The general patterns of the soil macrofaunal communities in the most common land use types (i.e., primary forests, rice crops, pastures and fallows) of the Brazilian Amazon are summarized. Modifications in the abundance and species diversity of communities according to land use changes and scales are particularly addressed. Also presented are case studies conducted in Benfica and Maraba (Para), Manaus (Amazonas) and Acre-Rondonia, illustrating the large and interregional variations of geological substrate, climate and land use impacts on macroinvertebrate communities.

In Brazilian Amazonia there are 26 million hectares of degraded pastures. One of the causes of degradation is assumed to be alterations in soil structure that is partly determined by a reduction in soil macrofauna diversity. The objective of this case study was to identify the consequences of altered macrofaunal diversity on soil structure and hydrological processes. After forest clearing for pasture establishment, the diversity of soil macroinvertebrates decreased strongly. Tensiometric measurements showed the formation of a superficial groundwater table in a 4-year-old pasture (P4), and a deeper groundwater table in an abandoned pasture (Pa), that was an exceptional situation. The forest soil, in contrast, was almost never water-saturated due to its microaggregate structure. Once the pasture was abandoned, the soil macrofauna was dominated by a single earthworm species, Pontoscolex corethrurus, which may modify, in the course of a few years, the morphology and hydrological functioning of soils. By placing very wet castings on the pasture soil surface, P. corethrurus can form a continuous layer that is relatively impermeable to air and water. Although initially limited to the surface layer, the episodic conditions of water saturation and anoxia may also occur in deeper horizons, favoring the development of hydromorphic conditions, even though the surface soil structure may be regenerating due to the activity of other macrofaunal groups such as termites, after reduction of P. corethrurus populations.

Soils play an important role in the carbon cycle, and deforestation in the tropics affects both soil carbon storage and CO2 release into the atmosphere. The consequences of deforestation and conversion to pasture for soil carbon content and dynamics were examined in two soil types differing mainly by their texture. Two chronosequences were selected, each consisting of an intact forest and three pastures of different ages (4, 8, 15 years and 3, 9, 15 years, respectively). One chronosequence is located in the central part of the Brazilian Amazon basin, where the soils are clayey ferralsols, and the second in the Eastern Brazilian Amazon Basin, where the soils are sandy clayey acrisols. In the upper layer the C content of clayey soils was three times higher than in the sandy soils, but despite the differences in soil texture, the C distribution in the particle-size fractions was quite similar. In the two chronosequences, the conversion to pasture induced a slight increase in C content. Bulk density increases were greater on soils with lower clay contents. The 13C measurements, which allowed to calculate the distribution of C derived from forest and from pasture, showed that all the particle-size fractions incorporated C derived from pasture and that a significant proportion of the young organic matter is rapidly trapped in the finest fractions. Although the proportions of pasture-derived C were higher in the sandy soils than in the clayey soils, the amounts of pasture-derived C in the particle-size fractions were 2–3 times larger in the clayey soils than in the sandy soils.

The aim of the study was to determine the effect of adding two tropical earthworm species, Rhinodrilus contortus and Pontoscolex corethrurus, to mesocosms on the availability of mineral N (NH4 + and NO3 − concentrations), soil microbial biomass (bio-N), and the decomposition rates of three contrasting leaf litter species, in a glasshouse experiment. The mesocosms were filled with forest soil and covered with a layer of leaf litter differing in nutritional quality: (1) Hevea brasiliensis (C/N=27); (2) Carapa guianensis (C/N=32); (3) Vismia sp., the dominant tree species in the second growth forest (control, C/N= 42); and, (4) a mixture of the former three leaf species, in equal proportions (C/N=34). At the end of the 97-day experiment, the soil mineral N concentrations, bio-N, and leaf litter weight loss were determined. Both earthworm species showed significant effects on the concentrations of soil NO3 − (p<0.01) and NH4 + (p<0.05). Bio-N was always greater in the mesocosms with earthworms (especially with R. contortus) and in the mesocosms with leaf litter of H. brasiliensis (6 µg N g−1 soil), the faster decomposing species, than in the other treatments (0.1–1.6 µg N g−1). Thus, earthworm activity increased soil mineral-N concentrations, possibly due to the consumption of soil microbial biomass, which can speed turnover and mineralization of microbial tissues. No significant differences in decomposition rate were found between the mesocosms with and without earthworms, suggesting that experiments lasting longer are needed to determine the effect of earthworms on litter decomposition rates.

