Results And Discussion

Microwatersheds presented various land use and management systems. Tables 24.3, 24.4, and 24.5 show the total C content aboveground (trees, shrubs plus grasses, litter, and/or stalks) and underground (roots and soil) from 0- to 105-cm depth for the main land use systems of the Mazateca, Cuicateca, and Mixe regions (Etchevers et al., 2003).

The C stock in the aboveground plus roots plus soil pools was highest (306 Mg ha-1) in the Mixe region, and lowest (54 Mg ha-1) in the Cuicateca region. Carbon stocks corresponding to various land uses (secondary-growth forest, permanent agricultural crops, milpa, and MIAF, etc.) were similar in each watershed. Soil and prevailing climate conditions appear to determine the amount of accumulated carbon in each region. In general, areas with deeper soils and higher precipitation presented higher C stocks. The data suggest that under hillside conditions, some agricultural systems accumulate as much C as secondary-growth forest systems under hillside conditions. In all cases, C stored underground pools was higher than that stored in aboveground pools. More than 90% of the C in agricultural systems was stored in the soil, yet this percentage was lower in the secondary forest. Carbon content decreased with soil depth, and about 60% of the C stock was found in the first 50 cm of the soil profile (Etchevers et al., 2001). Older secondary forest stored a large proportion of C in the aboveground pool. The estimated rate of C accumulation in the secondary forest was between 1.5 to 3 Mg ha-1 year-1. Greatest C accumulation occurred in the young acahual and the lowest accumulation in mature forests. Annual increments of C in the MIAF system trees were approximately 1 to 2 Mg ha-1 year-1. This rate of C sequestration is considered comparable to rates reported in the literature for tropical forests (Szott et al., 1994; Houghton, 1997).

Weeds growing on residual soil water after the milpa was harvested contributed between 1 and 2.5 Mg ha-1 yr-1 of C to both the traditional and conservation tillage systems. Crop residues contributed between 2 to 4 Mg C ha-1 yr-1 (data not shown).

Selected chemical properties associated with soil fertility in three microwatersheds are presented in Table 24.6. The surface soil layers (0- to 20-cm depth and 20- to 40-cm depth) had in all cases acid pH, high exchangeable acidity, and in

Table 24.3 Carbon Content in Aboveground (Litter, Grasses and Shrubs, Trees, and/or Stalks) and Underground (Roots and Soil, 0- to 105-cm Depth) Components, in Predominant Vegetation Systems, Mazateca Region, Oaxaca, Mexico (Mg ha-1)

Agricultural Systems®

Table 24.3 Carbon Content in Aboveground (Litter, Grasses and Shrubs, Trees, and/or Stalks) and Underground (Roots and Soil, 0- to 105-cm Depth) Components, in Predominant Vegetation Systems, Mazateca Region, Oaxaca, Mexico (Mg ha-1)

Agricultural Systems®

Forest Systems

Permanent

Mixed

Annual

Component

BL

BA15

BA10

CA

PR

Mv>30

Mv<30

LC>30

LC<30

LC>30

LC<30

Aboveground

99.5

46.3

31.0

34.5

5.4

2.6

1.6

0.6

1.2

1.0

1.4

Litter

5.9

8.4

12.6

9.2

NA

NA

NA

NA

NA

NA

NA

Grass and shrubs

0.5

1.00

3.0

0.7

5.4

1.3

1.6

0.6

1.2

1.0

1.4

Trees

93.1

36.9

15.4

24.7

NA

1.3

0

0

0

0

0

Straw

NA

NA

NA

NA

NA

a

a

a

a

a

a

Underground

155.3

158.9

244.3

151.7

175.2

159.4

131.1

267.0

276.0

235.9

200.3

Root

3.5

2.5

4.1

4.00

1.3

1.12

2.9

1.4

3.1

1.3

5.3

Soil

151.9

156.4

240.2

147.7

173.9

158.3

128.2

265.6

272.9

234.7

195

Total

255

205

275

186

181

162

133

268

277

237

202

SEb

(80)

(45)

(47

(69)

(43)

(13)

(15)

(27)

(99)

(44)

(38)

a Information was not available, as replanting occurred in 2001. b SE is the standard error of the mean of total carbon in the system.

Notes: BL = Liquidambar forest; BA15 = Alnus forest of 15 years; BA10 = Alnus forest of 10 years; PR = prairie; Mv = living wall; NA = not applicable; LC = minimum tillage; and LT = traditional tillage.

a Information was not available, as replanting occurred in 2001. b SE is the standard error of the mean of total carbon in the system.

Notes: BL = Liquidambar forest; BA15 = Alnus forest of 15 years; BA10 = Alnus forest of 10 years; PR = prairie; Mv = living wall; NA = not applicable; LC = minimum tillage; and LT = traditional tillage.

