INFLUENCE OF LANDUSE ON ORGANIC MATTER DISTRIBUTION IN SOIL AGGREGATE SIZE FRACTIONS IN ILE-IFE, SOUTHWESTERN

NIGERIA

ByOyedele, D.J.; Pini, R.; Sparvolli, E.,

Tijani, F.O. and Scatena, M.

INTRODUCTION• The importance of soil organic carbon (SOC) - Soils of tropical and subtropical forests account for almost 30% of total

global SOC (FAO, 2005)Three proposed mechanisms for SOC stabilisation are: (1) incorporation of SOC in soil aggregates that establishes a barrier

between microbes, microbial enzymes, and organic matter substrates; (2) preservation of SOC through inherent biochemical recalcitrance, or

selective degradation into chemically resistant materials during microbial decomposition; and

(3) sorption, precipitation, or complexation of SOC with the mineral matrix via intermolecular interactions that reduce the availability of substrate through changes in conformation and binding of functional groups.

(Christensen 1996; Sollins et al. 1996; Jastrow and Miller 1998; Baldock and Skjemstad 2000; Six et al. 2002a; Krull et al. 2003).

• Carbon sequestration in soil and quantification of for C trade schemes (CDM)

• The dynamics of SOC by physical protection.SOIL AGGREGATION• The Heirachical model (Oades, 1993).• This model has not been clearly confirmed by studies on

aggregate fractionation (de Sa et al., 2000), maybe due to variations in methods employed for soil fractionation (Ashman et al., 2003).

• Soils with different clay mineralogy were observed to respond differently to fractionation, and aggregate hierarchy exists only in soils where aggregate stability is controlled by organic materials (Oades and Water, 1991).

JUSTIFICATIONThere is currently limited knowledge of which mechanisms are most important for C storage under different soils and land-use systems, yet such knowledge is crucial for devising systems with efficient C sequestration, hence this study.

THE SPECIFIC OBJECTIVES• To evaluate the influence of land use type on organic carbon

distribution in soil• To investigate the potential of different soil aggregate

fractions to protect organic carbon in soils• To study the relationships between soil organic and the

stability of soil aggregates

MATERIALS AND METHOD• The experimental area was in the Teaching and Research Farm of Obafemi

Awolowo University, Ile Ife (7°25’N, 4°39’E), Nigeria. • The soil belong to the Iwo Association and were derived from coarse

gneiss and granite.• The texture varied from sandy loam to sandy clay loam.• Seven land use types were selected viz: Forest, Cacao, Teak, Oil palm,

Pasture, No Tillage (NT) and Continuous Conventional tillage (CT). • Sample locations were mid-slope positions in all land use types• Composite soil samples (40 subsamples) were taken in the different land

use types at depths of 0-15 cm (topsoil) and 15-30 cm (subsoil). • They were air dried, gently crushed by hand, and carefully sieved into size

fractions of 1-2, 0.5-1, 0.25-0.5, 0.125-0.25, 0.05-0.125, and <0.05 mm. • Organic C and total N were determined in each size class using the

Multiphase LECO RC-412 C analyzer and the FP-528 N analyzer respectively. • Water stable aggregates were evaluated by a modified Yoder method

RESULTS AND DISCUSSION

Land use0-15 cm

pH Sand (0.02-2 mm)

Silt (0.002-0.02 mm)

Clay (<0.002 mm)

Texture

No tillage 6.36 a 70 20 10 ab LSCont. Tillage 5.88 ab 78 14 8 b LSForest 6.76 a 66 21 14 a LOil palm 5.95 ab 63 22 15 a LTeak 6.12 a 69 20 11 ab LSCocoa 6.98 a 64 22 14 a LPasture 5.65 ab 64 23 13 ab LS

Table 1: The topsoil (0-15 cm) physical properties under different land use types

Means in the same coloumn followed by alphabets are statistically not different at 95% probability

Land use15-30 cm

pH Sand (0.02-2 mm)

Silt (0.002-0.2 mm)

Clay (<0.002 mm)

Texture

No tillage 6.20 a 77 b 14 bc 9 b LS

Cont. Tillage 5.82 a 87 a 11 c 2 c LSForest 6.76 a 64c 23 a 13 a L

Oil palm 6.00 a 64c 23 a 13 a LTeak 6.26 a 67c 19 ab 14 a LCocoa 6.95 a 67c 20 a 13 a L

Pasture 5.71 a 66c 24 a 10 b LS

Table 2: The subsoil (15-30 cm) physical properties under different land use types

Means in the same coloumn followed by alphabets are statistically not different at 95% probability

0

0.5

1

1.5

2

2.5

3O

rgan

ic C

(%

)

Forest Oilplam

Cocoa Teak Pasture NT CT

Land use types

0-15 cm

15-30 cm

LSD0. 05 (0-15 cm)

LSD0. 05 (15-30 cm)

Fig. 1: Distribution of soil organic C under different landuse types.

0

0.05

0.1

0.15

0.2

0.25

0.3

Tota

l N (

%)

Forest Oilplam

Cocoa Teak Pasture NT CT

Land use types

0-15

15-30

LSD0.05 (0-15 cm)

LSD0. 05 (15-30 cm)

Fig. 2: Soil distribution of total N under different landuse types.

