Conversion of forest into irrigated pasture II. Changes in the physical properties of the soil

The replacement of natural vegetation (NV) by agricultural species may cause important changes in the ecosystem. Land use and management result in modifications of soil properties and functions, especially with the application of irrigation. The aim here therefore, was to assess the physical properties that are indicative of soil quality in an area under irrigated pasture (PAST), and to compare the results with those obtained in an area under natural vegetation. In order to identify changes in the soil, some physical attributes {aggregate stability, soil water retention curve, Dexter soil physical quality index (S), and least limiting water range} were determined, together with the total amount of lipids and glomalin, which can act as structure-stabilizing agents for different classes of aggregate. Rhizodeposition, a characteristic of grasses, and associated with the supply of plant biomass from the management practices used in PAST, may explain the differences in aggregate stability in the studied areas. The results showed that, despite the highest levels of lipids being found in PAST, it can be inferred that, due to the sandy texture and trampling by animals, there has been no effective structuring of the soil on the surface. In contrast to the trend observed for the lipid content of the soil, glomalin levels were greater in NV, and it was demonstrated that the glomalin content reacts sensitively to changes in land use. The least limiting water range (LLWR) is narrower under PAST when compared to NV, and reflects the loss of the physical quality of the soil due to the management of the pasture. Under such conditions therefore, plants may be susceptible to water stress when there is no irrigation. It can be concluded that the management of animals and pasture adopted in this study negatively affected the physical properties under evaluation, despite the better chemical and organic conditions of the soil.