Soil quality and aggregation in runoff water harvesting forestry systems in the semi-arid Israeli Negev

Water availability is a major limiting factor for dryland afforestation. Earthworks that modify natural landforms for the formation of runoff harvesting systems are prevalent in the Israeli drylands, with the aim of establishing afforestation projects. However, serious concerns alarm that such earthworks have detrimental effects on the geo-ecosystem functioning. Therefore, the objective of this study was to examine the impact of the establishment of contour bench terrace (also called shichs/shychs/shikim) runoff harvesting systems on selected soil properties, with,particular focus on still aggregation. Thus, we assessed the effect of the establishment of contour bench terraces in a multi-aged forestry land, comprised of 2-year-old and 9-year-old afforestation areas, and compared them to 'natural' hillslopes as a reference treatment. The study was implemented in the Ambassadors' Forest, located in the semi-arid northern Negev of Israel, where we sampled the surface soil (at a depth of 0-5 cm) in north- and south-facing hillslopes. Considerable differences were recorded for the afforestation systems between the source (inter-terrace area) and sink (terrace-bottoms) areas. Data normalizing according to the relative cover percentage of the terraces and inter-terrace areas showed that the mean values of mean weight diameter (MWD), stable aggregate content, and particulate organic carbon in the natural hillslopes were 1.4%, 32.4%, and 20%, respectively, greater than in the 9-year-old afforestation systems, and 12.1%, 28.9%, and 31%, respectively, greater than in the 2-year-old systems. Means of clod content, aggregate slaking index, and clay dispersion index in the natural hillslopes were 623%, similar to twofold, and 35.0%, respectively, smaller than in the 9-year-old systems, and almost threefold, nearly twofold, and 46.2%, respectively, smaller than in the 2-year-old areas. The soil calcium carbonate content was similar in soils of the natural hillslopes and 9-year-old afforestation lands, which was similar to 17% smaller than in the 2-year-old afforestation systems. Considerable differences among the land-uses were also recorded for the soil texture. Mean coarse root biomass, despite revealing only a marginally-significant effect (p-value = 0.0765), was 40.0% greater in the natural hillslopes than in the 9-year-old systems, and more than twofold greater than in the 2-year-old systems. Hillslope aspect affected some of the measured properties, revealing only slightly better soil conditions in the (mesic) north- than in the (xeric) south-aspects. The data revealed that forestry-related earthworks degrade the soil quality and geo-ecosystem functioning in the short term. Yet, despite some discrepancies, the data also showed the occurrence of self-restoration processes of the geo-ecosystem over the long term. (C) 2016 Elsevier B.V. All rights reserved.