Soil macrofauna are sensitive to land use changes and this may have implications to soil functioning. The impact of the conversion of native ecosystems into extensive or intensive pastures on soil macrofauna were assessed with a standardised methodology in two neotropical phytogeographical regions, i.e. a tropical savanna area (Eastern Plains of Colombia) and a tropical rain forest area (Brazilian Amazon).In the savanna area, extensive cattle ranching only led to a slight enhancement of earthworm populations and to short-term fire-induced decreases of macrofaunal density. In intensive pastures, the initial taxonomic richness and composition of soil macrofauna were maintained, while native earthworm biomass was strongly increased. This may be explained by the similar mesologic conditions between these systems (similar vegetation structure) and by the higher quality of the organic inputs in the pastures (roots, litter and cattle faeces). Increased macrofaunal activity with a high taxonomic diversity is expected to have positive impacts on the sustainability of pastures in Colombian savannas.In the Amazon basin, slashing and burning of the forest for intensive pasture establishment resulted in more dramatic effects on native macrofauna. Taxonomic diversity was particularly strongly affected. Native earthworm species were largely depleted at the expanse of exotic peregrine species like, e.g. Pontoscolex corethrurus. These results are probably bound to the deep environmental changes that follow the conversion of forest into grassland ecosystems. Such modifications of macrofaunal communities are known to have potential negative effects on soil functioning and on the sustainability of agropastoral systems in this area. (C) 2004 Elsevier B.V. All rights reserved.

In the Brazilian Amazon region, millions of hectares of forest land have been converted into cattle pastures and then been abandoned. Agroforestry is a potential option for the transformation of in parts degraded lands into productive agricultural systems. The re-establishment of a diversified soil macrofauna can help in the process of recuperation of the often compacted soil structure of the pastures. The soil macrofauna community was studied during the rainy season in four different agroforestry systems near Manaus in Central Amazonia: 1. a high-input silvopastoral system (ASPh), 2. a low-input silvopastoral system (ASPI), 3. a palm based system with four tree crop species (AS 1) and 4. a high-diversity tree crop system with ten tree crop species (AS2), plus a spontaneous fallow for comparison. The sampling method recommended by the Tropical Soil Biology and Fertility Programme was used. The highest diversity of fauna groups was observed in the ASPh and ASPI where trees were associated with the leguminous cover crop, Desmodium ovalifolium. The cover crop exerted a favorable effect on the soil fauna presumably by maintaining the soil moist and shaded and providing litter as a substrate. Of the 15 soil fauna groups that were found in all systems, four were absent from AS1. Within the AS2 system a significantly greater density of the soil fauna was observed under peach palm (Bactris gasipaes) and cupuacu (Theobroma grandiflorum) (3107 and 524 ind.m(-2), respectively) than under the other three tree species. The soil under peach palm and cupuacu also tended to have a higher number of soil fauna groups. In AS1, the soil under peach palm had a higher fauna density than the soil under cupuacu, probably caused by the abundant residues of the heart of palm harvest on the soil. The earthworm biomass was particularly high in AS 1. Under cupuacu approximately 7 times more earthworms were found in AS1 (17.9) than in AS2 (2.4). The study of the macrofauna community, including both the litter layer and the superficial soil layers, allows to identify the plant species/management combinations which favour the increase of the diversity of the invertebrates.

Agroforestry systems are presented as a valuable alternative to pastures to sustain crop production in forested areas. In order to evaluate their effect on soil macroinvertebrate communities, sampling was conducted during the rainy season at four localities located in the Rondônia and Acre states of Brazil. Four land-use systems were selected (fallow, annual crop, agroforestry systems and pasture), and compared to nearby disturbed forests. We used the sampling method recommended by the Tropical Soil Biology and Fertility Programme. Soil macrofauna responded more readily than soil parameters to different cultivation practices. Co-inertia analysis, however, showed a relationship between soil parameters and soil macrofauna. Comparison of communities showed a significant impact of land-use practices. All systems had quite abundant invertebrate communities with relatively low densities in the forest (884 ind. m–2) and in pastures (841 ind. m–2), and higher densities in fallow, agroforestry system and annual crop (1,737–1,761 ind. m–2). Earthworms were dominant in pastures (155 ind. m–2 and 56.2 g m–2 on average), whereas termites thrived better in annual crops and fallows (with respective densities of 1,287 and 816 and biomasses of 2.32 and 1.38 g m–2). Macrofauna communities in agroforestry systems were rather similar to the forest, in spite of higher densities of social insects. The termite:earthworm ratios were very low in pastures (0.2), had similar values in the forest (7.9) and the agroforestry system (8.8), which is much lower than in fallows (20.4) and annual crops (21.4) This study showed that all land-use practices were able to sustain sizeable macrofaunal communities with agroforestry communities rather similar to the those of a disturbed forest.