0

5

10

15

20

25

C:N

rati

o

Forest Oilplam

Cocoa Teak Pasture NT CT

Landuse types

0-15 cm

15-30 cm

LSD0. 05 (0-15 cm)LSD0. 05 (15-30 cm)

Fig. 3: Soil C:N ratio distribution as influenced by land use types.

0

0.5

1

1.5

2

2.5

3

3.5

4

Orga

nic C

(%)

<0.05 0.05-0.125

0.125-0.25

0.25-0.5

0.5-1 1-2

Aggreate sizes (mm)

Cultivated

Uncultivated

LSD0. 05 (Cultivated)

LSD0. 05 (Uncultivated)

Fig 4: Organic C distribution in aggregate size fractions as influenced by cultivation in (a) topsoil (0-15 cm) and subsoil (15-30 cm)

0

0.5

1

1.5

2

2.5

3

3.5

4

Orga

nic C

(%)

<0.05 0.05-0.125

0.125-0.25

0.25-0.5

0.5-1 1-2

Aggregate sizes (mm)

Cultivated

Uncultivated

LSD0. 05 (Cultivated)

LSD0. 05 (Uncultivated)

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

Total

N (%

)

<0.05 0.05-0.125

0.125-0.25

0.25-0.5

0.5-1 1-2

Aggregate sizes (mm)

Cultivated

Uncultivated

LSD0. 05 (Cultivated)

LSD0. 05 (Uncultivated)

Fig 5: Total-N distribution in (a) topsoil (0-15 cm) and (b) subsoil (15-30 cm) aggregate size fractions as influenced by cultivation

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

Tota

l N (%

)

<0.05 0.05-0.125

0.125-0.25

0.25-0.5

0.5-1 1-2

Aggregate sizes (mm)

Cultivated

Uncultivated

LSD0. 05 (Cultivated)

LSD0. 05 (Uncultivated)

0

10

20

30

40

50

60

<0.05 0.05-0.125

0.125-0.25

0.25-0.5

0.5-1 '1-2

Aggregate sizes (mm)

Cultivated

Uncultivated

LSD0. 05 (Cultivated) LSD0. 05 (Uncultivated)

0

10

20

30

40

50

60

C:N ra

tio<0.05 0.05-

0.1250.125-

0.250.25-

0.50.5-1 1-2

Aggregate sizes (mm)

Cultivated

Uncultivated

LSD0. 05 (Cultivated)

LSD0. 05 (Uncultivated)

Fig 6: The distribution of C:N ratio in (a) topsoil (0-15 cm) and (b) subsoil (15-30 cm) aggregate size fractions as influenced by cultivation

0-15 15-300

5

10

15

20

25

30

35

0.25-0.5 mm0.5-0.1 mm1-2 mm

Soil Depth

WSA

%

Fig 7: Stability of different aggregate size fractions

Land useAggregate Size Fractions (mm)

0.25-0.5 0.5-1.0 1.0-2.0

Forest 51a 48a 27a

Teak 41ab 33b 11bc

Oil Palm 39ab 19bc 27a

Cocoa 37b 28b 11bc

Pasture 35b 21bc 18ab

No Tillage 34b 13cd 03c

Continuous Tillage 05c 04d 03c

Table 3: Effects of land use on water stable aggregates of different sizes fractions in the topsoil (0-15 cm).

Land useAggregate Size Fractions (mm)

0.25-0.5 0.5-1.0 1.0-2.0

Forest 22bc 20a 08b

Teak 19bc 08a 15b

Oil Palm 38a 14a 28a

Cocoa 29ab 13a 05b

Pasture 13dc 13a 10b

No Tillage 30ab 10a 09b

Continuous Tillage 05d 10a 09b

Table 4: Effects of land use on water stable aggregates of different sizes fractions in the subsoil (15-30 cm).

0.00 0.50 1.00 1.50 2.00 2.50 3.000

10

20

30

40

50

60

R² = 0.747352809257247

f(x) = 15.3034206529026 x + 2.3257788752832R² = 0.640365307578737

f(x) = 14.8215097982176 x − 4.10292408254658R² = 0.753778625185944

0-15

1-2Linear (1-2)0.5-1Linear (0.5-1)0.25-0.50Linear (0.25-0.50)

org C

WSA

Fig 8: The relationship between SOM and water stable aggregation in topsoil (0-15 cm) as influenced by cultivation

0.00 0.50 1.00 1.50 2.00 2.50 3.000

10

20

30

40

50

60

R² = 0.4398603383345

R² = 0.0185232777667255

R² = 0.712678287904635

15-30

1-2Linear (1-2)0.5-1Linear (0.5-1)0.25-0.50Linear (0.25-0.50)

org C

WSA

Fig 9: The relationship between SOM and water stable aggregation in subsoil (15-30 cm) as influenced by cultivation

CONCLUSIONS

• As expected, tillage and cultivation reduced the organic carbon in the soil.

• The consistently lower C:N ratio in the fine particle size fractions may indicate lower decomposition rates, thus suggesting a measure of protection of SOC by the fine sized soil particles.

• The water stability of the soil aggregates were mainly mediated by organic C.