Spatial and temporal variability of soil water repellency of Amazonian pastures

Fire is commonly used to establish and maintain pastures in the Amazon. Fire is also known to induce soil water repellency but few published data exist for the humid tropics. The objectives of this study were to characterise the intensity and spatial variability of water repellency on previously burned pasture soils in the Amazon, and its effect on the nutrient status of the forage grass Brachiaria brizantha (Hochst.) Stapf. Surface soils of pastures and forests in north-western Mato Grosso, Brazil, were found to exhibit soil water repellency using MED and WDPT tests. Soil water repellency was found only within 0-0.01 m of the mineral soil surface, with soil below 0.01 m found to be hydrophilic in all cases. Spatial variability of repellency was high for both pasture and forest soils. For pasture soils, soil water repellency was strongest on recently burned pastures, which exhibited some extremely high values (MED >8 M). Repellency decreased rapidly with time following burning. Increasing soil water repellency was associated with decreasing N:P ratios or B. brizantha above-ground biomass ([r.sup.2] = 0.66, P= 0.004). These findings indicate that soil water repellency and pasture productivity are inversely related. Since pasture abandonment fuels continued deforestation, disrupting the processes causing pasture degradation may lead to more sustainable land use in the Amazon. Additional keywords: hydrophobicity, Brachiaria brizantha, burning, N : P, Oxisol, Ultisol.
Introduction Over 75% of Amazonian deforestation in the past 30 years has been directed towards the establishment of pastures (Fearnside 1996). The productivity of Amazonian pastures declines within 4-8 years [...]