Materials And Methods

This study was carried out in three microwatersheds (Maza-teca, Cuicateca, and Mixe regions) in the northern sierra of Oaxaca State, Mexico (Table 24.1). The hillside area represented by these microwatersheds is approximately 1 million ha in size. Names and biophysical characteristics of these experimental watersheds are San Jerónimo Tecoatl (319 ha, 1300 to 1900 m.a.s.l.) in the Mazateca region (latitude 18° 08' 57" and 18° 10' 13" north; and longitude 96° 53' 30" and 96° 54' 43" west); Concepción Pápalo (147 ha, 1700 to 2200 m.a.s.l.) in the Cuicateca region (latitude 17° 50' 20" and 17° 51' 25" north; and longitude 96° 51' 55" and 96° 52' 35" west); and Tzompantle (32 ha, 1280 and 1520 m.a.s.l.) in Cacalotepec in the Mixe region (latitudes 17° 00' 45" and 17° 01' 16" north; and longitudes 95° 53' 53" and 95° 54' 10" west). Principal land use systems in these regions are secondary-growth forest (Alnus, Liquidambar, and Quercus spp. mixed with Pinus spp.); coffee (Coffea arabica); permanent milpa (traditional practice of cultivating maize [Zea mays L.] interspersed with other food crops such as beans [Phaseolus vulgaris or Phaseo-lus coccineus] and Cucurbitacea); and milpa for 2 or 3 years followed by a variable number of years of acahual. Acahual is a local name for a secondary-growth vegetation characterized by a mixture of young tree species, herbaceous species, and shrubs, which are usually more than 3 years old, that

Table 24.1 Characteristics of Experimental Regions and Microwatersheds

Region

Table 24.1 Characteristics of Experimental Regions and Microwatersheds

Region

Characteristics

Mazateca

Cuicateca

Mixe

Area (km2)

2301.33

1329.67

6470

Dominant Soils (a)

Luvisol, Rendzina,

Luvisol,

Acrisol and

Feozem, and

Rendzina,

Cambisol

Acrisol

Feozem, and

Acrisol

Land use

Agriculture, forest

Forest (pine-

Agriculture,

(pine-oak) high

oak), sylva,

low sylva,

sylva, pasture

pasture

pasture,

forest (pine)

Altitude (m)

200-3250

200-3000

200-3200

Slope gradient (%)

>15 (25-45)

>15

>25

Precipitation (mm)

>2000

500-700

1500-2000

Temperature (°C)

16-27

18-20

17-27

Microwatersheds

San Jerónimo

Concepción

Tecoatl

Pápalo

Zompantle

Area (ha)

319

149

32

Altitude (m)

1380-1910

1700-2200

1280-1525

Slope gradient (%)

30 to 60

30 to 65

30 to 65

Land use (ha)

Milpab

41.04

65.31

3.00

Pasture

16.49

27.66

Forest

20.29

49.00

0.52

Coffee

169.14

8.39

Acahual (second

72.79

20.77

growth)

Bushes

5.20

a The Food and Agriculture Organization-UNESCO and Mexican soil classification systems are similar, but they are not directly equivalent. The following are approximations: Luvisol = Alfisol; Rendzina = Borol; Cambisol = Inceptisol; Feozem = Mollisol; and Acrisol = Alfisol. b A traditional Mexican practice of cultivating maize (Zea mays L.) interspersed with other food crops such as beans (Phaseolus vulgaris or Phaseolus coccineus) and Cucurbitacea.

a The Food and Agriculture Organization-UNESCO and Mexican soil classification systems are similar, but they are not directly equivalent. The following are approximations: Luvisol = Alfisol; Rendzina = Borol; Cambisol = Inceptisol; Feozem = Mollisol; and Acrisol = Alfisol. b A traditional Mexican practice of cultivating maize (Zea mays L.) interspersed with other food crops such as beans (Phaseolus vulgaris or Phaseolus coccineus) and Cucurbitacea.

grow after farmers cultivate the land using the traditional slash-and-burn system. Burning the biomass accumulated during the fallow cycle contributes to soil fertility. If slash-and-burn practices are eliminated, the acahual may become secondary-growth forests comprising Liquidambar, Alnus, or Quercus spp.

Observation sites representing various combinations of secondary forest vegetation, dominant perennial crops (coffee and pastures), milpa, and acahual were selected for study in the watersheds. In addition, technologically improved production systems (plots with peach or coffee plantings, depending on the altitude, in rows 9 m apart) were established at a very high density of 1400 trees ha-1. Milpa cropping was located between tree rows either under traditional (LT) or minimum tillage (LC) management. This improved technology was named "MIAF" (Cortés et al., 2004).

In the experimental secondary-growth forest and perennial agricultural crop sites, five 4 x 25-m plots were established at random (Woomer and Palm, 1998; Kotto-Same et al., 1997). Three plots 9 m long and of variable width (4 to 8 m) were established for each of the milpa, acahual, and MIAF systems. In each plot, the aboveground (tree, bush plus weeds, and litter) and underground biomass was measured. Detailed procedures for sampling were described by Acosta et al. (2001a, 2001b, 2002). Soil samples were collected from 0 to 1.05 m deep in 15-cm increments at 18 to 20 georeferenced points within each plot. The latter permits monitoring changes in C pools over time (Acosta, 2001b).

Soil samples were taken with a handheld cylindrical sampler of 19-cm length and 4.17-cm internal diameter. The soil sampler allowed taking uncompressed soil cores (15 cm long) that were used to determine bulk density.

Soil samples were air dried and sieved to less than 2 mm before chemical analyses. Carbon concentration was measured in aboveground (tree, bush plus weeds, litter) and underground biomass, and in soil samples after removing roots and stones. A Shimadzu 5050A automatic carbon analyzer was used to measure total C. In this study, total C represents SOC, as soil pH was less than 7 (Table 24.5).

Table 24.2 Assumed Optimal Values for Selected

Properties

Indicator Ideal Value pH 7.0

Exchangeable acidity, c mole kg-1 0.2

Organic matter, % 4.2

Olsen-P, ppm 12.0

Exchangeable K, c mole kg-1 0.3

Exchangeable Ca, c mole kg-1 10

Exchangeable Mg, c mole kg-1 3

Cation exchange capacity, c mole kg-1 30

Bulk density, g cm-3 1.1

Available soil moisture, % 20

Hydraulic conductivity, cm hour -1 10 Microorganism activity, ^L 02 hour-1 100 g-1 150

Source: From Vergara S., 2003. Identificación y Selección de Indicadores de Calidad del Suelo y Sustentabilidad en Sistemas Naturales y Agrícolas de Ladera en Oaxaca. Ph.D. thesis, Colegio de Postgraduados, Montecillo, Mexico.

A separate independent soil-sampling scheme was implemented in selected plots to measure chemical properties associated with soil fertility, and to assess spatial variability of SOC. Table 24.2 presents assumed optimal levels of various properties obtained from the literature, as cited by Vergara (2003). Assumed optimal levels for each characteristic (assigned a 100% value) were used to build radar-type graphs and to compare different systems. Eighteen soil samples were taken for this study from the 0- to 20-cm and 20- to 40-cm depths in each plot. The sampling design permitted the calculation of some semivariograms and spatial variability maps (Vergara et al., 2003).

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