Table 24.4 Carbon Content in Aboveground (Litter, Grasses and Shrubs, Trees, and/or Stalks) and Underground (Root and Soil, 0- to 105-cm Depth) Components in Predominant Vegetation Systems of Cuicateca Region, Oaxaca, Mexico (Mg ha-1)

Forest

Agricultural Systems

Table 24.4 Carbon Content in Aboveground (Litter, Grasses and Shrubs, Trees, and/or Stalks) and Underground (Root and Soil, 0- to 105-cm Depth) Components in Predominant Vegetation Systems of Cuicateca Region, Oaxaca, Mexico (Mg ha-1)

Forest

Agricultural Systems

Systems

Permanent

Mixed

Annual

Component

BE

PR

Mv(M)

Mv(G)

LC(M)

LC(G)

LT(M)

LT(G)

Aboveground

37.6

2.2

4.3

3.4

4.2

3.8

3.3

2.7

Litter

7.6

NA

NA

NA

NA

NA

NA

NA

Grass and shrubs

0

2.2

0.2

0.5

0.6

0.6

0.3

0.4

Trees

30.0

NA

0.7

0.1

NA

NA

NA

NA

Straw

0

0

3.4

2.8

3.5

3.1

3.0

2.2

Underground

58.9

97.2

63.5

113.9

67.5

50.1

57.8

65.7

Root

14.3

6.2

0.7

1.0

1.9

1.0

0.6

0.6

Soil

44.6

91.0

62.8

112.8

65.6

49.1

57.2

65.1

Total

97

99

68

117

72

54

61

68

SEa

(20)

( (23)

(7)

(46)

(11)

(8)

(4)

(3)

a SE is the standard error of the mean of total carbon in the system.

Notes: BE = Quercus forest; LC = minimum tillage; LT = traditional tillage; Mv = living walls; NA = not applicable; PR = prairie.

05 o

S o r a SE is the standard error of the mean of total carbon in the system.

Notes: BE = Quercus forest; LC = minimum tillage; LT = traditional tillage; Mv = living walls; NA = not applicable; PR = prairie.

Table 24.5 Carbon Content in Aboveground (Litter, Grasses and Shrubs, Trees, and/or Stalks) and Underground (Root and Soil, 0- to 105-cm Depth) Components in Predominant Vegetation Systems of Mixe Region, Oaxaca, Mexico (Mg ha-1)

Agricultural Systems

Table 24.5 Carbon Content in Aboveground (Litter, Grasses and Shrubs, Trees, and/or Stalks) and Underground (Root and Soil, 0- to 105-cm Depth) Components in Predominant Vegetation Systems of Mixe Region, Oaxaca, Mexico (Mg ha-1)

Agricultural Systems

Forest Systems

Permanent

Mixed

Annual

Component

AC1G

AC7

AC2

CA

Mvc

LC

LT

Aboveground

25.G

24.1

9.9

11.2

5.6

3.1

4.8

Litter

7.3

6.7

3.3

2.G

NA§

NA

NA

Grass-shrubs

4.3

1.9

6.6

G.3

G.9

1.G

1.G

Trees

13.4

15.5

G

8.9

G.2

NA

NA

Straw

NA

NA

NA

NA

4.5

2.1

3.8

Underground

128.1

174.5

123.2

163.7

267.9

281.G

3GG.3

Root

7.8

5.1

4.G

4.G

1.9

2.8

2.3

Soil

12G.4

169.3

119.2

159.7

266.G

278.1

298.G

Total

153

199

133

175

273

284

3G5

SEa

(21)

(23)

(15)

(31)

(8)

(23)

(17)

a SE is the standard error of the mean of total carbon in the system. Notes: AC2, AC7, and AC10 = Acahuales (second growth) aged 2, 7, and 10 years, respectively; CA, LC = minimum tillage; LT = traditional tillage; Mvc = living walls of coffee plants; NA = not applicable.

a SE is the standard error of the mean of total carbon in the system. Notes: AC2, AC7, and AC10 = Acahuales (second growth) aged 2, 7, and 10 years, respectively; CA, LC = minimum tillage; LT = traditional tillage; Mvc = living walls of coffee plants; NA = not applicable.

general low percentage base saturation and Olsen-P. Maza-teca and Mixe region soils presented similar chemical characteristics. Soils in the Cuicateca region differed somewhat due to high erosion. Low soil productivity in the region is associated with low soil fertility. An appropriate management of soil fertility results in significant yield increments in the Cuicateca region (Cortés et al., 2004).

