Influence of types of restorative vegetation on the wetting properties of aggregates in a severely degraded clayey Ultisol in subtropical China

Vegetation may affect soil physical behaviour because of increases in hydrophobic organic compounds that bind soil particles and reduce the rate of wetting by inducing a low level of water repellency. These processes have not been isolated previously, so the aim of this study was to evaluate how different types of restorative vegetation influence the water repellency and pore structure of soil. We measured an index of water repellency, R, as the ratio between water and ethanol sorptivities on the surfaces of different size soil aggregates from a severely degraded clayey Ultisol. R is proportional to the decrease in water sorptivity. The treatments were (1) eroded bare land, two types of restorative vegetation-(2) deciduous Camphor trees (Cinnamomum camphora) and (3) leguminous Lespedeza shrubs (Lespedeza bicolor), (4) Masson pine (Pinus massoniana), and (5) a vegetable garden with large annual inputs of pig manure. R increased from <1.5 in eroded bare land up to the range from 2.0 to 4.4 when this land was planted with vegetation. Undisturbed soil under Masson Pine had an R congruent to 3. Large (20-50 mm) aggregates had greater water repellency and porosity than small (10-20 mm) aggregates, particularly under Camphor trees. The type of vegetation and pore structure therefore affects water repellency. Isolating these properties will help us understand how plant selection and soil management influence soil physical behaviour and structure dynamics. (C) 2003 Elsevier Science B.V. All rights reserved.