Carbon sequestration in sub-tropical soils under rubber plantations in

north-east India

Debasis Mandal and K. R. Islam

Indian Rubber Research Institute, Tripura, andUniversity South Centers, USA

Widespread deforestation for fuelwood, timber and forage is an on-going process in the south-east Asian countries(Islam et al 1999).

Deforestation transfers Cdirectly to the atmosphere, and affects the mechanism to hold C in soil (Chambers et al. 2001).

Rubber plantations were raised as a secondary forest in the Indian state of Tripura since 1975 to improve degraded lands, generate income for locals, and meet increasing demand for rubber.

It is expected that reforestation of degraded lands may act as tropospheric CO2 sink (Wallace 1994, Islam et al. 1999).

This study was conducted to evaluate the impact of rubber plantations on C fractions, stocks, and sequestration in reforested degraded sub-tropical forest lands in Tripura, India.

Materials and methods

Site: 10,412 ha of rubber plantationsof different ages at

Kunjaban,Agartala, NE India

Soil UltisolTexture Clay loamPorosity 0.41 m3 m-3

pH 4.5+0.3

Sample plots from 5, 10, 15, 20, 25, and 30 yrs old rubber plantations with a control (initial).

Standing biomass and annual C flow as litter-fall

Soil sampled at 0 - 15 and 15 - 30 cm depthsOxidizable organic C (COX)Non-oxidizable C (CNOX)Total C (CT)Bulk densityC concentration and stocks

Data analysis by SAS as 7 (age) x 2 (soil depth) factorial combination at p<0.05.

Results and Discussion

Table 1: Standing biomass of different ages of rubber plantations_______________________________________Plantation Shoot Root Totalage (year) ____ biomass (Mg ha-1)___________________________________________0 (initial) 16.8c 4.2d 21.0c5 29.3c 13.6c 42.3c10 121.3b 26.6b 145.9b15 156.0ab 29.4ab 185.5ab20 160.4ab 32.4a 193.4a25 160.7ab 31.9a 192.9a30 161.7a 29.9ab 191.6a______________________________________LSDP>0.05 39.8 7.1 41.6

Table 2: Biomass C of different ages of rubber plantations___________________________________________Plantation Shoot Root TotalAge (year) _____ biomass C (Mg ha-1) _________________________________________________0 7.2b 1.9d 9.1b5 12.6b 5.7c 18.4b10 52.3a 11.1b 63.3a15 67.1a 13.2ab 80.3a20 69.2a 14.5a 83.7a25 69.2a 14.3a 83.5a30 69.5a 13.3a 82.9a___________________________________________LSDP>0.05 19.7 3.2 20.9

Years after plantation0 10 20 30

Litte

r-fa

ll (M

g ha

-1 y

r-1)

0

2

4

6

8

Biomass Carbon

Years after plantation0 10 20 30

Bul

k de

nsity

(g c

m-3

)

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

0-15-cm depth; y = 1.6 - 0.003*Xr2=0.57*

15-30-cm depth: y = 1.61- 0.002*Xr2=0.37ns

Years after plantation 0 10 20 30

CO

X co

nc (g

kg-

1 )

0

5

10

15

20

250-15 cm: y = 7.47 + 0.41*Xr2=0.84** 15-30 cm: y = 4.83 + 0.40*X - 0.008*X2

r2=0.76*

Years after plantation 0 10 20 30

CN

OX

conc

(g k

g-1 )

0

5

10

15

20

25

0-15 cm: y = 10.6 + 0.33*X - 0.01*X2

r2=0.33ns15-30 cm: y = 12.5 + 0.5*X- 0.01*X2

r2=0.57*

Years after plantation0 10 20 30

CT c

onc

(g k

g-1 )

0

5

10

15

20

25

30

35

0-15 cm: y = 18.1 + 0.74*X - 0.01*X2

r2=0.88**15-30 cm: y = 17.3 + 0.89*X - 0.018*X2

r2=0.63*

Years after plantation0 10 20 30

CO

X st

ock

(Mg

ha-1

)

0

10

20

30

40

50

600-15-cm depthy = 17.85 + 205.6*(-0.0046*X)r2 = 0.94***15-30-cm depthy = 10.94 + 12.8*(0.113*X)r2 = 0.73*

Years after plantation0 10 20 30

CN

OX

stoc

k (M

g ha

-1)

0

10

20

30

40

50

60

0-15-cm depthy = 26.3 + 3.13*(1-exp(-0.19*X)r2 = 0.1ns15-30-cm depthy = 30.32 + 15.13*(1-exp(-0.088*X)r2 = 0.55*

Years after plantation0 10 20 30

CT s

tock

(Mg

ha-1

)

0

20

40

60

80

0-15-cm depthy = 43.02 + 34.49*(1-exp(-0.055*X)r2 = 0.91***15-30-cm depthy = 41.35 + 27.88*(1-exp(-0.097*X)r2 =0.76*

Years after plantation0 10 20 30

Prof

ile C

sto

ck (M

g ha

-1)

0

20

40

60

80

100

120

140

160

COxi = 29 +56.9*(1-exp(-0.037*X), r2=0.93***CNOxi = 56.45 + 17.22*(1-exp(-0.122), r2=0.37nsCT = 84.35 + 60.2*(1-exp(-0.076*X, r2=0.85**

Years after plantation 0 10 20 30

Cox

seq

uest

ratio

n (k

g ha

-1 y

r-1)

0

200

400

600

800

1000

1200

1400

0-15 cm: y = 872.8 - 2.33*Xr2=0.99***15-30 cm: y = 1354.2 - 56.4*X + 0.84*X2

r2=0.98***

Years after plantation 0 10 20 30

CN

ox s

eque

stra

tion

(Mg

ha-1

yr-1

)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4 0-15 cm: y = 1.36 - 0.026*Xr2=0.99***15-30 cm: y = 1.13 - 0.06*X + 0.001*X2

r2=0.98***

Years after plantation 0 10 20 30

CT s

eque

stra

tion

(Mg

ha-1

yr-1

)

0.0

0.5

1.0

1.5

2.0

2.5

3.00-15 cm: y = 2.23 - 0.028*Xr2=0.99***15-30 cm: y = 2.49 - 0.12*X + 0.002*X2

r2=0.98***

Table 4: Profile-wise C sequestration rates in soils under different ages of rubber plantations______________________________________________Plantation COx CNOx CTAge (year) _________ (Mg ha-1 y-1) ________________________________________________________0 ----- ----- -----5 1.9+0.2a 1.7+0.6a 3.5+0.4a10 1.7+0.1ab 1.1+0.4ab 2.8+0.3ab15 1.6+0.1abc 0.8+0.3ab 2.4+0.2bc20 1.5+0.1c 0.6+0.2ab 2.1+0.1bc25 1.4+0.1c 0.5+0.2b 1.9+0.1c30 1.3+0.1c 0.4+0.1b 1.7+0.1c______________________________________________LSDP>0.05 0.32 1.15 0.83

ConclusionsStanding biomass, biomass C content, and annual litter-fall of rubber plants quadratically and significantly increased.

Greater accumulation litter-fall on the surface linearly and significantly decreased soil compaction. The beneficial effects of litter-fall on decreasing compaction was greatest on surface soil.

Conclusions……..The COX stock increased linearly in surface soil compared to a non-linear increase in subsurface soil. However, both CNOX and CTstocks increased quadratically over time.

Summing the C content over soil depth, the profile-wise accumulation of various C fractions increased non-linearly over time.

Thanks