Variability of soil aggregation in a hilly semi-arid rangeland

Aggregation is an important process related to structure of the soil. This study examined soil aggregation indices in a semi-arid rangeland in the northern Negev region of Israel. The purpose of the study was to investigate to what extent the soil aggregation is affected by hillside aspect, surface cover and grazing. The soil of four types of surface cover - shrub patch, intershrub area, flock trampling route and rock fragment cluster - was sampled in an area subject to grazing and in grazing exclusion plots. At the peak of the growing season and at the end of the dry season, sampling was conducted from two depths (0-2 and 5-10 cm) on a north- and south-facing hillside. We determined the aggregation indices of <250 μm, >2000 μm, >8000 μm and the mean weight diameter. Differences were found between hillside aspects as well as among the types of cover. Grazing affected the fraction >8000 μm. The aggregation indices of the trampling routes were distinct from those of the intershrub area. It is proposed that in semi-arid rangelands, uneven grazing pressure in the intershrub area creates spatial heterogeneity in soil aggregation properties, which affect the spatial redistribution of soil resources and primary production.