and CO2 = increased air temperature and atmospheric CO2 concentration (5), and Tf= fur- ther-increased air temperature (6).

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-5 23.08.91

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- - ~ ~ / ~ , ~ ~ ' ~ ' ~ l 0 01.12.91 11.03.92 19.06.92 27.09.92 05.01.93

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temperature

soil respiration, incr. temperature & CO2

Fig. 4. Soil respiration vs. temperature as measured at the soil surface in relation to time. Increased temperature and CO2 concentration (5).

Discussion

The mean values and the fitted curves indicate that soil respiration in the open control (1) differed from that in chambers; i.e. the analyses of mean values gave a con- siderable peak for the summer but in the fitted curves, however, CO2 evolution in the open control (1) was the highest in all temperatures. These differences can be explained by the slight over-estimations of curves under low temperatures, which is not necessarily the same for every treatment. Nevertheless, the chamber

Table 1. Results of analysis of variance for mean soil respiration. *** indicates statistically significant differences, a) Fall 1991 (Sept. - Nov. 1991), b) Winter 1992 (Dec. 1991 - F e b . 1992), c) Spring 1992 ( M a r c h - May 1992), d) Summer 1992 (June - Aug. 1992), and e) Fall 1992 (Sept. - Nov. 1992)

Chamber Incr. Incr. Incr. Further

control temp. CO 2 temp. incr.

and CO2 temp.

a. Fall 1991

Open control * ** * * *

Chamber control * **

Incr. temp. *** ***

Incr. CO2 *** ***

Incr. temp. and CO2 *** ***

b, W i n t e r 1992

Open control

Chamber control

Incr. temp.

Incr. CO2

Incr temp. and CO2

c. Spring 1992

Open control

Chamber control

Incr. temp.

Incr. CO2

Incr. temp. and CO2

d. Summer 1992

Open control *** *** *** ***

Chamber control * ** ** *

Incr. temp. *** ***

Incr. CO2 *** ***

Incr. temp. and CO2 *** ***

e. Fall 1992

Open control ***

Chamber control

Incr. temp.

Incr. CO2 ***

Incr. temp. and CO2

567

effect is evident; i.e. the chamber itself changed the conditions by increasing the temperature despite the ventilation arranged, and by altering the movement of water and thereby soil moisture.

Even though moisture seemed to be sufficient for tree growth, it could have become a limiting factor for

soil respiration in the temperature-treated chambers, as Virzo de Santo et al. (1993) as well as Rout and Gupta (1989) observed under elevated temperatures. Thus, the chamber treatments should be compared with each other, not necessarily with the open control (1) when soil moisture is not considered.

568

250

2OO

150

i~ lOO

5O

250

200

150

~ 100

r~ 5O

open etrl chamber etrl T CO2 T & CO2 Tf Fall 1991

open c~rl chamber etrl T CO2 T & CO2 Tf Spring 1992

c

250

200

150

~' 100

50

25o[ 200

150

i lOO

5O

open elrl chamber etrl T CO2 T & CO2 Tf

Winter 1992

clrl chamber etrl T CO2 T & CO2 Summer 1992

b

d

250

200

150

.=.

~' 100

5O

1

open ctrl chamber eU'I T CO2 T & CO2 Fall 1992

Fig. 5. Seasonal mean soil respiration, a) Fall 1991 (Sept - Nov 1991), b) Winter 1992 (Dec, 1991 - Feb, 1992), c) Spring 1992 (March - May 1992), d) Summer 1992 (June - Aug. 1992), and e) Fall 1992 (Sept - Nov, 1992). Legend : see Figure 3.

Both analyses show that an elevation in the concen- tration of atmospheric CO2 raised soil respiration in the increased- CO2 treatment (4), and brought it nearer to the level of the open control (1); indeed, it was even higher than in the open control (1). Temperature ele- vation seemed to restrict the rise possibly due to the imbalance between temperature and moisture suitable for microorganisms (Virzo de Santo et al., 1993). Thus,

the increment in soil respiration in the increased-CO2 treatment (4) was probably caused by stimulated root respiration (Norby et al., 1987; Rogers et al., 1992). However, the decrease in soil respiration by the end of the observation period in the further-increased tem- perature treatment (6) indicates that the intensive CO2 evolution in the soil was caused by increased decom- position of easily decomposable material.

Table 2. Seasonal means for soil respiration (rag C02 m -2 h -1) by treatment

Soil Open Chamber Incr. Incr. Incr. Further

respiration control control temp CO2 temp. Incr.

mgCO2 m -2 h -1 and temp.

CO2

Fall 1991 57.1 54.8 43.8 66.7 53.1 69.1

Winter 1992 22.8 20.1 24.3 26.7 27.2

Spring 1992 50.1 53.9 38.5 84.7 46.6 56.8

Summer 1992 166.6 100.4 91.2 139.7 133.4 111.3

Fall 1992 85.7 54.7 54.0 110.4 59.3 66.7

569

Table 3. Estimates for parameters a, b and c. Differences between values designated by *** are statistically significant

Chamber Incr. Incr. Incr. Further

control t emp. CO2 temp. incr.

and temp.

CO2

Estimates for parameter a 21.6

Open control 27.8

Chamber control 21.6

Incr. temp 17.4

Incr. CO2 20.9

Incr. temp. and CO 15.3 ***

17.4 20.9 15.3 14.5

Estimates for parameter b

Open control 6.8

Chamber control 6.5

Incr. temp 5.1

Incr. CO2 6.8

Incr. temp and CO 8.1

6.5 5.1 6.8 8.1 9.5

Estimates for parameter c

Open control 0.20

Chamber control 0.24

Incr. temp. 0.12

Incr. CO2 0.19

Incr. temp and CO2 0.24

0.24 0.12 0.19 0.24 0.35

A c k n o w l e d g e m e n t s

This research is a part of a project "Response of bore-

al forest ecosystem to changing climate and its sil-

vicultural implications" conducted by Dr Seppo Kel-

lom~.ki. The funding for this research was provided

by the Academy of Finland via the Finnish Research

Programme on Climate Change. I wish to thank Seppo

Kellom/aki and Kari T Korhonen for their construc-

tive criticism of the manuscript, Eeva-Maiju Aulin for

assisting with data processing, Riitta Honkanen for

drawing the figures, Erkki Pekkinen for revising the

language, and the staff of Mekr i j~v i Research Station

for assisting with the measurements.

570

F/g. 6.

250

200

"t~ 150 c- o

100

o t ~

50

0 I I I I

-5 0 5 10 15 20 25 Temperature. oC

4- open ctrl -,- chamber ctrl , T

-o- CO2 ~ T & CO2 * Tf

The fitted curves describing soil respiration in relation to temperature. Legend: see Figure 3.

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