Soil quality in native forest and pasture eco-systems in the eastern Acre region of Western Amazon / Qualidade do solo em ecossistemas de mata nativa e pastagens na região leste do Acre, Amazônia Ocidental

AUTOR(ES)
DATA DE PUBLICAÇÃO

2008

RESUMO

The state of Acre is presently at the initial stage of converting its forests into pastures. Around 12% of its total area (19.200 km2) has been deforested. Out of this, approximately 81% (13.352 km2) have been utilized by several productive segments in Acre (farmers, land settlers, extractivists, riverine farmers and cattle producers), with extensive pasture ecosystems. It is estimated that around half (667.000 ha) is at some stage of degradation. This gradual deforestation process has made the public government and the scientific community as a whole aware of the need to measure the impacts caused, especially on the soil, and to propose alternatives for the use, management and recovery of these ecosystems. However, in Amazon few studies have emphasized the assessment of soil quality /degradation due to pasture use. Within this context, this study aimed to select, characterize, and evaluate the physical and chemical indicators of soil quality in pasture ecosystems, compared to correlated native forest ecosystems, in two eastern Acre areas so as to subsidize sustainable soil management, particularly with respect to forest-pasture succession. Thus, relevant conceptual aspects of soil quality, as well as procedures and indicators used to measure this quality in agro-ecosystems are initially discussed in a generic way followed by some soil and pasture degradation concepts and impacts caused by the conversion of native forest into pasture ecosystems in the Amazon. Morphological, physical, physical-chemical, chemical and biological soil attributes that can be used in an integrated way to identify and measure the various forms of pasture degradation occurring under edapho-climatic conditions in Acre are also discussed, as well as some recovery alternatives for these areas. Some soils and environments in the municipalities of Rio Branco and Senador Guiomard are then characterized. The soils described in Rio Branco were classified as Dystrophic Red Argisol (PVd), Red-Yellow Argisol, Red- Yellow Epieutrophic (PVAe), alitic plintic Red-Yellow Argisol (PVAal) and Dystrophic Argiluvic Plintosol (FTd). In Senador Guiomard, a Dystrophic Red Yellow Latosol (LVAd) was described and characterized. Rio Branco soils are located predominantly in relief areas of more movement (Rio Acre-Rio Javari Deppression), are more diversified, have solum varying from shallow to deep, are imperfectly to badly drained, present better natural fertility and signs of expansive clay-minerals, with silt content on higher surfaces than that of clay.. In Senador Guiomard, the pedo-systems are located in plain areas of Western Amazon high plains, whose soils are well drained, deep, kaulinitic, and of low natural fertility. Finally, the impact of deforestation and the introduction of Brachiaria brizantha, cv. Marandu pastures in relation to the physical and chemical attributes of soil, on C stock and dynamics, humic substances and on the intensity of alteration in soil quality/degradation are evaluated. The first succession is located in the municipality of Rio Branco and consists of an open forest area, with bamboo and palm trees, and two (03- and 10- year-old) pastures in alitic, plintic, RedYellow Argisol (PVAal) area. The second is located in the municipality of Senador Guiomard on Dystrophic Red-Yellow Latosol (LVAd) area under dense forest, with plain relief and 20-year-old pasture. In each site, soil samples were collected at depths of 0 to 5, 5 to 10, 10 to 20 and 20 to 40 cm from the surface. Soil physical and chemical characteristics were determined in the samples. Soil quality was estimated at depth of 0-20 cm from the surface, by means of the Soil Degradation Index (SDI) on the pasture ecosystem, using the native forest as reference. The two chronological sequences soils had their physical and chemical properties altered differently. In many situations, there was an improvement in chemical properties, such as pH and exchangeable bases, as a result of ash incorporation in soil. Besides, increase of C stock and addition of Ca-derived C were favored, most expressively in Latosol, indicating that this soil is storing more C than Argisol. Increased b13C values were confirmed in the soils, along pasture utilization time in both successions, being more evident in the 20-year-old pasture. The b13C values for the humic substances indicated that humic acid (FAH) fraction had a greater C4-derived C participation. The use of soil with pasture altered the relative proportion of the humic fractions of organic C, with the structural stability of carbon (HUM/AF+AH relation) tending to decrease. Considering the functionality aspects of soil, soil SDI was more deleterious for the 3-and 10-year-old pastures.

ASSUNTO(S)

solos do acre native forest-pasture conversion soils of acre ciencia do solo conversão floresta-pastagem western amazon amazônia ocidental

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