Changes in soil organic matter associated with pig rearing: Influence of stocking densities and land gradient on forest soils in central Italy

Outdoor pig rearing was reintroduced in Italy, as in the rest of Europe as a consequence of a European policy aimed to pay attention to animal welfare and food quality. Despite these advantages, outdoor pigs rearing may lead to serious damage of both vegetation and soil because of animals eating habits and behavior. In a forest ecosystem the effects of pig impact could be really negative, reducing the natural ability of soil to recover itself and causing the loss of organic matter. The aim of this study was to evaluate the effect of outdoor pig rearing in forest ecosystems in Mediterranean areas on soil, in order to identify the critical thresholds for soil organic matter quality and functionality in terms of: animal pressure (animal density); grazing livestock area (slope, depth and soil characteristics). Two farms, similar for type of vegetation (forest environment) and soil characteristics were chosen. In each farm three different situations of animal pressure (high, low animal pressure and undisturbed), combining to territorial and morphologic characteristics used for pig rearing (high, low slope and flat soil), were identified. For each experimental situation, chemical and chemical-structural parameters of soil were investigated. Grazing at high animal pressure showed a reduction of all parameters related to organic matter content, in addition to a worsening of chemical-structural characteristics, thus resulting in a loss of soil quality. Grazing at low animal pressure generally seemed not to affect chemical fertility of soil, preserving organic matter content, even though a significant worsening of chemical-structural characteristics was identified. Soil damage including mineralization of stable organic matter was greatest on the steeper slopes for both upper and lower soil layers.