Soil fertility and 137Cs redistribution as related to land use, landscape and texture in a watershed of ParaÃba state / Fertilidade do solo e redistribuiÃÃo de 137Cs em funÃÃo da cobertura vegetal, relevo e classes texturais, em uma microbacia hidrogrÃfica do estado da ParaÃba

AUTOR(ES)
DATA DE PUBLICAÇÃO

2003

RESUMO

Intensive land use and growing deforestation of the natural vegetation in Northeastern Brazil have contributed to the degradation of resources, particularly the decrease of soil fertility. As a result, biodiversity and ecosystem capacity to restore its resources after disturbances have been diminished. The decrease in soil fertility is more substantial in areas dominated by an undulating topography. In these areas, erosion is intensified when crops or pasture replaces natural vegetation. Even though degradation processes are reflected in environmental, social, and economical changes, there is a lack of information regarding the interrelationship between these changes and soil fertility and erosion. Thus, the âVaca Bravaâ watershed (14,04 km2), located in the âAgresteâ region of ParaÃba State, was selected to study the interrelationships between land use, landscape, particle size distribution, soil fertility and erosion using 137Cs redistribution. Small farms, where subsistence agriculture is intensive, are common in this watershed, as well as areas for environmental protection. A georreferenced survey of the watershed topography was initially carried out. Based on the survey data, the watershed was digitalized using a scale of 1:5000, and a 3-D map was created. Each landform element had its area determined on a area (absolute value) and percentage (relative value) basis. Shoulder, backslope and footslope positions represented 83% of the cultivated area in the watershed. A data base of 360 georreferenced soil samples (0 â 20 cm), collected using a stratified sampling scheme, was further created. Sites were stratified based on their landscape position (summit, shoulder, backslope, footslope, and toeslope) in factorial combination with land use (annual crops, pasture, Pennisetum purpureum, Mimosa caesalpiniaefolia, bush-fallow, and native forest). Physical analyses of the soil samples included particle size distribution and bulk density, whereas soil chemical analyses were total C and N, extractable P (Mehlich-1), pH (water), exchangeable acidity and cations). The soils of the watershed generally exhibited low fertility, regardless of their landscape position (p<0.05). However, land use and texture contributed significantly to the soil fertility (p<0.05). The contents of C, N, Ca, Mg, and ECEC were significantly higher (p<0.05) for the forest soils, whereas P, K, and exchangeable acidity were higher (p<0,05) in the cultivated soils. Forest soils were classified as sandy clay and sandy clay loam (65%), whereas cultivated soils were identified as sand and sandy loam (64%). Particle size distribution and topography were interrelated as sand contents decreased with the increase in slope. Overall, soils with finer texture showed higher fertility levels, regardless of the landscape position. Catchment sites were not discriminated as areas of significant gain of nutrients. It was observed a strong P deficiency, regardless of the landscape position, land use or particle size distribution. The catena segmentation, based on landform elements, land use and soil depths, was an efficient tool to understand the erosion/sedimentation processes by using the 137Cs redistribution approach. Soils of the topossequences under native forest (n = 2) and pasture (n = 3) were sampled for this purpose. It was observed a significant correlation between clay content and 137Cs activity in the soil (r = 0.75, p<0.01). At the summit positions of the forest soil, the average 137Cs stock down to a depth of 28 cm was 38,7 Bq m-2. At the backslope positions the 137Cs stock was similar for the same depth range. On the other hand, the average 137Cs stock in soils under pasture at the summit landscape position was 58,0 Bq m-2, with a maximum of 74,9 Bq m-2. Levels of 137Cs were below detection limit (0,03 Bq kg-1) on soils under pasture from the shoulder and backslope positions. The redistribution of 137Cs within the shoulder landscape position resulted in an estimated loss of 219,7 kg soil m-2, which is equivalent to a soil layer of 20,0 cm. Soil losses were higher on the backslope position, which reached a value of 268 kg m-2. This loss corresponds to the removal of a 24,5 cm soil layer. Given that the topossequences were representative of the area, these results indicate intense sediment redistribution within the watershed, which probably caused the transport of organic matter and nutrients associated with the clay fraction into the fluvial system

ASSUNTO(S)

funÃÃo da cobertura vegetal microbacia hidrogrÃfica engenharia nuclear fertilidade do solo

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