Case study. Destruction and restoration of soil structure and pasture production in kaolinitic soils of the Amazon Basin (Barros, 1999; Chauvel et al., 1999)
In the Brazilian Amazon, 95% of the deforested area is converted into pastures, and of these, about 50% can be considered degraded due to mismanagement, phyto-sanitary problems, poor soil fertility and soil structural modification (linked to faunal activity). The kaolinitic soils that predominate in the Amazonian region have a favorable micro-aggregate structure yet it is very fragile, due to low oxy-hydroxide metal contents. When the forest is converted to pasture, the machines and later the cattle trampling the soil lead to severe soil compaction, particularly in the 5-10 cm layer.
However, more importantly, the native soil macro-faunal communities are radically altered, most of the native taxa disappearing (151 vs. 48 morphospecies). These are replaced by an opportunistic invading species Pontoscolex corethrurus, which occupies the empty niches and increases in biomass up to >450 kg ha-1, and representing nearly 90% of the total soil faunal biomass. This species produces more than 100 Tons ha-1 of castings, dramatically decreasing soil macroporosity down to a level equivalent to that produced by the action of heavy machinery on the soil (2.7 cm3 100g-1). During the rainy season these casts plug up the soil surface, saturating the soil and producing a thick muddy layer, where anaerobic conditions prevail (increasing methane emission and denitrification). In the dry season, desiccation cracks the surface and the inability of roots to extract water from the soil causes the plants to wilt and die, leaving bare patches in the field.
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Figure 3. Structural changes in an Amazonian soil after one year in soil monoliths (25 x 25 cm) taken from the forest and placed into a Brachiaria sp. pasture and vice-versa (modified from Barros, 1999). |
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An experiment performed North of Manaus, Brazil demonstrated the role of P. corethrurus and a diverse assemblage of soil organisms (other earthworm species, termites, millipedes, isopods, ants, etc.) in creating and destroying soil structure. Soil monoliths 25 x 25 cm square were removed from the pasture and placed into the forest and similar blocks were taken from the forest and placed into the pasture. After one year, the structure of the compacted pasture soil was completely restored to levels of those typical in native forest soils by the action of the diverse community of forest soil organism. Meanwhile the macroaggregate structure of the forest soil was completely destroyed by P. corethrurus, reaching compaction and porosity levels similar to those of the degraded pasture (Figure 3). This work highlights not only the extremely important role of a diverse assemblage of macro-organisms to the maintenance of soil structure (especially in these kaolinitic soils), but also the problems associated with management practices that are not well adapted to the environment (pastures in the Amazon), and the role of invading species on ecosystem properties and processes.
Two other cases of biodiversity "accidents" have been reported:
- The first comes from the rice fields in the Northern Province of Corrientes, Argentina (see Folgarait, 1998). At this site, the mechanical destruction of the soil structure for constructing the rice paddies, has led to important changes in the soil biotic community, with ant (Camponotus sp.) populations tremendously increasing and creating up to 2000 large (1 to 2m tall) mounds per ha. This action completely eliminates the possibility of further agricultural uses on the land, without large and expensive measures to destroy the ant nests and the ant populations.
- The second case was observed in Holland, where farmers contacted scientists because the invasion of lumbricid earthworms (Aporrectodea caliginosa and A. rosea, dominant) in reclaimed polders was creating a massive structure (similar to that created by P. corethrurus in the Amazon) in potato fields (Ester and van Rozen, 2000). Problem areas in the fields with cloddy, compacted structure were identified and these were always associated with high earthworm numbers. When the potatoes growing in these problem areas were harvested, large soil clods, up to 50cm in size were harvested together with the potatoes, rendering the harvesting process impossible and reducing the quantity and quality of the potatoes.
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