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, intermediateaggregate 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.
Land-use change and specifically a change in the type of vegetation cover affects soil morphology, chemistry, biology, and nutrient regimes. Numerous studies have documented that in land-use conversions from agricultural land to forest, or from plantations to restored natural savanna most soil attributes and functions undergo changes. The purpose of the present study was to evaluate the changes brought about by afforestation of degraded croplands and to understand the impact of forest vegetation on soil evolution in a semiarid region where soils originally co-evolved with a savanna biotope. We used long-term experiments (>40 years) of five tree species: Pinus halepensis (PH), Pinus halepensis inoculated with ecto-mycorrhiza at planting (PM), Pinus pinea (PP), Eucalyptus spp. (E), and Gleditsia triacanthos (G) and compared these to an agricultural soil (A) at the same site near Santa Rosa, La Pampa in the semiarid center of Argentina. Soil profiles were described, and samples taken for chemical and physical analyses of soil properties [organic matter (OM), pH, cation exchange capacity (CEC) and exchangeable cations, particle size distribution (texture), aggregate stability (MWD), bulk density (BD), porosity (TP), and water holding capacity (WHC)]. We found a strong effect of tree species on soil profile morphology, even taxonomy, and on all studied variables. PM and G had highest OM, CEC, neutral pH, higher TP, WHC, while PH, PP, and E had acid pH, lower base saturation, OM, TP, and WHC. The effect of tree species on the soil profile was noticeable a depth of about 40 cm, comprising the A and AC, but not the C horizons. The results showed that to obtain reasonable results of OM sequestration under forest systems, tree species should be chosen to include legumes to improve C/N stoichiometry for C fixation, or inoculation with mycorrhiza to promote microbial transformation of forest litter.
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