Impacts of Forest-to-Pasture Conversion on Soil Water Retention in the Amazon Biome

Land-use conversion from forest-to-pasture in the Amazon can affect soil physical quality and hydraulic functioning. The study evaluates the effects of land use (forest and pasture) and soil texture (fine and coarse) on soil structure and hydraulic properties, using the soil water retention curve as an integrative indicator. The study was conducted with soil samples from the Tapaj&oacute:s National Forest region, Par&aacute: State, Brazil, with eight sites (four forest and four pasture), balanced by texture. Undisturbed samples were collected from five profile layers (0&ndash:10, 10&ndash:20, 20&ndash:30, and 30&ndash:40 cm) for each site, totaling 160 samples. Samples were saturated and measured at soil water matric potentials from &minus:0.1 to &minus:15,000 hPa to obtain the soil water retention curve, which was fitted using the van Genuchten&ndash:Mualem model. Pore size distribution was derived from the relationship between soil water matric potential and equivalent pore diameter. Results are reported for the 0&ndash:40 cm soil profile (integrating the four sampled layers). Forest-to-pasture conversion altered soil pore structure and water retention in a texture-dependent manner. For fine-textured soils, bulk density increased from 1.03 to 1.31 Mg m&minus:3 (+27%) from forest to pasture. In coarse-textured soils, the drainable pore volume up to &minus:15,000 hPa, equivalent diameter &gt: 0.2 &micro:m) decreased from 0.296 to 0.147 m3 m&minus:3 (&minus:50%) from forest to pasture. Plant-available water across the 0&ndash:40 cm profile ranged from 0.107 m3 m&minus:3 (pasture, fine-textured) to 0.137 m3 m&minus:3 (forest, coarse-textured). Coarse-textured soils showed a marked reduction in macroporosity, water retention, and plant-available water, whereas fine-texture soils showed smaller changes in water availability but reduced aeration associated with macropore reduction. These results indicate higher physical quality vulnerability of coarse-textured soils following forest-to-pasture conversion.