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IX. Ecological approach to sustainable land use in the humid tropics

A. Soil erosion control
B. Soil organic matter maintenance
C. Soil structure
D. High soil biodiversity
E. Mineral cycling
F. Soil resilience
G. Ecological compatibility

Attempts at "modernizing" traditional agriculture in the humid tropics have met with limited success. Drastic perturbations in the ecosystem are likely to cause serious disruptions in cycles of H₂O, C, N. S. P. and other elements and disturb the energy balance. Resource-based and low-input systems, although subsistence and less productive, have been ecologically compatible because they do not overly disturb the ecological processes (e.g. energy balance, hydrological balance? and mineral cycling). The successful transformation of subsistence agriculture to productive systems must be based on understanding and the ability to manipulate processes governing: (i) energy and biomass flow from and into the ecosystem; (ii) mineral recycling; and (iii) the water balance of the ecosystem at micro- and meso-scales. A sustainable system must have favorable attributes and desirable characteristics (Fig. 21).

Maintenance of soil quality is critical to the success and sustainability of land-use and farming systems. The soil quality depends on several attributes, described below.

A. Soil erosion control

Accelerated soil erosion must be controlled. Because the nutrient reserves of most soils are mostly concentrated in the upper 10 to 15 cm layer of soil developed in situ (e.g., Oxisols, Ultisols, Alfisols), the loss of top soil has drastic impacts on productivity. Soil erosion control is the most important attribute of an ecologically compatible and productive land-use system.

Fig. 21 Attributes and characteristics of a successful land-use or farming system

B. Soil organic matter maintenance

Maintenance of a high level of soil organic matter content is crucial to the enhancement of soil structure and other life-support processes and in environmental regulatory mechanisms. Although soil organic matter content is not a nutrient, its concentration must be maintained above the critical level for maintaining and enhancing soil properties and processes. The critical level of soil organic matter content varies among soil types, ecoregions, and land-use systems.

C. Soil structure

Soil structure must be maintained to allow the functioning of the dynamic processes (e.g., aeration, water transmission and retention, root growth). Different manifestations of soil structure include aggregation and its stability; porosity and pore-size distribution, especially macropores; water retention and transmission properties; and compatibility. Once destroyed, these favorable attributes of soil structure are difficult to restore.

D. High soil biodiversity

High soil organic matter content and favorable soil structure depend on the activity and species diversity of soil fauna. The activity of soil macrofauna (e.g., earthworms, termites) is critical to the maintenance of favorable soil structure and mineral cycling. Management practices with detrimental effects on soil biodiversity lead to a rapid decline in soil structure and degradation of the resource base.

E. Mineral cycling

It is important that the nutrient reserves of the ecosystem are not depleted but are enhanced and recycled. The recycling mechanisms (e.g., a combination of deep-rooted and shallow-rooted species, returning biomass to the soil surface, high soil biodiversity) must be strengthened. The loss of nutrients by erosion, leaching, and volatilization must be minimized.

F. Soil resilience

A sustainable system must enhance soil resilience or the soils ability to restore its life-support processes. A resilient soil must not only resist degradative processes but also respond to science-based inputs and judicious management. Soil resilience also depends on land use, and the choice of an appropriate land use is crucial to a soils ability to withstand perturbations.

G. Ecological compatibility

Environmental compatibility is an important characteristic of a sustainable system. Deforestation of the TRF ecosystem supposedly contributes 1.1 to 3.6 pg-C/yr as a principal greenhouse gas into the atmosphere (Houghton, 1991). The TRF ecosystem should be managed to sequester carbon into the soil and biomass rather than release carbon and nitrogen into the atmosphere, improve water quality rather than accentuate sediment load and eutrophication of surface and ground waters, and increase soil biodiversity within a simplified and managed ecosystem. Next to production, environmental concerns are very important for the sustainable management of the humid tropics.

Fig. 22 An ecological approach to sustainable land use in the humid tropics

The sustainability of a land-use system depends on a set of science based agronomic practices. These practices involve appropriate methods of forest conversion, erosion preventive and control measures, nutrient management techniques involving nutrient cycling through returning biomass and organic wastes to the soil, and production systems that optimize the use of the most limiting resources. A flow chart depicting the interaction among these technological options is depicted in Fig. 22. These practices interact with soil properties in determining productivity and impact on the environment, and are very specific to soil types and ecoregions.

The adoption of improved technologies on existing land can reduce pressure and the need for deforestation and clearing new land. Sanchez (1993) estimated that the adoption of science-based improved technologies on I hectare of land can save between 5 and 11 hectares of TRF from deforestation (Table 49). However, these technologies have to be adapted for soil and environmental conditions specific to each ecoregion. Developing locale-specific packages for alleviating specific soil and environmental constraints to sustainable land use requires fine-tuning and adaptive research for specific regions. In fact, it is difficult to provide blueprints for soil- or site-specific cropping/farming systems. Some systems have proven successful on research farms and under farmers, conditions and can be applied to similar soils and ecological conditions elsewhere in the humid tropics. Examples of such systems are outlined in Table 50.

Table 49 Estimates of TRF saved from deforestation by adoption of improved technology

(Sanchez. 1993)

Improved germplasm plays an important role in the development of sustainable land-use systems. It is important to select new crops and develop improved varieties that can tolerate acid soil conditions and high concentrations of Al and Mn in the sub-soil, and grow under continuously warm and humid environments with high pressure from diseases and pests. The positive role that genetic resources and improved germplasm can play in developing sustainable systems cannot be overemphasized.

Table 50 Examples of potentially sustainable land-use systems in the humid tropics

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