Changes in physico-chemical properties andmetabolic activity of soil in poplar plantations

replacing natural broad-leaved forests in KumaunHimalaya

Mukesh Joshi*, Kiran Bargali & S.S. Bargali

Department of Forestry/Botany, Kumaun University, Nainital-263002,(U.P.) India

(Received 8 December 1994, accepted 5 January 1996)

Physico-chemical properties and metabolic activity of soil were measuredduring the first 8 years of new plantations of Populus deltoides Bartr. raisedafter the clear-felling of natural mixed broad-leaved forests in the subtropicalzone of the Kumaun Himalaya. Results were compared with similarmeasurements under the original forest cover. Several soil physical propertieschanged detrimentally with increasing plantation age; soil chemical proper-ties, notably organic carbon, total N, P and K concentration decreased as aresult of reforestation with Populus and further decreased with increasing ageof the plantation. Soil metabolic activity also decreased as a result ofreforestation with Populus and there was a further decrease with plantationdevelopment.

©1997 Academic Press Limited

Keywords: metabolic activity; plantation development; Populus deltoides;mixed broad-leaved forest; Kumaun Himalaya

Introduction

Soil properties which affect plant growth are a complex combination of physical,chemical and biological processes (Coleman et al., 1983; Elliott & Coleman, 1988).However, few studies on changes in physico-chemical and microbiological propertiesof soil following the establishment of tree plantations have been conducted in thehumid tropics (Stewart & Kellman, 1982; Kadeba & Aduayi, 1985; Bargali et al.,1992; Bargali et al., 1993).

Soil respiration results from the metabolic activities in and on the soil surface due tomicrobes, roots and fauna, as a result of which CO2 is released (Singh & Gupta, 1977;Tewary et al., 1982). Attempts to partition soil respiration into microbial, microfaunaland root respiration for the purpose of estimating carbon balance have provenproblematic (Minderman & Vulto, 1973). However, measurement of carbon dioxide

*Address for correspondence: Dr Mukesh Joshi, c/o Sri H.S. Bisht, 223, Naya Bazar, Tallital, Nainital —263 002, (U.P.) India.

Journal of Arid Environments (1997) 35: 161–169

0140–1963/97/010161 + 09 $25.00/0/ae960149 © 1997 Academic Press Limited

D

40

J

Tem

pera

ture

(°C

)

JF M A M J A S O

700

500

600

400

300

200

100

Rai

nfa

ll (

mm

)

N

20 50

evolution as a diagnostic tool for the overall assessment of relative biological activitieshas enjoyed continued attention over the years (Reiners, 1968; Tewary et al., 1982;Weber, 1985; Joshi et al., 1991; Joshi, 1994). Thus, soil respiration is a major index oftotal metabolic activity in the soil (Edwards, 1982; Tuben, 1991).

In the Himalayan tarai belt, a level area with abundant surface and pronounceddeposition of finer materials, monocultures of fast growing trees such as poplars(Populus deltoides Bartr.) and eucalypts (Eucalyptus) are being raised after clear fellingof uneven aged natural, mixed broad-leaved forest. It is generally assumed that the soilcharacteristics may prevent the plantation forest from growing to maturity and/orhamper the regeneration of the natural forest.

In the present study, physico-chemical properties and metabolic activity in terms ofsoil respiration of soil under 1 to 8-year-old P. deltoides plantations and natural mixedbroad-leaved forest were measured to find out whether soil change occurred followingthe establishment of Populus in low-montane subtropical forests in the KumaunHimalaya when the starting conditions of all plantations were the same (Bargali et al.,1992, 1993).

Materials and methods

Study sites

The study was conducted in the tarai belt of Kumaun Himalaya, India (29° 3' to 29°12' N and 79° 20' to 79° 23' E) at an altitude of 230–280 m. Total basal cover, that isthe ground surface occupied by stems of trees (51 m2 ha–1), in the natural broad-leavedforest was comparable with that of other similar forests of the region (Bargali et al.,1993).

The climate of the region is subtropical monsoon with a long dry season from earlyOctober to mid-June and a wet season from mid-June to early October. Monthlyrainfall and temperature are indicated in Fig. 1. Of the annual rainfall (1593 mmaverage from 1985–1989), about 89% occurs from mid-June to September. Meanmonthly temperature ranges from 14·4 °C (January) to 31·3 °C (June).

Geologically, tarai is characterised by tertiary sediment consisting of lowersandstones of old tarai deposits washed from the Himalayan mountains. The soil isdeep and fertile, moist alluvial loam conspicuously free from boulders and gravels.

Figure 1. Climatic conditions in the study area. (s) = temperature; (d) = rainfall.

