Grazing-induced spatial variability of soil bulk density and content of moisture, organic carbon and calcium carbonate in a semi-arid rangeland

The landscape of many semiarid rangelands is characterized by a two-phase, shrub-intershrub vegetation mosaic, each phase having different soil properties. However, this broad subdivision groups together types of intershrub surface cover that may also differ in their soil properties and play important roles in ecosystem functioning. In the northern Negev region of Israel, we examined the soil properties associated with flock trampling routes and rock fragment clusters, as well as those associated with the remainder of the intershrub area and shrub patches. Moisture content, organic carbon content, bulk density and calcium carbonate content of the soil were determined for the above four types of cover, inside and outside long-term grazing exclosures. Soil was sampled in the peak of the growing season and in the end of the dry season, on a north- and a south-facing hillside, and from two depths. The shrub patches exhibited the highest soil moisture and organic carbon contents, and the lowest bulk density and calcium carbonate contents. The trampling routes showed opposite trends. The rock fragment clusters and the remainder of the intershrub area did not generally differ and had intermediate values of these properties. Grazing did not have a significant effect on soil properties at the whole-plot scale, but there were highly significant interactions between grazing and type of cover. Compared with the former trampling routes in the exclosures, the active trampling routes outside them had higher bulk density and lower moisture and organic carbon contents. The intershrub area had higher moisture and organic carbon contents under grazing than in the exclosures. Grazing increased the spatial heterogeneity of the soil properties examined via the creation of a network of trampling routes on the hillsides. The routes themselves, which constituted over 20% of the landscape cover, had degraded soil properties but they led to the improvement of the properties of the remainder of the intershrub area via functionally important source-sink relationships. The study of the soil of regions in which such networks are apparent should be duly cognizant of this intershrub subdivision in addition to the widely recognized shrub-intershrub dichotomy.