Abstract The composition and functional structure of soil invertebrate communities are discussed, as well as their effects on soil fertility. The effects of agroforestry practices on soil invertebrate communities are analysed. Finally, the techniques to assess their composition, abundance and activity are described.

In a central Amazonian pasture, a single earthworm species, Pontoscolex corethrurus, becomes very abundant (400 ind. m(-2)) after forest clearing. Its casts form a compact, continuous and impermeable crust with a thickness of 20 cm. To analyze the structural modifications, we established a field experiment in which soil blocks from the forest were implanted in the pasture, and soil blocks from the pasture were implanted in the forest. The objectives were (1) to verify the formation of the compact crust at the soil surface in pasture environment, (2) to evaluate the time necessary for the formation and the destruction of this crust, and (3) to find out if the crust formation was a reversible process. We used quantitative morphology to identify the biogenic structures formed by different fauna groups and to quantify the modifications in the solid phase as well as the resulting porosity. For the soil depth 0-5 cm, the measured porosity was 48% in the forest and 16% in the pasture. After 1 year, the blocks of forest soil installed in the pasture presented a porosity of 26%, and the blocks of pasture soil installed in the forest presented a porosity of 34%. There were significant differences between the control blocks and the exchanged blocks. The results demonstrate that the processes of formation and destruction of the biogenic structures are reversible. Approximately 1 year is necessary to re-establish the equilibrium between the exchanged blocks and the control blocks.This experiment illustrates the compacting effect of P. colethrurus. In addition, the small millimetric pores, which are formed by termites in the blocks of pasture soil implanted in the forest, show the decompacting effect of certain termite groups. (C) 2001 Elsevier Science B.V. All rights reserved.

This paper synthesises information on the food requirements of soil macroinvertebrates and some of their effects on soil organic matter dynamics. Some clues to techniques that would optimise their activities through organic matter management are suggested. Soil macroinvertebrates can consume almost any kind of organic residues in mutualistic association with soil microflora. Significant amounts estimated at several T per ha of predominantly easily assimilable C are used yearly in natural ecosystems as energy to sustain these activities. Sources of C used are highly variable depending on the feeding regime. The largest part of the energy assimilated (e.g., 50% by the tropical earthworm Millsonia anomala) is actually spent in burrowing and soil transport and mixing. Bioturbation often affects several thousand tons of soil per hectare per year and several tenth of m3 of voids are created in soil. A great diversity of biogenic structures accumulate and their nature and persistance over time largely controls hydraulic soil properties. The OM integrated into the compact biogenic structures (termite mounds, earthworm globular casts) is often protected from further decomposition. Most management practices have negative effects on the diversity and abundance of macroinvertebrate communities. Structures inherited from faunal activities may persist for some weeks to years and the relationship between their disappearance and soil degradation is rarely acknowledged. When SOM supply is maintained but diversity is not, the accumulation in excess of structures of one single category may have destructive effects on soil. It is therefore essential to design practices that provide the adequate organic sources to sustain the activity and diversity of invertebrates. Special attention should also be paid to the spatial array of plots and rotations in time.

Almost all cultivated soils undergo some reduction in the porosity of the surface layers, and nowhere is this more evident than in tropical rainforests that have been converted to pastures. Following deforestation in an area of Costa Rica, soil bulk density has been shown to increase rapidly after conversion to pasture, leading to poor drainage and a reduced rate of gaseous diffusion 1 . These factors limit methane consumption and promote the anaerobic production of methane. A similar effect on methane flux has been found in upland soils in the Brazilian Amazonian basin after conversion from forest to pasture 2 , 3 . Increases in atmospheric methane are therefore not limited to emissions from flooded soils 4 , as forest-to-pasture conversion promotes the anaerobic mineralization of organic matter by changing the physical properties of soil.