M. JOSHI ET AL. 162

The forest vegetation which is being replaced by poplar plantations consists ofdeciduous and evergreen broad-leaved species such as Acacia catechu, Adina cordifolia,Albizia spp., Bauhinia spp., Butea monosperma, Bombax ceiba, Mallotus phillippinensis,Shorea robusta, Syzygium cumni, Terminalia bellirica and Toona ciliata.

Immediately after clear-felling of the natural forest, normal site preparationtechniques were followed (Bargali & Singh, 1991; Bargali et al., 1992) and ploughingwas done by tractor.

Methods

Soil samples (in triplicate) were collected from poplar plantations, varying in age from1 year to 8 years, and from adjacent natural forest. A soil corer (surface area 132·8cm2) was inserted at three depths (0–10 cm, 10–20 cm and 20–30 cm) in January, Mayand September.

Soil texture (sand > 0·02 mm, silt 0·002–0·02 mm, clay < 0·002 mm), bulk density,water-holding capacity and porosity (dry weight basis under laboratory conditions)were determined according to Misra (1968). Total nitrogen was determined by micro-Kjeldahl technique (Peach & Tracey, 1956); total phosphorus with a spec-trophotometer; total potassium with a flame photometer; and organic carbon by thewet oxidation method (Jackson, 1958).

Soil respiration was determined in situ by using the alkali absorption technique(Gupta & Singh, 1977). Rates of soil respiration were measured within the intact litterlayer using aluminium cylinders (13 3 23 cm) inserted 10 cm into the ground and a1:20 ratio of alkali absorption area to soil surface area as recommended by Kirita(1971). The respiration study was performed under: (i) poplar plantations varying inage from 1 year to 8 years; and (ii) adjacent natural forest. Three replicates ofexperimental cylinders, and one set of three control cylinders (13 3 13 cm, equivalentto the above-ground part of the experimental cylinders), capped with airtight lids atboth ends, were set up in each of the eight plantations and on the natural forest. Beforeeach cylinder was fixed, green vegetation within the cylinder was clipped. A 50 mlbeaker containing 20 ml 0·5 N NaOH (determined as given in Gupta & Singh, 1977)was hung on a thin wire in each cylinder. The alkali was titrated against N HCI aftera 24 h absorption period to avoid diurnal variations (Harris & Van Bavel, 1957). Thecylinders were placed randomly irrespective of the distance from the trees and on eachsampling date the soil temperature was measured with a soil thermometer. CO2

evolved during the experiment was calculated by the formula of Misra (1968).

Statistical analyses

Seasonal respiration rates, expressed as the production of CO2 m–2 h–1, were subjectedto linear regression analysis in order to establish a relation between soil characteristics(dependent variable, Y) and plantation age (independent variable, X). The linearequation used was of the type Y = a + bX, where a is the Y-intercept and b is slope ofregression coefficient (r). Data from the natural broad-leaved forest and different ageplantations were compared using analysis of variance (ANOVA) and students t-testfollowing Snedecor & Cochran (1967).

SOIL CHANGES DUE TO SECONDARY FORESTS 163

Results

Physico-chemical properties

Results of the soil analyses presented below refer to the entire depth studied, i.e.0–30 cm. In all the poplar plantations the clay content was lower than in the naturalmixed broad-leaved forest. The natural forest had 59%, and this value decreasedsignificantly (p < 0·01) with age down to 45% (Fig. 2). Soil bulk density increasedwith plantation age, from 1·09 to 1·22. Water-holding capacity, porosity and soilmoisture content all decreased with plantation age (Fig. 2). Water-holding capacitydecreased only slightly from 84% to 82%, while porosity changed more from 58% inthe 1-year-old plantation to 53% in the 8-year-old one. Soil moisture decreasedconsiderably from 38% in the natural forest to 32% in the 1-year-old plantation andwith a further decrease to 29% in year 8 (Fig. 2).

Organic carbon values were all low, but slightly higher in eutrophic soils of thenatural forest compared to the poplar plantation (3·9% vs. 3·7% in the youngestplantation). In the older plantations, there was a steady decrease to 2·5%. Values forN, P and K were all lower in plantations as compared to the natural forest. Values forN dropped from 0·25% to 0·21% in the plantations compared to 0·26% in the naturalforest. For P the corresponding figures are 0·010 to 0·007% compared to 0·012%, andfor K 0·16 to 0·12 compared to 0·17%.

Seasonal patterns showed that values for soil moisture, organic carbon and all thenutrients (N, P and K) were highest in the rainy season and lowest in the winter season(Fig. 3), while soil temperature was highest in the summer season (Fig. 3).

