The objective of the present study was to evaluate the effect of afforestation of degraded agricultural soils on soil organic carbon (OC) stocks and related soil physical properties. We assumed that forest species and soil texture would have a strong effect on C stabilization and thus would also affect some physical properties. The study was composed of two experiments to evaluate (i) the effect of different tree species and (ii) the effect of soil texture on OC and physical properties. In the first case, soil samples were taken in 40-year-old plantations of Pinus haleppensis, P. haleppensis inoculated with ectomycorrhiza at planting, Pinus pinea, Eucalyptus spp., and Gleditsia triacanthos. For the second experiment, soils under three land-use types, namely, agriculture, P. haleppensis forest, and native savanna virgin were sampled at seven different sites in La Pampa, Argentina, that represented a climatic gradient from 850- to 600-mm annual rainfall and a textural gradient ranging from loam, sandy loam, to sand. The measured variables were bulk density, particle density, water retention at field capacity water content, structural stability index, total OC, large-aggregate C, intermediate-aggregate C, and small-aggregate C. Forest species had a differential effect on physical properties, especially bulk density, and OC contents. The forest species with the most favorable effects were P. haleppensis inoculated with ectomycorrhiza and G. triacanthos, whereas P. pinea and Eucalyptus were least effective, confirming the beneficial role of mycorrhizal fungi and symbiotic N fixation for restoring C stocks in a semiarid temperate environment. In all sites, we found significant differences in total OC and large-aggregate C contents between agriculture and forest treatments, thus indicating that afforestation with P. haleppensis would favor C sequestration in any of the three textural classes.

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