The spatial variability of most soil chemical properties was high in the 0- to 20-cm depth, as indicated by Vergara (2003). Figure 24.1 illustrates the spatial variability of soil C. The high spatial variability of C necessitates redesigning sampling strategies specifically designed for obtaining soil samples under hillside conditions and other ecosystems with variable soils. The SOC measurements were not always spatially correlated, as indicated by geostatistical parameters (Vergara et al., 2003). The extrapolation of data from a

Table 24.6 Mean Values of Selected Chemical Properties Determined in Soil Samples from Experimental Plots in Mazateca, Cuicateca, and Mixe Microwatersheds, Oaxaca, Mexico

Cation

Table 24.6 Mean Values of Selected Chemical Properties Determined in Soil Samples from Experimental Plots in Mazateca, Cuicateca, and Mixe Microwatersheds, Oaxaca, Mexico

Cation

Exchange

Organic

Organic

AI+3

Ca+2

K+

Mg+2

Na+

Total

Capacity

Base

Region and

Samples

Matter

Carbon

N

Olsen-P

(c mole

(c mole

(c mole

(c mole

(c mole

(c mole

(c mole

Saturation

Watershed

(n)

pH

(%)

(%)

(%)

(ppm)

(+)/kg)

(+)/kg)

(+)/kg)

(+)/kg)

(+)/kg)

(+)/kg)

(+)/kg)

(%)

Means by MicroWatershed (0-

to 40-cm

depth)a

Mazateca

372

5.2 b

6.4 a

3.9 a

0.3 a

5.8 a

1.9 b

3.9 b

0.2 b

0.7 b

0.28 a

5.1 b

7.1 b

63.3 b

Cuicateca

166

5.6 a

3.1 b

1.9 b

0.2 b

6.0 a

0.4 c

9.6 a

0.7 a

6.5 a

0.01 b

16.7 a

17.1 a

94.8 a

Mixe

214

4.9 c

6.9 a

4.2 a

0.3 a

4.4 b

3.9 a

2.6 c

0.2 b

0.7 b

0.05 b

3.6 c

7.4 b

41.6 c

LSD

0.1

0.5

0.3

0.3

0.7

0.3

0.9

0.1

0.9

0.08

1.5

1.4

5.6

Means by Microwatershed and Depth (0- to 20-cm and 20- to 40-cm depths)3

Mazateca

186

5.2 b

7.7 b

4.7 b

0.4 b

6.5 a

1.7 b

4.8 b

0.3 b

0.9 b

0.29 a

6.3 b

8.0 b

73.1 b

(0-20 cm)

Cuicateca

96

5.7 a

3.7 c

2.3 c

0.2 c

7.2 a

0.2 c

9.7 a

0.7 a

5.2 a

0.01 b

15.6 a

15.8 a

97.3 a

(0-20 cm)

Mixe (0-20 cm)

107

5.0 c

9.4 a

5.6 a

0.5 a

5.2 b

3.8 a

3.5 c

0.3 b

1.1 b

0.07 b

5.0 b

8.8 b

51.7 c

LSD

0.14

0.74

0.44

0.03

1.08

0.45

1.22

0.10

0.95

0.12

1.82

1.64

6.57

Mazateca

186

5.2 b

5.1 a

3.1 a

0.3 a

5.1 a

2.2 b

3.1 b

0.2 b

0.4 b

0.27 a

3.9 b

6.2 a

53.6 b

(20-40 cm)

Cuicateca

70

5.4 a

2.2 c

1.4 c

0.1 c

4.3 ab

0.7 c

9.4 a

0.7 a

8.2 a

0.01 b

18.3 a

19.0 b

91.3 a

(20-40 cm)

Mixe (20-40 cm)

07

4.9 c

4.4 b

2.7 b

0.2 b

3.6 c

4.0 a

1.6 c

0.1 b

0.4 b

0.03 b

2.1 b

6.1 b

31.5 c

LSD

0.16

0.61

0.36

0.03

1.0

0.49

1.19

0.15

1.52

0.11

2.43

2.27

Car rb

o er rt a Means followed by a different letter in the same column (0 to 40-cm) and (0 to 20-cm and 20 to 40-cm) are significantly different (p = 0.05), according to Tukey test. Note: LSD = least significant difference (p = 0.05).

% carbon

% carbon

10 12 14 16

20 22

Figure 24.1 Spatial variability of soil carbon content for the 0- to 20-cm depth in a coffee plantation. (From Vergara et al. 2004. Terra Latinoamericana, 22:359-367.)

10 12 14 16

20 22

Figure 24.1 Spatial variability of soil carbon content for the 0- to 20-cm depth in a coffee plantation. (From Vergara et al. 2004. Terra Latinoamericana, 22:359-367.)

point/pedon level must be evaluated carefully to avoid misinterpretations.

The chemical characteristics of the soil were used to generate graphical indexes of actual quality for the various systems. This graphical index is useful to compare the different management systems. Near-optimum values for each variable were calculated based on data from the literature. Figure 24.2 is an example of this type of exercise. In general, severe deviations from ideal values were observed. Correction of these deviations may lead to improved C sequestration in the long run.

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