Soil respiration

The rate of soil respiration was lower in all the poplar plantations (1-year-old to 8-year-old), than in natural forest, declining from 247 mg CO2 m–2 h–1 in natural forest to 239mg CO2 m–2 h–1 in the youngest plantation, and there was a marked decrease(p < 0·01) to 169 mg CO2 m–2 h–1 with plantation age.

Soil respiration followed similar seasonal pattern (Fig. 3) in all the plantations andnatural forest, the value being highest during the rainy season (459 mg CO2 m–2 h–1

in natural forest to 326–447 mg CO2 m–2 h–1 in plantations) and lowest during thewinter season (91 mg CO2 m–2 h– 1 in natural forest to 50–87 mg CO2 m–2 h–1 inplantations). Rainy season value was approximately twice the summer value in all theplantations, and five to six times higher than the winter value. The difference betweenthe rainy and the winter season was greater in older plantations. ANOVA on soilrespiration data showed that the rate of soil respiration was significantly different foreach plantation age and among seasons. The interaction between plantations andseason was also significant (p < 0·01).

Discussion

Physico-chemical properties

Results of the soil physical analysis show that soil texture changed in the poplarplantations. The decrease in the proportion of fine soil particles and increase in coarseparticles during the early stage of plantations may be explained by an eroding influenceof the rains on the fine soil particles (Bargali et al., 1992). The loss of fine soil particlesin turn will lead to a reduction of the moisture retention capacity of the soil and trophicstructure of soil microbes and hence the rate of decomposition and mineralization

M. JOSHI ET AL. 164

Age

(ye

ars)

0.01

0

0.00

6

Total P (%)

51

23

46

7

0.00

9

0.00

8

0.00

7

r =

–0.9

44a

= 0.

01b

= 0.

004 8

1.16

0.12

Total K (%)

51

23

46

7

0.15

0.14

0.13

r =

–0.9

10a

= 0.

16b

= 0.

005 8

240

160

Soil respiration (mg CO2 m

–2 h

–1)

51

23

46

7

220

200

180

r =

–0.9

97a

= 24

7.47

b =

–10.

26 8

66 50

Porosity (%)

62 58 54

r =

–0.9

91a

= 62

.56

b =

–0.8

4

28 20

Soil moisture (%)

26 24 22

r =

–0.9

92a

= 27

.30

b =

–0.6

2

4.0

2.0

Organic C (%)

3.5

3.0

2.5

r =

–0.9

98a

= 3.

89b

= –

0.17

0.25

0.21

Total N (%)

51

23

46

7

0.24

0.23

0.22

r =

–0.9

47a

= 0.

25b

= –0

.004 8

52 44

Clay (%)50 48 46

r =

–0.8

98a

= 52

.24

b =

– 0.

83

28 20

Sand (%)

26 24 22

r =

0.73

8a

= 0.

55b

= 19

.97

85 81

Water-holding capacity (%)

84 83 82

r =

–0.9

44a

= 83

.72

b =

–0.6

6

1.18

0.94

Bulk density (g cm–3

)

1.12

1.06

1.00

r =

0.99

8a

= 0.

97b

= 0.

02

Fig

ure

2.

Rel

atio

nsh

ip b

etw

een

pla

nta

tion

age

an

d c

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(%),

san

d (

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wat

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capa

city

(%

), b

ulk

den

sity

(g

cm–3),

por

osit

y (%

),so

il m

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ure

(%

), o

rgan

ic c

arbo

n (

%),

tot

al N

(%

), t

otal

P (

%),

tot

al K

(%

), a

nd

tot

al s

oil

resp

irat

ion

(m

g C

O2

m–2

h–1).

Cor

rela

tion

coef

fici

ent

r, i

nte

rcep

t a

and

slo

pe b

are

give

n.

All

regr

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ons

are

of t

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ype

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bX

and

sta

tist

ical

ly s

ign

ifica

nt

(p<

0·01

).

SOIL CHANGES DUE TO SECONDARY FORESTS 165

Age

(ye

ar)

85

12

34

67

0

35 10

Soil temperature (°C)

30 25 20 15

0.18

0.10

Total K (%)

0.16

0.14

0.12

40 15

Soil moisture (%)

35 30 25 20

5 0Organic C (%)

4 3 2 1

0.40

0.15

Total N (%)

0.35

0.30

0.25

0.20 500 0

Soil respiration (mg CO2 m–2

h–1

)

400

300

200

100

0.01

5

0.00

5

Total P (%)0.01

3

0.01

1

0.00

9

0.00

7

85

12

34

67

0

85

12

34

67

08

51

23

46

70

Fig

ure

3.

