Carbon Sequestration under Tropical Homegardens and Related Land-use Systems in Kerala, India
Subhrajit K. Saha1*, P.K. Ramachandran Nair1, Vimala D. Nair2, B. Mohan Kumar3
1School of Forest Resources and Conservation, 2Soil and Water Science Department, University of Florida, Gainesville, FL
3Department of Silviculture and Agroforestry, Kerala Agricultural University, Kerala, India
Introduction
Results and Discussion
Objective:
To compare soil C storage within a meter profile in whole- and size-fractionated soils of homegardens with four
other common land-use systems (natural forests, rubber plantations, sole stand of coconuts, and rice-paddy fields)
in Kerala, India.
Materials and Methods
Study Sites
Forest
Coconut Stand
Rubber
Plantation
Selected Land-use Systems (Treatments)
Location: Three villages (Pandiparmbu, Chirakkakode,
and Vellanikkara) and an adjacent forest area from the
Madakkathara subdivision (Panchayat),Thrissur district
(10°0' – 10°47' N; 75°55' – 76°54' E), Kerala State,
India.
N
Land-Use
Types
Thrissur
Kerala
Study Location in India
Soils: Inceptisols and Ultisols.
Climate: Mean annual rainfall: 2783 mm.
Mean annual temperate: 27.7°C.
Homegardens (HG): An agroforestry system where
multipurpose trees are grown in intimate association
with shrubs and herbaceous species, and livestock
around the homesteads (Fernandes and Nair, 1986).
In this study, homegardens are categorized into two
classes: large (≥ 0.4 ha/ 1 acre) (HGL) and small (<0.4
ha/ 1 acre) (HGS).
Primary Forest (PF): Undisturbed patches of moist
deciduous forests located near the (other) study sites.
Approximately 25% of the state is under forest.
Coconut Stands (CN): Kerala Agricultural University’s
experimental plantings of 30-year old coconut palms
(Cocos nucifera) spaced 8m × 8m.
Rubber Plantations (HB): Rubber trees (Hevea
brasiliensis) planted at 6 m x 6 m spacing; the age and
size of selected plantations varied at different sites.
Rubber is the fastest growing cash crop in Kerala
Rice-Paddy Fields (OS): Rice (Oryza sativa)-paddy
fields in close proximity to the HG sites. Approximately
14% of the state is under the rice-paddy cultivation.
Large
Homegarden
Small
Homegarden
Rice-Paddy
Field
Depth
(cm)
Soil Particle Size Density
Soil pH Total N (g
(%)
N/kg soil)
Clay
Silt
Sand
0 - 20
6.1
20 – 50
5.9
1.34
39.24
20.76
40.00
50 - 80
5.8
1.07
38.82
20.23
40.95
80 - 100
5.7
0.71
37.49
20.94
41.57
0 - 20
5.8
1.02
36
13.9
50.1
20 – 50
5.7
0.84
48.2
15.2
36.6
50 - 80
5.7
0.55
50.4
17.2
32.3
80 - 100
5.6
0.48
48.1
16.9
35
0 - 20
5.5
1.54
21.7
15.4
62.9
20 – 50
5.1
1.01
23.4
16.8
59.8
50 - 80
5.1
0.92
29.5
15.8
54.6
80 - 100
5.2
0.68
36.4
17.8
45.8
0 - 20
6.4
0.89
25.69
23.15
51.16
Wet sieving of air-dried, 2 mm sieve samples, through
250 and 53 µm sieves to obtain three fraction size
classes (250 – 2000 µm, 53 – 250 µm, and <53 µm)
(Elliot 1986). The whole and fractionated soil samples
were finely crushed to homogenize them for analyses.
Soil Collection
46.41
0.79
38.79
13.17
48.04
50 - 80
6.1
0.72
31.14
22.96
45.90
80 - 100
6
0.71
38.73
9.60
51.68
0 - 20
6.4
1.03
29.73
13.66
56.60
20 – 50
6.3
0.82
35.78
11.73
52.49
50 - 80
6.2
0.76
37.10
10.33
52.57
80 - 100
6.1
0.66
37.36
9.57
53.08
0 - 20
6.2
1.46
28.78
17.34
53.88
20 – 50
6.5
0.52
23.54
11.32
65.14
50 - 80
6.5
0.23
19.02
10.48
70.50
80 - 100
6.1
0.24
9.53
4.91
Forest
Coconut Stand
Rubber
Plantation
Rubber
0
Rice-Paddy
Field
85.56
-----20
Homegardens had less C in top
soil compared to forests and
rubber plantations, which could
be a result of soil disturbance
(periodic tillage) consequent to
cultivation of herbaceous crops.
-----30
-----40
-----50
-----60
-----70
------80
------90
-----100
HG Soil Profile
analyzer.
Fig. 1: Mean of Total C from All Study Villages (Whole Soil) in
Thrissur, Kerala, India.