Sea

son

al p

atte

rn o

f so

il m

oist

ure

(%

), o

rgan

ic C

(%

), t

otal

N (

%),

tot

al P

(%),

soi

l te

mpe

ratu

re (

°C),

tot

al K

(%

), a

nd

soi

lre

spir

atio

n (

mg

co2

m–2

h–1).

(d

––d

)=

rain

y se

ason

; (d

––

–d)

=su

mm

er s

easo

n;

(s––

s)

=w

inte

r se

ason

.

M. JOSHI ET AL. 166

(Coleman et al., 1968, 1989; Elliott et al., 1980; Bargali et al., 1992, 1993). A similareffect of soil compaction has been reported by Babalola & Samie (1972), and Bargaliet al. (1992, 1993) for Eucalyptus plantations.

From the onset of establishment of the poplar plantations, organic matter andnutrient levels tend to decline, especially in the early stages of development. Mostprobably the site preparation technique prior to planting will also have induced someinitial soil changes (Bargali et al., 1992). In most reports, soil changes following thereplacement of the indigenous forest canopy with exotics in tropical environmentsusually occur during the first 10 years of plantation development (Lundgren, 1978;Chijoke, 1980; Kadeba & Aduayi, 1985). These soil changes are due to the vigorousgrowth of the exotic trees and rapid nutrient uptake from the top soil as the canopy androot system of the young plantation develop.

Removal of natural vegetation cover is a major ecological change, since aconsiderable part of the nutrient capital of the ecosystem is kept in the vegetation.When the vegetation is removed the soil tends to rapidly lose the fertility (Kang & Juo,1982). There is evidence that when land is cleared and replanted with fast growingexotic trees in mono-culture, changes in organic matter and soil properties occur(Babalola & Samie, 1972; Lundgren, 1978). These changes frequently result inreduced suitability of the soil for tree growth.

Erosion and loss of soil occur due to the deep ploughing and/or clear-cutting. In thepresent study, soil nutrient losses mostly occurred immediately after clear-cutting ofthe natural forest when poplar trees were young and the tree canopy was not closed.Soil erosion is a major destabilizing force in disturbed ecosystems, particularly duringthe phase following disturbance. During this phase, referred to as the ‘reorganizationphase’ of ecosystem development (Bormann & Likens, 1979), the ecosystem loses totalbiomass and nutrients.

Soil respiration

The most commonly used abiotic variables explaining observed respiration patternsare temperature and moisture (Singh & Gupta, 1977; Weber, 1985; Joshi et al., 1991).These two environmental variables exert their influence by having a direct effect onCO2 evolution and also by indirectly altering other abiotic parameters such as gaseousdiffusion (Weber, 1985). In the present study, high moisture and moderatetemperature during the rainy season resulted in markedly higher soil respiration. As theplantations aged, soil respiration decreased with decreasing soil moisture (p < 0·01).Lowered soil respiration as a result of lower soil moisture regimes in the presence ofoptimal temperature has been documented previously (Singh & Gupta, 1977; Weber,1985; Joshi et al., 1991; Bargali et al., 1993). The available soil moisture enhances soilrespiration by increasing microbial activity and decomposition of organic matter(Tewary et al., 1982; Joshi et al., 1991; Joshi, 1994).

Conclusion

To conclude, the decrease in fine soil particles, water-holding capacity, moisturecontent, organic carbon, nutrient concentration and the rate of soil respiration, withincreasing plantation age suggest that poplar plantations may indeed lead to soildegradation. Modification in physico-chemical and metabolic activity of soil asdescribed in this study are likely to occur in connection with clear-cutting of naturalforests (Bormann & Likens, 1979; Bargali et al., 1993). Subsequent to deforestation,decomposition rates of organic matter, both on the soil surface and within the top soillayers, are enhanced, rendering the system vulnerable to leakage of nutrients. The

SOIL CHANGES DUE TO SECONDARY FORESTS 167

significant relationship between soil respiration and edaphic conditions reflects themajor role played by micro-organisms such as soil fungi and bacteria in soil respiratoryactivities. The habitat having greater organic carbon, nutrients, moisture and porosityand lesser bulk density showed greater respiration. Comparison between plantationsand natural forest suggest that the ecosystem properties that develop subsequent toclear-cutting of a natural forest bring about marked changes in metabolism of the soilsubsystems. It can also be concluded that intensive clear-cutting of natural broad-leaved forests and establishment of mono-culture of poplar in a short rotation of 8years will lead to measurable change in site productivity as a result of nutrientdepletion.

We thank Prof. S.P. Singh and Dr R.P. Singh for valuable suggestions and for providing theessential research facilities. The research was funded by the Council of Scientific and IndustrialResearch (CSIR), New Delhi.

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SOIL CHANGES DUE TO SECONDARY FORESTS 169