The overall trend of total soil C in the three aggregate size classes was similar to that of total soil C: forest and rubber plantation having higher amounts and
rice-paddy the lowest (p< 0.0001). Also, across systems, the amount of C decreased with depth in all three aggregate size classes. No difference was
observed among large and small homegardens and coconut stands. In all three aggregate size classes, the amount of C decreased with depth. Rice-paddy
soils had a sharp decrease in the amount of C from top soil to sub-soil in small aggregate size class (<53 µm).
Small
Homegarden
Lowest amount of soil C was
found in the rice-paddy systems
(p< 0.0001). Thus, soil C content
was directly related to tree
density and long-term
undisturbed nature of the
systems.
-----10
Fig. 2: Mean Soil C of All Study Sites (Fractionated Soil) in Thrissur, Kerala, India.
Large
Homegarden
Among the land-use types, total
soil C was highest in the forest
(p< 0.0001), followed by the
rubber plantations.
Analysis: Total soil C determination by dry combustion on an automated FLASH EA 1112N C elemental
Statistical Analysis: Waller-Duncan K-ratio T test was performed following ANOVA.
21.46
6.4
-----0
cm
Particle Size Fractionation
32.13
20 – 50
Soil Sampling
• Collected from four replications of each treatment
(land-use systems).
• Each sample, a composite of three sub-samples from
three random sampling points.
• Samples from four soil depths (0 – 20, 20 – 50, 50 – 80
, 80 – 100 cm) at each sampling site.
1.98
Land-Use
Types
Across systems, highest total C
was found in the top soil (0 – 20
cm) ( p< 0.0001), which
gradually decreased with depth
resulting in lowest total C at 80 –
100 cm depth in all land-use
systems.
Table 2: Mean of Total Soil C from All
Study Villages (Whole Soil) in
Thrissur, Kerala, India.
Land-use
Types
Soil C (Mg/ha) at
different depths (cm)
0 – 50
50 – 100
Forest
126.8a
67.02a
Coconut
56.39c
33.33c
HGL
58.65c
43.82b
HGS
59.05c
42.83b
Rubber
77.85b
49.08b
RicePaddy
44.65d
14.25d
Lower case letters next to the mean values
indicate significant differences in soil C among
land-use types (within a given depth).
The overall pattern in total soil C was,
Primary Forest> Rubber Plantations>
Homegardens> Coconut Stands>
Rice-Paddy fields.
Total C (g/ kg soil)
10
20
30
Rice-Paddy
40
0
0
0
10
10
20
20
30
30
40
50
60
70
Soil Depth (cm)
Hypotheses:
1. Tree based land-use systems promote soil C sequestration.
2. Carbon content is higher in smaller soil aggregate size classes than in larger aggregates in tree-based systems.
Table 1: Soil Characteristics of Pandiparambu (Site 1), Thrissur, Kerala, India.
Soil Depth (cm)
Soil is a major sink of terrestrial carbon (C). Tree-based land-use systems such as forests and agroforests are
likely to retain substantial stable C in the soil as a consequence of their litter and root dynamics, the smaller soil
aggregates retaining relatively more stable C than larger aggregates. The perceived carbon sequestration
potential of agroforestry systems, however, remains largely unexplored, especially in systems like tropical
homegardens (HG).
Total C (g/ kg soil)
10
20
30
40
40
50
60
70
80
80
90
90
100
100
Fig. 3: Mean Soil C in Three Fraction Size Classes across Five Land-use Systems in Thrissur, Kerala, India.
Amount of C was higher in the small aggregate size class (<53 µm) in all selected land-use types and at all depth classes. Carbon is known to be present in
most stable form in small soil fractions (Six et al. 2002). No difference was found between other two aggregate size classes (250 – 2000 µm and 53 – 250 µm).
Conclusions
Land-use systems with higher tree density and less soil disturbance contributed to greater soil C sequestration. Homegardens retained more C in soil than
agricultural systems such as rice-paddy. Across the systems, the stable soil fraction (<53 µm) contained the highest amounts of C. Carbon content in soil
profiles decreased with soil depth; but lower depths up to 1 m contained substantial amount of C, indicating the importance of considering the soils below the
surface horizon in soil C studies.
References
1.
2.
3.
Elliot E.T. 1986. Aggregate structure and carbon, nitrogen and phosphorus in native and cultivated soils. Soil Sci. Soc. Am. J. 50:627-633.
Fernandes, E.C.M. and Nair, P.K.R. 1986. An Evaluation of the Structure and Function of Tropical Homegardens. Agroforest. Syst. 21:279-310.
Six, J., R.T. Conant, E.A. Paul, and K. Paustian. 2002. Stabilization mechanism of soil organic matter. Implications for C-saturation of soils. Plant Soil
241:155-176.
Acknowledgments
We thank the Kerala Government Departments of Agriculture and Forest for data sources, the farmers and land-owners for their cooperation in the study, and
the Kerala Agricultural University staff for their help with soil sampling and land-use survey.